Comparison of Ipad Pro®'S Lidar and Truedepth Capabilities with An

technologies

Article Comparison of iPad Pro®’s LiDAR and TrueDepth Capabilities with an Industrial 3D Scanning Solution

Maximilian Vogt 1,* , Adrian Rips 2 and Claus Emmelmann 3

1 Fraunhofer Research Institution for Additive Manufacturing Technologies IAPT, 21029 Hamburg, Germany 2 School of Mechanical Engineering, Hamburg University of Technology TUHH, 21073 Hamburg, Germany; [email protected] 3 Institute of Laser and System Technologies, Hamburg University of Technology TUHH, 21073 Hamburg, Germany; [email protected] * Correspondence: [email protected]

Abstract: Today’s smart devices come equipped with powerful hard- and software-enabling pro- fessional use cases. The latest hardware by Apple utilizes LiDAR and TrueDepth, which offer the capability of 3D scanning. Devices equipped with these camera systems allow manufacturers to obtain 3D data from their customers at low costs, which potentially enables time-efﬁcient mass customization and product differentiation strategies. However, the utilization is limited by the scanning accuracy. To determine the potential application of LiDAR and TrueDepth as a 3D scanning solution, in this paper an evaluation was performed. For this purpose, different Lego bricks were scanned with the technologies and an industrial 3D scanner. The results were compared according to shape and position tolerances. Even though the industrial 3D scanner consistently delivered more

accurate results, the accuracy of the smart device technologies may already be sufﬁcient, depending on the application.

Citation: Vogt, M.; Rips, A.; Emmelmann, C. Comparison of iPad Keywords: 3D scanning; reverse engineering; iPad Pro; TrueDepth Camera; LiDAR Pro®’s LiDAR and TrueDepth Capabilities with an Industrial 3D Scanning Solution. Technologies 2021, 9, 25. https://doi.org/10.3390/ 1. Introduction technologies9020025 To ensure the future competitiveness of manufacturing companies, it is necessary to face constantly changing customer requirements and market turbulence. Consequently, a Academic Editor: Manoj Gupta shortening of the product development cycle is constantly pursued in order to adapt to the dynamic market efficiently and quickly [1]. Manufacturers today tend to adopt product Received: 14 February 2021 differentiation strategies and more customer-centric approaches to remain competitive. Accepted: 3 April 2021 Hence, mass customization and product diversification are one of the most commonly Published: 7 April 2021 implemented business models. As part of the digital transformation and Industry 4.0, these objectives are being realized. Additive manufacturing technologies offer high potential for Publisher’s Note: MDPI stays neutral individualization at low cost in a short period of time [2]. Cost-effective and high-quality with regard to jurisdictional claims in published maps and institutional affil- mass customization requires technological resources such as reverse engineering and 3D iations. scanning. Currently, smartphones and tablets can be utilized as 3D scanners as well [3]. This lowers the entry barrier for both private users and industrial users to digitize objects. While the customer only needs to be provided with the hardware, the manufacturer can realize the product design by means of reverse engineering (RE). This means that the basic scanning can be performed by customers themselves so that the manufacturer can offer Copyright: © 2021 by the authors. them a product tailored to their needs. An example for this is the business model of the Licensee MDPI, Basel, Switzerland. company HEXR, which produces custom-fit bike helmets with additive manufacturing. In This article is an open access article order to scan the shape of the customers’ heads, a smartphone application and a fitting cap distributed under the terms and conditions of the Creative Commons are needed. The application uses the standard smartphone camera and the random texture Attribution (CC BY) license (https:// of the fitting cap to model the shape of the head [4]. However, with LiDAR and TrueDepth creativecommons.org/licenses/by/ technologies included in the newest devices by Apple, even more enhanced possibilities to 4.0/). digitize real objects are offered.

Technologies 2021, 9, 25. https://doi.org/10.3390/technologies9020025 https://www.mdpi.com/journal/technologies Technologies 2021, 9, 25 2 of 13

Ref. [5] defines the processes of designing, manufacturing, assembling and maintain- ing products and systems as engineering and divides it into two different types. While forward engineering implies the process of the approach from a highly abstracted idea to the physical implementation of a system, RE refers to already existing objects. Thereby, an existing part, subassembly or product without drawings and documentations is duplicated. Furthermore, RE is a process to obtain a 3-dimensional computer model from an existing object through measurements without consideration of functionalities. This process is also known as Computer Aided Reverse Engineering (CARE), which involves the steps of data collection, mesh construction and surface fitting [6]. For data collection, different hardware can be used, whereas mesh construction and surface fitting focus mainly on software solutions. The hardware is divided into noncontact and contact-based methods, which can be used for data acquisition [7]. When the highest scan accuracy is required, coordinate measuring machines (CMM) can be used. These are based on the principle of contact by measuring the surface to be tested with a probe. Due to the high precision manufacturing processes and built-in reference standards, CMMs provide the highest accuracy in inspection processes [6]. In addition, the results are independent of the reflection rate of a surface. Limited operating space is a point of emphasis because the object needs to be accessed and touched by the probe. The amount of contact points determines the accuracy. Therefore, the process can get time-consuming by inspecting complex structures as well as unknown shapes [6–8]. An alternative to this procedure is the noncontact method, in which no physical contact to the surface is needed. Noncontact methods are subdivided into different scan technologies including photogrammetry, structured light and Time of Flight (ToF). Those techniques are used by a variety of scanning systems with different capabilities and limitations [7,9]. Based on the field of application, it is important to select a suitable scanner that meets the requirements. Besides a relatively short processing time, noncontact 3D scanners cost much less compared to a CMM [8]. In addition, a high point cloud density of some 3D scanners can be used to measure complex and deep features with high accuracy. Thereby, the point density is defined by the quantity of emitted points and the obtained measured values. As a result, a higher point density provides more measured values for the same scan area [10]. Portable scanners are also becoming increasingly interesting for the manufacturing industry. The scanning method Structured Light is based on the principle of triangulation, while incident laser lines are projected onto the object to be scanned. A common industrial 3D scanner is the Artec Space Spider, which uses structured light as scan technology. The Artec Space Spider uses a blue LED with a narrow wavelength, which allows better filtering of interference from ambient light [11]. From the narrowband of laser stripes with known angular width, many points of the stripe are reflected by the surface. These points are detected by a charge-coupled device (CCD) sensor and transformed to 3D coordinates via the triangulation principle [8]. In total, the Artec Space Spider uses three cameras at various angles and depths. In the center of the device, an RGB camera is installed surrounded by LED flash light to capture textures without the need for special light setup. Thereby the Artec Space Spider is advertised with a pinpoint accuracy of 0.05 mm and 3D resolution of 0.1 mm [11]. Recent developments of commercial devices such as smartphones and tablets have shown that in addition to photogrammetry, scanning is also feasible with LiDAR (light detection and ranging) and TrueDepth. LiDAR includes ToF measurements, which determine the time it takes for an object, particle or wave to travel a distance. Therefore, LiDAR emits a pulse or modulated light signal and measures the time difference in the returning wavefront [12]. TrueDepth uses vertical-cavity surface-emitting laser (VCSEL) technology and consists of a traditional camera, an infrared camera, a proximity sensor and a dot projector as well as a flood illuminator. The system is named and patented by Apple. The dot projector emits more than 30,000 points of infrared light, which are reflected on the surfaces. Consequently, the infrared camera picks up these light dots again and the pattern Technologies 2021, 9, x FOR PEER REVIEW 3 of 15

Technologies 2021, 9, 25 3 of 13

surfaces. Consequently, the infrared camera picks up these light dots again and the pat- ternis analyzed is analyzed by softwareby software to createto create a depth a depth map. map. Using Using this this depth depth map, map, aa mathematicalmathematical model is generated by machine learning algorithms [9,13–15]. [9,13–15]. InIn iOS,iOS, thethe operating operating system system of Apple’sof Apple’ smartphoness smartphones and tablets,and tablets, TrueDepth TrueDepth is mainly is mainlyused for used 3D facefor 3D authentication face authentication and recognition, and recognition, while LiDAR while enablesLiDAR enables new features new fea- for turesAugmented for Augmented Reality by Reality accelerating by accelerating plane detection. plane detection. To scan To objects, scan objects, it is therefore it is therefore neces- necessarysary to install to install an additional an additional application. application. Heges Heges is such is such an iOS an applicationiOS application that exploitsthat ex- ploitsTrueDepth TrueDepth and LiDAR. and LiDAR. The authors The authors of [16] of evaluated [16] evaluated different different smartphone smartphone applications, applications,showing showing that Heges that has Heges one has of the one finest of the 3D finest resolutions 3D resolutions (0.5 mm). (0.5 The mm). application The application can be canused be to used scan to objects scan objects and export and scansexport as scans STL andas STL polygon and polygon (PLY) files, (PLY) while files, colors while can colors also canbe captured. also be captured. In contrast In tocontrast other applications,to other applications, Heges is alsoHeges not is limited also not to limited scan faces to scan only. facesIn addition, only. In a addition, screen sharing a screen function sharing facilitates function scanning, facilitates and scanning, a one-time and paymenta one-time allows pay- mentunlimited allows export unlimited of these export scan filesof these [16]. scan Apple files itself [16]. does Apple not specifyitself does the accuracynot specify of the accuracyrespective of technologies the respective or technologies hardware. or hardware. While the the hardware hardware and and the the software software of of th thee device device determine determine the the scan scan accuracy accuracy internally,internally, there there are are also also external external factors factors influe influencingncing scan scan quality. quality. A A literature literature research research has shownshown that factors are reflectance, reflectance, shape and color of the object as well as surface texture and ambient lightning. In In addition, addition, the the distance distance between object and scanner, scanning strategystrategy and scanning movements influencesinfluences scanscan quality [[7–9,17].7–9,17]. During During post-processing, post-processing, thethe meshing and and surface surface fitting fitting ha haveve also also shown influences influences [17] [17].. Depending Depending on on the scan accuracy, different fieldsfields ofof applicationsapplications cancan bebe exploited.exploited. To determine the potential use of LiDAR an andd TrueDepth included in the recent iPad Pro and iPhone 1212 ProPro lineup lineup as as a a 3D 3D scanner, scanner, this this study study was was conducted. conducted. For For the the evaluation, evaluation,an iPad an iPad Pro (2020)Pro (2020) [3] was [3] was used used and and was was compared compared to the to the industrial industrial Artec Artec Space Space Spider Spi- derHandheld Handheld 3D Scanner3D Scanner [11]. [11]. Therefore, Therefore, different different Lego Lego bricks bricks were were scanned scanned and comparedand com- via the software GOM Inspect. The aim of this study was to determine the scan accuracy of pared via the software GOM Inspect. The aim of this study was to determine the scan LiDAR and TrueDepth as 3D scanning technique. accuracy of LiDAR and TrueDepth as 3D scanning technique. 2. Materials and Methods 2. Materials and Methods The procedure of this study is shown in Figure1 and can be divided into three steps: The procedure of this study is shown in Figure 1 and can be divided into three steps: (1) Scanning • (2) (1)Measurement Scanning • (3) (2)Comparison Measurement • (3) Comparison

Real Lego Brick

3D Scanning Measurement Industrial 3D-Scanner Heges Application Keyence VHX-5000 Artec Space Spider iPad Pro 2020 Blue-Light Technology TrueDepth LiDAR

Scanned Scanned Scanned Reference Data 1 Data 2 Data 3 CAD Model Comparison Artec GOM MeshLab Studio Inspect

Figure 1. Schematic illustration of the procedure to evaluate the accuracy of TrueDepth and light detection and ranging Figure 1. Schematic illustration of the procedure to evaluate the accuracy of TrueDepth and light detection and ranging (LiDAR) using an iPad Pro (2020) in comparison to an industrial 3D scanning system. (LiDAR) using an iPad Pro (2020) in comparison to an industrial 3D scanning system.

Technologies 2021, 9, 25 4 of 13

2.1. Scanning The ﬁrst step is to scan an object. Thereby, it is important to control the factors that

Technologies 2021, 9, x FOR PEER REVIEWinﬂuence the scan quality and only modify the factors that should be4 of analyzed. 13 In this Technologies 2021, 9, x FOR PEER REVIEW 4 of 13 Technologies Technologies 2021, 9, x FOR 2021 PEER, 9, x REVIEWFORstudy, PEER REVIEW the factors to be analyzed are the inﬂuence of different colors4 of and 13 shape4 of 13 toler- Technologies Technologies 2021, 9, x FOR 2021 PEER, 9, x REVIEWFOR PEER REVIEW 4 of 13 4 of 13

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OverviewofOverview the used of ofofScanned Lego thethe usedusedbricksTolerance Lego LegoLego for the Bricksbricksbricks inspection Type forfor by thethe Shape inspectioninspectionof geometric Tolerances Lego ofof Brick geometriccharacteristicsgeometric According characteristicscharacteristics and to tolerances DIN and andEN tolerances[19].tolerancesISO 1101 Control[19]. [19]. Summary SymbolTable EvaluationGeometric 1. OverviewTableCharacteristicEvaluation Characteristicof1. of OverviewScanned the used of ofScanned LegoLego theTolerance usedbricksBricks Lego Lego for by Type the Bricksbricks Shape inspection for by Tolerances Legothe Shape ofinspection Brickgeometric Tolerances According of Elementcharacteristicsgeometric According to NumberDIN characteristics and EN to tolerancesNumber ISODIN1 Control 1101 andEN [19].1tolerancesISO Summary 1101 [19]. TableSymbolEvaluation 1. OverviewTable EvaluationGeometric 1.of Overviewof Scanned the used Characteristicof ofScanned LegoLego the used bricksBricks Lego Lego for Toleranceby thebricksBricks Shape inspection for by TolerancesType the Shape inspection ofof geometricgeometric TolerancesLego According of Brick geometriccharacteristicscharacteristics According to ElementDINcharacteristics andand EN to tolerancestolerances ISONumberDIN and1101 EN tolerances [19]. [19].ISO 1 Control 1101 [19]. Summary Symbol SymbolGeometric Geometric Characteristic Characteristic Tolerance Tolerance Type TypeLego BrickLego BrickElement NumberElement Number1 Control 1 SummaryControl Summary EvaluationEvaluationEvaluation of Scanned ofof LegoScannedScanned Bricks LegoLego by BricksBricks Shape by byTolerances ShapeShape TolerancesTolerances According AccordingAccording to DIN EN toto ISO 1DINDIN 1101 ENEN ISO1ISO 11011101 Symbol SymbolEvaluationGeometric EvaluationGeometricEvaluation Characteristicof Scanned Characteristicof of Scanned LegoScanned Tolerance Bricks Lego Lego Toleranceby Type Bricks BricksShape by byTolerancesTypeLego Shape Shape Brick Tolerances TolerancesLego According BrickElement According According to DINElementNumber EN to to ISODINNumber DIN Control 1101 EN EN ISO ISO SummaryControl 1101 1101 Summary Symbol SymbolSymbolGeometric GeometricGeometric Characteristic CharacteristicCharacteristic Tolerance ToleranceTolerance Type TypeTypeLego BrickLegoLego BrickBrickElement NumberElementElement 1NumberNumber Control 11 SummaryControlControl SummarySummary Symbol SymbolGeometric StraightnessGeometric Characteristic Characteristic Tolerance Tolerance Type TypeLego BrickLego ElementBrick4,181,139/44,237 NumberElement 11Number Control1 1 SummaryControl Summary Symbol SymbolGeometric GeometricStraightness CharacteristicStraightness Characteristic Tolerance Tolerance Type TypeLego BrickLego BrickElement NumberElement4,181,139/44,2374,181,139/44,237 Number Control SummaryControl Summary StraightnessStraightness 4,181,139/44,2374,181,139/44,237 StraightnessStraightness 4,181,139/44,2374,181,139/44,237

Straightness StraightnessStraightness 4,181,139/44,237 4,181,139/44,2374,181,139/44,237 Straightness StraightnessStraightness 4,181,139/44,2374,181,139/44,2374,181,139/44,237 Controls form or sur- Form (No Rela- Controls form or sur- Flatness Form (No Rela- 300,123/3001 Controlsfaces of aControlsform feature or sur-Controls inform or sur- form or Flatness Formtion between(NoForm Rela- (No Rela- 300,123/3001Controls Controlsformfaces or of sur- aform feature or sur- in FlatnessFlatness Flatness tion between 300,123/3001300,123/3001300,123/3001 faces of afaces feature of ain feature in Form (NoForm Rela- (No Rela- Controlsrelation form ControlsControlsto its or own sur- formform surfaces oror sur-sur- of a Flatness Flatness FormtionFormFeatures) between(No (NoFormForm Rela-tion Relation between (No(No Rela-Rela- 300,123/3001300,123/3001 Controlsfaces of aControlsformControlsfaces relationfeature or of sur- aformin formto feature its or orown sur- sur-in Flatness FlatnessFlatness Formtion between(NoForm Rela-tionFeatures) between (No Rela- 300,123/3001 300,123/3001300,123/3001 facesrelationperfect of a to facesfeaturerelation form. its ownoffeature ina to feature its own in relationin Flatness Flatness betweenFeatures)Form Features) (No Rela- 300,123/3001 300,123/3001 facesrelation of a to facesfeaturerelation itsperfect own of in ato featureform. its own in Roundness (Circularity)Flatness tion betweentiontion betweenbetween 6,322,842/98,138300,123/3001 faces of a facesfeaturefacesperfect of of a ina feature featureform. in in Roundness (Circularity)tiontionFeatures) betweenbetween tiontionFeatures) between between 6,322,842/98,138relationperfect torelation relationform. its own to toto its itsits own ownown perfect RoundnessRoundness (Circularity) (Circularity) Features) Features)Features) 6,322,842/98,138 6,322,842/98,138 relationperfect torelation relationform. itsperfect own to to form. its its own own Roundness Features)Features) Features) perfect form.perfectperfect form.form.form. Roundness Roundness (Circularity) (Circularity) 6,322,842/98,138 6,322,842/98,1386,322,842/98,138 perfect form.perfectperfect form. form. Roundness(Circularity)RoundnessRoundness (Circularity) (Circularity)(Circularity) 6,322,842/98,138 6,322,842/98,1386,322,842/98,138 Roundness CylindricityRoundnessRoundness (Circularity) (Circularity) (Circularity) 6,322,842/98,13817,715/87,9946,322,842/98,1386,322,842/98,138 Cylindricity 17,715/87,994 CylindricityCylindricity 17,715/87,994 17,715/87,994 CylindricityCylindricity 17,715/87,994 17,715/87,994

CylindricityCylindricityCylindricityCylindricity 17,715/87,994 17,715/87,99417,715/87,99417,715/87,994 CylindricityCylindricityCylindricity 17,715/87,99417,715/87,99417,715/87,994 Position 300,123/3001 Position 300,123/3001 Position Position 300,123/3001300,123/3001 Position Position 300,123/3001300,123/3001 Position PositionPosition 300,123/3001 300,123/3001300,123/3001 Position PositionPosition 300,123/3001 300,123/3001300,123/3001 Controls size, form Position 300,123/3001 Controls size, form Controlsand orientation Controlssize, form of size, form Profile of a surface 230,223 Controls Controlssize,and orientationform size, form of Profile of a surfaceLocation 230,223Controlsandscanned orientation size, Controlsandsurfaces orientationformControls of size, form size, of form Profile ofProfile a surface of a surface Location 230,223 230,223Controlsand orientation size,Controlsandscanned orientationform of size,surfaces form of Location Controls Controlssize,Controlsscanned form size, size, surfaces form form Profile ofProfile a surface of a surfaceLocation 230,223 230,223basedandscanned onorientation datum surfacesandand orientation orientationrefer-and of orientation ofof of Profile of ProfileProfilea surface ofof aa surfacesurfaceLocation Location 230,223 230,223230,223andscanned orientationbased andsurfacesscannedand onorientation orientation of datum surfaces refer- of of Profile of aProfile surface of a surfaceLocation Location Location 230,223 230,223basedscanned onences. bdatumased surfacesscanned on refer-scanned datum surfaces refer- surfaces Proﬁle Profile of a surface of a surfaceLocation Location 230,223230,223basedscanned on bdatumased surfacesscanned on refer-ences. datum surfaces refer- Location LocationLocation scanned surfacesscannedscannedences.based surfaces surfaces on datum based onences. bdatumbasedased onon refer- datumdatum refer-refer- based onences. bdatumbasedased on onrefer-ences. datum datumreferences. refer- refer- Profile of a line 230,223 ences. ences.ences. Profile of a line 230,223 ences. ences.ences. Profile ofProfile a line of a line 230,223 230,223 Profile of a line 230,223 Profile of a line 230,223 ProfileProﬁle of ProfileProfilea of line a line ofof aa lineline 230,223 230,223230,223230,223 Profile ofProfile Profilea1 line Element of of a anumber line line of the Lego brick in the Lego online store 230,223 [20]. 230,223230,223 1 Element number of the Lego brick in the Lego online store [20]. 1 Element number1 Element of number the Lego of bric thek Lego in the bric Legok in online the Lego store online [20]. store [20]. 1 Element number1 Element of number the Lego of bric thek Lego in the bric Legok in online the Lego store online [20]. store [20]. 1 1 1 Element1 Element number1 ElementElement number of number numberthe ofLego the ofof bricLego thethek Lego Legoin brick the bricbric Lego inkk the inin online Legothethe LegoLego store online onlineonline [20]. store storestore [20 [20].].[20]. 11 ElementElement number1number1 Element Element ofof number numberthethe LegoLego of of bricbric the thek Lego Legoin the bric bric Legokk in in online the the Lego Lego store online online [20]. store store [20]. [20].

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ToTo control control influencing influencing factors, factors, the thefollowing following setup setup was waschosen: chosen: one object one object was placed was placed onon a rotary a rotary table table with with an anirregular irregular surface surface texture, texture, which which increases increases the theobject object tracking tracking due due to tothe the high-contrasthigh-contrast colorcolor design.design. Radially the the scanning scanning devices devices were were stationary-mounted stationary-mounted with witha constant a constant distance distance of of 300 300 mm mm to to the the middlemiddle ofof the table (object). (object). To To control control the the scan- scanning ningspeed, speed, the the table, table, including including the the irregular irregularsurface surface texturetexture and the the object, object, was was rotated rotated one onefull full rotation rotation per per minute. minute. OneOne scanner was was activated activated for for one one minute minute scanning scanning the theobject object in in360 360°.◦. AfterAfter aa scan,scan, thethe processprocess waswas repeatedrepeated analogouslyanalogously withwith thethe nextnext scanningscanning technologytech- nologybefore before the next the objectnext object was placed.was placed. This ensuredThis ensured that thethat placement the placement of the of Legothe Lego brick was brickalways was thealways same the for same each for scan each technology scan technology and each and scanning each scanning session. session. In total, In eachtotal, object eachwas object scanned was by scanned each scan by technologyeach scan technology three times. three All threetimes. sensors All three were sensors aligned were so that a alignedscan angle so that of a 65 scan◦ was angle set of with 65° respectwas set towith the respect surface. to For the TrueDepth,surface. For theTrueDepth, iPad was the laid flat iPadon was its display laid flat on on a its stand, display because on a stand, it is a because front-facing it is a sensor. front-facing In addition, sensor. In the addition, device had to thebe device rotated had 180 to◦ .be rotated 180°. ToTo ensure ensure the the same same post-processing, post-processing, the automatic the automatic meshing meshing in Artec in Artec Studio Studio 15 was 15 was used every time. For the scans with Heges, the files were exported as stereolithography used every time. For the scans with Heges, the files were exported as stereolithography files (*.STL) and cleaned in MeshLab the same way. files (*.STL) and cleaned in MeshLab the same way.

2.2.2.2. Measurement Measurement TheThe digital digital microscope microscope Keyence Keyence VHX-5000 VHX-5000 was was used used to obtain to obtain the theactual actual geometry geometry of of an object using the optical metrology with 200× magnification. Based on those meas- an object using the optical metrology with 200× magnification. Based on those measure- urements, the bricks were reconstructed in Autodesk Inventor 2020 and saved as inventor ments, the bricks were reconstructed in Autodesk Inventor 2020 and saved as inventor part part files (*.ipt). The scanned meshes and designed components were imported in GOM files (*.ipt). The scanned meshes and designed components were imported in GOM Inspect Inspect Suite 2020. This Software offers a wide range of inspection tools with high accu- Suite 2020. This Software offers a wide range of inspection tools with high accuracy and a racy and a low standard deviation, as tests performed by [21] showed. low standard deviation, as tests performed by [21] showed. 2.3. Comparison 2.3. Comparison The generated meshes were examined for deviations in mm as absolute values as The generated meshes were examined for deviations in mm as absolute values as shown in Figure 2. While the color influence was investigated by a flatness test (a), shape shown in Figure2. While the color influence was investigated by a flatness test (a), shape tolerances were examined with inspection tools from GOM Inspect (b). In this analysis no tolerances were examined with inspection tools from GOM Inspect (b). In this analysis no relation between features was considered. To compare the size, form and orientation of relation between features was considered. To compare the size, form and orientation of scanned surfaces based on datum references, a target-actual comparison was established (c).scanned Therefore, surfaces the scanned based on Lego datum bricks references, were aligned a target-actual with the reconstructed comparison wasone establishedin GOM (c). Inspect.Therefore, Then, the the scanned deviation Lego between bricks the were sca alignednned mesh with to the the reconstructed constructed part one was in GOM deter- Inspect. mined.Then, For the the deviation profile of between a line th thee edge scanned of a curved mesh surface to the constructedwas used. part was determined. For the profile of a line the edge of a curved surface was used.

(a) (b) (c)

FigureFigure 2. 2. ExaminationExamination of of deviation deviation wi withth GOM GOM Inspect Inspect Suite Suite 2020: 2020: (a) flatness (a) flatness test by test a byscanned a scanned Lego Legobrick with brick the with Artec the Artec SpaceSpace Spider Spider whether whether colors colors influence influence the the scan scan quality quality scanned; scanned; (b) form (b) form tolerance tolerance analysis analysis of a Lego of a Legobrick scanned brick scanned by the by the TrueDepthTrueDepth camera; camera; (c ()c )target-actual target-actual comparison comparison of a of Lego a Lego brick brick scanne scannedd by the by TrueDepth the TrueDepth camera camera for the for examination the examination of of scan accuracy of a profile on a surface. scan accuracy of a profile on a surface.

Technologies 2021, 9, 25 6 of 13 Technologies 2021, 9, x FOR PEER REVIEW 6 of 15

3. Results Tables A1–A6A1–A6 (in(in Appendix Appendix A)A) list list the the determined determined values, values, whereby whereby the thedata data are sub- are dividedsubdivided according according to factors to factors and and scanning scanning technologies. technologies. Furthermore, Furthermore, the thetables tables lists lists the threethe three determined determined values, values, the mean the mean and andstandard standard deviation deviation as well as wellas the as variance. the variance. It was It foundwas found that LiDAR that LiDAR as a scanning as a scanning technique technique on the iPad on the Pro iPad was Pronot applicable was not applicable for scanning for smallscanning objects small such objects as a Lego such asbrick. a Lego Figure brick. 3 shows Figure that3 shows the generated that the generated mesh was mesh not accu- was ratenot accurateenough enoughfor an evaluation. for an evaluation. Consequent Consequently,ly, the LiDAR the LiDAR scanning scanning technique technique was wasnot listednot listed in the in following the following results. results.

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Figure 3. ((a)) Screenshot Screenshot of the the camera camera preview preview of a Lego Lego brick brick on on an an iPad iPad Pro Pro (2020); (2020); ( (bb)) Screenshot Screenshot of of the the scanning scanning preview preview via LiDAR in the same scenarioscenario usingusing thethe HegesHeges AppApp onon anan iPadiPad ProPro (2020).(2020).

3.1. Impact Impact of of Color Color The first first factor to be modified modified is the colorcolor provided by seven different-colored Lego bricks, which are shown in Figure4 4.. UnderUnder bothboth scanscan techniques,techniques, thethe white,white, blue,blue, andand turquoiseturquoise Lego bricks were scanned with the highest flatness flatness represented by the smallest deviations. The smallestsmallest deviationdeviation between between the the two two scanning scanning techniques techniques was was shown shown by theby thewhite white brick. brick.

Figure 4. The seven different colored Lego bricks used in the ﬁrstfirst experiment: black, red, oran orange,ge, green, blue, turquoise, and white (left to right).

The results ofof thethe experimentexperiment areare shown shown in in Figure Figure5. 5. Thereby, Thereby, the the average average deviation deviation of of0.14 0.14 mm mm (Artec (Artec Space Space Spider) Spider) was was very very similar similar to to 0.16 0.16 mm mm (TrueDepth).(TrueDepth). InIn contrast,contrast, the green and red Lego brick using TrueDepth showed the greatest deviations with 0.35 mm and 0.32 mm.mm. UsingUsing thethe ArtecArtec Space Space Spider Spider and and the the iPad iPad Pro Pro TrueDepth TrueDepth camera, camera, it was it was not notpossible possible to track to track or scan or thescan black the black Lego brick.Lego Thebrick. red The Lego red brick Lego could brick also could not also be scanned not be scannedwith the Artecwith the Space Artec Spider Space but Spider with TrueDepth, but with TrueDepth, where the highest where standardthe highest deviation standard of deviation0.104 mm of was 0.104 obtained. mm was In obtained. all three cases, In all thethree Lego cases, brick the was Lego not brick even was displayed not even on dis- the playedscreen, makingon the screen, scanning making impossible. scanning The impossible. data obtained The can data be foundobtained in Tables can be A1 found and A2 in Tablesin Appendix A1 andA .A2 in Appendix A.

Technologies 2021, 9, x FOR PEER REVIEW 8 of 15

Technologies 2021, 9, 25 7 of 13

0.45 0.40 0.35 0.30 0.25

[mm] 0.20 0.15 0.10 0.05 0.00 White Orange Red Green Blue Black Turquoise

Technologies 2021, 9, x FOR PEER REVIEW Mean Deviation of Arctec Space Spider Mean Deviation of iPad Pro TrueDepth 9 of 15

FigureFigure 5. Comparison 5. Comparison of the of determined the determined flatness flatness values values of seven of different seven different colored coloredLego bricks Lego bricks scanned scanned by Artec by Artec Space Space Spider Spider and and the theiPad iPad Pro Pro (2020) (2020) using using TrueDepth. TrueDepth. 3.2. Scanning Accuracy 3.2. Scanning Accuracy The scanning accuracy was determined in two different ways as summarized in FiguresThe scanning6 and7. Figureaccuracy6 provides was determined the results in two of shape different tolerances, ways as while summarized Figure7 inshows Fig- uresthe 6 resultsand 7. Figure of the target-actual6 provides the comparison. results of shape Objects tolerances, scanned while by the Figure Artec 7 Spaceshows Spider the resultsshowed of the similar target-actual accuracy comparison. in straightness Object (0.15s scanned mm in average), by the Artec flatness Space (0.14 Spider mm in showed average), similarand cylindricityaccuracy in (0.17straightness mm in average),(0.15 mm wherebyin average), roundness flatness had(0.14 much mm in greater average), deviations and cylindricity(0.96 mm (0.17in average). mm in average), The best whereby results usingroundness TrueDepth had much were greater achieved deviations in straightness (0.96 mm(0.44 in average). mm in average) The best andresults flatness using (0.41TrueDe mmpth in were average), achieved and in the straightness highest deviations (0.44 mm in in average)cylindricity and (0.82 flatness mm (0.41 in average) mm in average) and roundness, and the (1.17 highest mm deviations in average). in cylindricity The standard (0.82deviation mm in average) varied from and roundness 0.006 mm to(1.17 0.197 mm mm in average). for the Artec The standard Space Spider deviation and 0.049 varied mm fromto 0.2680.006 mm to for 0.197 TrueDepth. mm for Thethe Artec data obtainedSpace Spider can beand found 0.049 inmm Tables to 0.268 A3 andmm A4for in TrueDepth.Appendix TheA. data obtained can be found in Tables A3 and A4 in Appendix A.

1.80 1.60 1.40 1.20 1.00

[mm] 0.80 0.60 0.40 0.20 0.00 Straightness Flateness Roundness Cylindricity

Mean Deviation of Arctec Space Spider Mean Deviation of iPad Pro TrueDepth

Figure 6. Comparison of the determined form tolerance values of different Lego bricks scanned by Figure 6. Comparison of the determined form tolerance values of different Lego bricks scanned by Artec space spider and the iPad Pro (2020) using TrueDepth. Artec space spider and the iPad Pro (2020) using TrueDepth.

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During the evaluation of the target-actual performance comparison, the geometric characteristic position showed the smallest deviations. However, the results of the TrueDepth scans (1.03 mm in average) were more than twice as high as those of the Artec Space Spider (0.43 mm in average), as shown in Figure 7. When scanning a profile of a surface, a mesh with higher deviations was generated (0.68 mm in average for Artec Space Spider and 1.13 mm in average for iPad Pro). Nevertheless, the highest overall values of deviations were determined during the examination based on the profile of a line (4.51 Technologies 2021, 9, 25 8 of 13 mm in average for Artec Space Spider and 4.92 mm in average for iPad Pro). The data obtained can be found in Tables A5 and A6 in Appendix A.

6.00

5.00

4.00

3.00 [mm]

2.00

1.00

0.00 Position Profile of a surface Profile of a line

Mean Deviation of Arctec Space Spider Mean Deviation of iPad Pro TrueDepth

FigureFigure 7. Comparison 7. Comparison of ofthe the determined determined position position and and profile profile tolerance tolerance values values of of different Lego bricks bricksscanned scanned by Artec by Artec Space Space Spider Spider and theand iPadthe iPad Pro (2020)Pro (2020) using using TrueDepth. TrueDepth. During the evaluation of the target-actual performance comparison, the geomet- 4. Discussion ric characteristic position showed the smallest deviations. However, the results of the TrueDepthIn this study, scans an (1.03 iPad mm Pro in (2020) average) was were used more as a than3D scanner twice as to high evaluate as those the of accuracy the Artec of Spacethe built-in Spider LiDAR (0.43 mmand TrueDepth in average), technologies as shown in in Figure comparison7. When to an scanning industrial a profile 3D scan- of a ningsurface, system. a mesh Therefore, with higher Heges deviationswas chosen was as th generatede application (0.68 offering mm in average one of the for finest Artec scan Space resolutionsSpider and (0.5 1.13 mm). mm As in scan average objects, for iPadLego Pro). bricks Nevertheless, of different shapes the highest were overallused. By values scan- of ningdeviations the same were objects determined simultaneously during thewith examination the industrial based 3D on scanner the profile Artec of Space a line (4.51Spider, mm thein results average were for Artecthen analyzed Space Spider and andcompared 4.92 mm in inGOM average Inspect. for iPad Pro). The data obtained canBased be found on a inliterature Tables A5 research, and A6 therein Appendix are differentA. kinds of measuring methods, pre- senting processes to determine the scanning accuracy [7–9,17]. The VDI 2634, for instance, defines4. Discussion a process for the verification of optical 3D measuring systems. However, this methodIn is this for study, 3D scanners an iPad using Pro (2020) triangulation, was used whereas as a 3D scanner LiDAR tois evaluatebased on the ToF accuracy and TrueDepthof the built-in on VCSEL LiDAR technology. and TrueDepth Since there technologies is no official in comparisonmethod to determine to an industrial the scan- 3D ningscanning accuracy, system. this study Therefore, evaluates Heges the was scanning chosen accuracy as the application due to shape offering tolerances. one of the finest scanThe resolutions LiDAR technology (0.5 mm). proved As scan to objects,be imprac Legotical bricks for scanning of different small shapes objects were such used. as LegoBy scanningbricks. For the this same reason, objects no simultaneouslyresults could be with obtained the industrial to determine 3D scanner the accuracy. Artec Space Li- DARSpider, is based the results on the were ToF thentechnique, analyzed which and ma comparedkes it possible in GOM to Inspect.measure the time differential inBased the returning on a literature wavefront research, independentl there are differenty of scene kinds textures. of measuring Therefore, methods, full frame present- real-timeing processes depth toestimates determine are the po scanningssible [12]. accuracy In addition, [7–9, 17the]. Thedata VDI are 2634,contaminated for instance, with de- randomfines a systematic process for measurement the verification errors. of optical A me 3Dthod measuring for improving systems. the However, depth map this methodis superresolution.is for 3D scanners Therefore, using a triangulation,high-resolution whereas RGB camera LiDAR is is combined based on with ToF anda depth TrueDepth camera. on AnVCSEL algorithm technology. is used to Since combine there those is no two official imag methodes [12,15]. to determine So far (iOS the 14), scanning Apple does accuracy, not providethis study the raw evaluates LiDAR the depth scanning data accuracyfor additional dueto applications shape tolerances. on an iOS device. Instead, The LiDAR technology proved to be impractical for scanning small objects such as Lego bricks. For this reason, no results could be obtained to determine the accuracy. LiDAR is based on the ToF technique, which makes it possible to measure the time differential in the returning wavefront independently of scene textures. Therefore, full frame real-time depth estimates are possible [12]. In addition, the data are contaminated with random systematic measurement errors. A method for improving the depth map is superresolution. Therefore, a high-resolution RGB camera is combined with a depth camera. An algorithm is used to combine those two images [12,15]. So far (iOS 14), Apple does not provide the raw LiDAR depth data for additional applications on an iOS device. Instead, the LiDAR depth data is merged with the accompanying color (RGB) data using AI to create a depth map. Furthermore, the mesh of the depth map consists of coarse meshed triangles. In effect, LiDAR might be used to assist Augmented Reality or to scan large objects, for example, Technologies 2021, 9, 25 9 of 13

rooms, but it is not applicable to scan small objects such as Lego bricks. The depth image detected with TrueDepth shows a much ﬁner mesh. Thereby, the dots of the infrared pattern are more closely arranged with each other than emitted dots with LiDAR are. As a result, TrueDepth is much more suitable for scanning small objects.

4.1. Impact of Color By evaluating the flatness of different-colored Lego bricks, it can be demonstrated that color has an influence on the surface quality of the generated CAD models, using TrueDepth and the Artec Space Spider. In total, the objects scanned via TrueDepth show less accurate surfaces than objects scanned by the industrial scanner. However, no measurement exceeds the set accuracy of 0.5 mm in the application. Thus, the results are satisfying. Nevertheless, it is important to include the influencing factor color when high-precision scans are required. Black-colored surfaces, for example, were not able to be scanned. Black objects absorb visible light, but in the near infrared from about 1 µm it reflects the radiation. The TrueDepth camera uses infrared sensors that capture light in the wavelength range of 800 nm to 1300 nm to achieve pattern-based depth mapping. Therefore, the black object absorbs and reflects a high proportion of infrared emitted by TrueDepth and the object cannot be scanned. The same applies to the Artec Space Spider, whose blue structured light pattern is absorbed. In addition, the red brick showed the highest deviation of 0.044 mm scanned by TrueDepth, while the Artec Space Spider was not even able to track the object at all. Due to the blue light LED illumination of the Artec Space Spider, red is not visible for the scanner because the light gets highly absorbed. At the same time. the highest standard deviation of 0.104 mm was achieved by scanning the red brick with the TrueDepth technology. This case shows how much influence disturbances can have. Disturbance variables could be environmental, which have not been eliminated yet, or the post-process could have a large influence. Thus, further experiments could show the influence of meshing and surface fitting. When scanning dark surfaces, antiglare spray or ambient light might im- prove the scan quality [7,8]. Since the blue and white objects obtained the highest accuracy with both scan technologies, those colors were chosen to determine the scanning accuracy.

4.2. Scanning Accuracy The scanning accuracy determines the potential use of a 3D scanner and thus fields of applications. In order to make a generally valid statement about the scan accuracy of arbitrarily formed objects, shape tolerances were evaluated. Although the objects scanned by Artec Space Spider showed small deviations for straightness, flatness and cylindricity, these shape tolerances have a great influence on the scanning accuracy with TrueDepth. When considering straightness and flatness, four of six measurements had deviations of more than 0.5 mm. In fact, the set parameter of 0.5 mm accuracy was therefore not fulfilled. However, the high standard deviation of 0.161 mm (straightness) and 0.105 mm (flatness) showed the potential of more accurate CAD models. Thereby, it was observed that both the scan strategy and the scan movements can lead to much more accurate results. For example, it was found that two rotations of the scanned object actually improved the scanning accuracy. In comparison to the Artec Space Spider one can assume that hardware and software are further developed by producing more repeatable results. In fact, this can be observed by evaluating cylindricity. When using TrueDepth, the live stream of capturing the Lego brick geometry showed that the object was lost and was captured again several times during one scan. This led to greater deviations of the generated mesh. Analogously, the same effect can be observed when testing roundness. Thereby, the Artec Space Spider showed greater deviations as well. Even though the CAD models have certain shape tolerances, the accuracy of the size, form, and orientation were not verified. Therefore, the target-actual comparison has to be analyzed. Especially in this analysis, the highest deviations were found. Considering the position and profile of a surface, the ratio of deviations between the Artec Space Spider and Technologies 2021, 9, 25 10 of 13

TrueDepth showed similar behavior. The scans taken with TrueDepth were almost twice as high as those of the Artec Space Spider. It is worth mentioning that the most accurate CAD model generated by Heges had a more accurate surface than the CAD model of Artec Space Spider. Notable were deviations of 4.51 mm (Artec Space Spider) and 4.92 mm (TrueDepth) in average by inspecting the proﬁle of a line. However, the highest deviations were formed by small deviating areas. Also, in those cases the standard deviations were very high leading to the conclusion that more accurate scans can be achieved.

5. Conclusions Manufacturers today tend to adopt product differentiation strategies and more customer- centric approaches to ensure future competitiveness. Mass customization and product diversification are key parts of these trends that require new hardware and software solutions. In this context, RE provides a method to individualize a product in a cost- and time-efficient way. In this case, the customer must be provided with hardware for 3D scanning. The LiDAR and TrueDepth technologies included in the newest devices by Apple offer such a deployment. However, the scanning accuracy determines the potential application and possible use cases. In this work, we have highlighted the potential of a smartphone and tablet as a cost-efficient 3D scanner. Additionally, we have pointed out influencing factors during 3D scanning. Based on the color influence as well as shape and position tolerances, we evaluated the scanning accuracy of an iPad Pro with the LiDAR and TrueDepth camera and compared it with an industrial 3D scanner, the Artec Space Spider. In principle, the industrial scanner should still be preferred if it is available. The advantage of smart devices, however, is that they are widely distributed among potential customers. Although scanned objects obtained with the iPad Pro had higher deviations than objects scanned by the Artec Space Spider, the iPad Pro offers a way to scan at lower costs. However, certain color ranges do not pick up well. Specifically, black tends to be filtered out. Use of a coating can mitigate this. In a scenario in which a customer is preparing the scans, this seems to be impractical, though. Depending on the use case, it must be evaluated whether the scanning accuracy of an iPad Pro meets the requirements. In addition, more accurate CAD models can be achieved through a specific and adaptive scanning strategy. In addition, asymmetric and detailed objects can lead to more accurate CAD models. Due to the fact that the TrueDepth sensor is front-facing, it is currently suitable for scanning objects only to a limited extent. Scanning objects is made difficult due to the location of the sensor and the handling required as a result. Instead, it can be used to design products that are tailored to the customer’s head, for example. The integrated LiDAR scanner would provide a way out. However, the available accuracy is not yet sufficient for the manufacturing of customized products due to the low mesh resolution. A new iteration of integrated sensor technology or a software update granting more access to developers may provide a remedy here. Future work will show how the technologies perform under different lighting conditions and how different scan strategies or scan times affect the accuracy.

Author Contributions: Conceptualization, M.V. and A.R.; methodology, M.V.; validation, A.R. and M.V.; formal analysis, A.R.; investigation, M.V. and A.R.; resources, C.E.; data curation, A.R.; writing— original draft preparation, M.V. and A.R.; writing—review and editing, M.V.; visualization, M.V. and A.R.; supervision, C.E.; project administration, M.V.; funding acquisition, C.E. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data sharing not applicable. Information provided by Marek Simonik (Developer Heges) is contained within this article. Technologies 2021, 9, 25 11 of 13

Acknowledgments: Many thanks are given to Marek Simonik, developer of the Heges iOS application. He provided us with detailed information on the functionality of his application and the limitations of software and hardware. Conﬂicts of Interest: The authors declare no conﬂict of interest.

Appendix A

Table A1. Determined ﬂatness values of seven different colored Lego bricks scanned by Artec Space Spider.

Artec Space Spider with Blue Light Technology 1 Brick Color Value 1 Value 2 Value 3 Mean Standard Deviation Variance White 0.18 0.07 0.18 0.14 0.064 0.004 Orange 0.14 0.15 0.18 0.16 0.021 0.000 Red N/A 2 N/A 2 N/A 2 --- Green 0.14 0.20 0.14 0.16 0.035 0.001 Blue 0.21 0.11 0.10 0.14 0.061 0.004 Black N/A 2 N/A 2 N/A 2 --- Turquoise 0.10 0.11 0.10 0.10 0.006 0.000 1 All values are in mm. 2 Scanner could not track and scan that object.

Table A2. Determined ﬂatness values of seven different colored Lego bricks scanned by iPad Pro (2020) using TrueDepth.

iPad Pro 2020 with TrueDepth Camera 1 Brick Color Value 1 Value 2 Value 3 Mean Standard Deviation Variance White 0.12 0.21 0.14 0.16 0.047 0.002 Orange 0.25 0.35 0.30 0.30 0.050 0.003 Red 0.24 0.44 0.29 0.32 0.104 0.011 Green 0.37 0.40 0.27 0.35 0.068 0.005 Blue 0.19 0.28 0.19 0.22 0.052 0.003 Black N/A 2 N/A 2 N/A 2 --- Turquoise 0.24 0.33 0.26 0.28 0.047 0.002 1 All values are in mm. 2 Scanner could not track and scan that object.

Table A3. Determined form tolerance values of different Lego bricks scanned by Artec Space Spider.

Artec Space Spider with Blue Light Technology 1 Form Value 1 Value 2 Value 3 Mean Standard Deviation Variance Tolerance Straightness 0.15 0.14 0.15 0.15 0.006 0.000 Flatness 0.12 0.17 0.14 0.14 0.025 0.001 Roundness 1.19 0.87 0.83 0.96 0.197 0.039 Cylindricity 0.20 0.14 0.16 0.17 0.031 0.001 1 All values are in mm.

Table A4. Determined form tolerance values of different Lego bricks scanned by the iPad Pro (2020) using TrueDepth.

iPad Pro 2020 with TrueDepth Camera 1 Form Value 1 Value 2 Value 3 Mean Standard Deviation Variance Tolerance Straightness 0.42 0.29 0.61 0.44 0.161 0.026 Flatness 0.42 0.30 0.51 0.41 0.105 0.011 Circularity 0.87 1.27 1.38 1.17 0.268 0.072 Cylindricity 0.79 0.88 0.80 0.82 0.049 0.002 1 All values are in mm. Technologies 2021, 9, 25 12 of 13

Table A5. Determined position and proﬁle tolerance values of different Lego bricks scanned by Artec Space Spider.

Artec Space Spider with Blue Light Technology 1 Target-Actual Standard Value 1 Value 2 Value 3 Mean Variance Comparison Deviation Position 0.50 0.37 0.41 0.43 0.067 0.004 Proﬁle of a surface 0.60 0.78 0.67 0.68 0.091 0.008 Proﬁle of a line 4.63 4.24 4.67 4.51 0.238 0.056 1 All values are in mm.

Table A6. Determined position and proﬁle tolerance values of different Lego bricks scanned by the iPad Pro (2020) using TrueDepth.

iPad Pro 2020 with TrueDepth Camera 1 Target-Actual Standard Value 1 Value 2 Value 3 Mean Variance Comparison Deviation Position 1.10 0.88 1.10 1.03 0.127 0.016 Proﬁle of a surface 1.25 1.58 0.57 1.13 0.515 0.265 Proﬁle of a line 4.51 5.04 5.20 4.92 0.361 0.130 1 All values are in mm.

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