digital film technology Scanity multi application film scanner white paper standing the test of time

multi application film scanner Scanity >>>

In the last few years, both digital intermediate (DI) postproduction systems and digital projection have advanced to full 4K resolution, avoiding the generation losses of traditional photo-chemical workflows. This leads to the question of how high a resolution is necessary to scan the film at the beginning of such a production chain. In fact, with the possibility that further improvements in both the DI and projection stages even beyond 4K could occur in future, it seems reasonable to conclude that the only limitation that should be applied at the scanning stage is the information-carrying capacity of the film itself.

This paper examines the questions of just what that limit is, what the required parameter values are – both analog and digital – to capture it, and what practical issues are involved in designing a film scanner that pursues such values.

2 www.dft-film.com 3 standing the test of time

MTF on Camera MTF on Camera Negative Film Negative Film Figure 1 Figure 2 Figure

(Eastman Kodak Vision 3 5207 250D)1 (Figure based on ITU test results)2

1. The “horizontal lines / picture height” scale is format-sensitive here based on a frame 20.96 mm x 11.33 mm (1.85:1). Slightly higher numbers would occur with a Super 35 mm frame, but the added line pairs/mm The Information Capacity MTF on film it from a computer file), scale is absolute and can be compared with the scale in Figure 1. of 35mm Motion Picture we have to get it there via 2. The “Normal Lens“ means spherical, as opposed to an anamorphic lens used in another part of the test Film First, we can consider the a camera and lens. Lenses sequence. published data for the MTF have MTF responses, too; the Although larger optical formats of a particular film stock. image resulting on the film is The ITU’s report also included “all” of the information on theoretical limiting resolution can capture more spatial We can assume we will find therefore a convolution of the information on the camera the highest quality 35 mm of the film stock; the response information, the predominance the highest readings on a lens characteristics with the lenses used. Although these film. However, this has to be at much lower spatial of 35 mm in motion picture camera negative. The example film’s own response. A useful were set to a fixed aperture for considered in the context of frequencies – 20 to 50 lp/ mm cinematography leads us to shown in Figure 1 is taken test that incorporated this fact optimum MTF, i.e. minimizing three inter-related parameters: – has a better correlation with the assumption that a film from the manufacturer’s data was conducted by the ITU resolution losses from both limiting resolution, sharpness, perceived sharpness. scanner should primarily for Eastman Kodak 5207 in 2001-2002 as part of a aberrations and diffraction and aliasing, because these consider the image frame 250D color camera negative. project known as “Large Scale (a constraint that might not are the factors that concern us But what is important about formats associated with this Modulation in the three color Digital Imagery”. Although always be possible in a normal when we make the transition the limiting resolution is the gauge. 35mm film can hold layers is plotted out to 80 line the overall objective was to production). from the analog information potential for any modulation an extremely high density of pairs/millimeter (lp/mm) at a examine film answer print and on the film to a digital at this frequency to induce information. In calculating level falling to just under 50% release print resolution, the It could therefore appear that representation in the scanner. visible aliasing when scanned how much in digital terms, in the green recording layer, data that was collected also rather than the highest spatial with a digital sensor. Avoiding it is necessary first to begin with blue a bit higher and red included measurements of the resolution indicated on the Limiting Resolution, such aliasing is the most with the Modulation Transfer somewhat lower. MTF of the original camera manufacturer’s data sheet of a Sharpness and Aliasing important factor in deciding the Function (MTF) characteristics negative (OCN). In this test, current film stock (80 lp/mm necessary digital resolution of However, this is not necessarily of a particular sample. This the ITU measured a close-to- at 50%), the close-to-limiting At an MTF of 6%, the ITU’s the scanner. the level of modulation that complex analog quantity must limiting* modulation depth resolution of 106 lp/mm at 6% measured result at 106 lp/ would be obtained in practice. then be transformed into a of 6% at 106 lp/mm, in the found by the ITU might be a mm on the developed OCN Nevertheless, the conclusion To expose an image on film digital equivalent, via the OCN (Eastman Kodak 5274). possible target for capturing was evidently close to the has to be that reading film application of sampling theory. (other than film-recording

4 www.dft-film.com 5 information at 106 lp/mm is We must therefore consider all the variations occurring not of the highest significance, how many millimeters’ total within the window. The because it is at too low a distance we are scanning window therefore has its own level either to be visually horizontally. For Super 35 MTF, which is convolved with significant or to trigger visible mm format, this is 24.92 the MTF of the information aliasing. Instead, the ITU’s mm across the exposed frame on the film (which is itself measurement at 80 lp/mm in width, meaning that we need a convolution, as discussed the same curve seems more enough horizontal pixels to earlier), reducing the detected meaningful, because it is read 80 x 24.92 or 1994 line level of the detail on the film higher in level at about 17%*, pairs total. Sampling theory even further. In the context and also because it confirms tells us that since a “line pair” of an image sensor, this the validity of the limit of 80 is one complete cycle of a sine function is also known as the lp/mm plotted on the Kodak wave, Nyquist frequency for “geometric MTF” of the sensor curves in Figure 1. the sensor will equal the line (to distinguish it from other pair count per scan line, and sources of resolution loss in Limiting the target resolution therefore pixel frequency will solid state image sensors). to 80 lp/mm rather than 106 be a minimum of twice this, or Like the MTF curves for the lp/mm, therefore, makes 3988 pixels per scan line. Is film, the geometric MTF curve sense, because the reduced our answer therefore that we has a limiting frequency and MTF via the camera lens need at least a “4K” scanner? a shape. Unlike the film MTF, operates under completely operate with a fairly small of field. In total, therefore, the has already led to a much however, the geometric MTF fixed geometry, with a aperture, thus making any convolution of the scanner lower modulation level, so Before we conclude that “4K” curve has a very regular shape magnification factor close to lens aberrations insignificantly lens MTF with the other MTFs that any aliasing that does is indeed the answer, let’s and a very clearly defined unity, and with very favorable small, while the relatively large can be designed to be quite occur from frequencies look a little more closely at the limiting frequency. Because lighting conditions, it can optical format of the film and insignificant (see Figure 5). beyond this will effectively scanning function. sampling is involved, the avoid the optical compromises sensors means that diffraction be invisible (provided the geometric MTF curve also inherent in most camera losses are also very small. Another MTF to Consider! scanning sensor’s pixel layout has an alias curve associated lenses. Furthermore, defocusing loss is appropriately chosen - see with it, also of very regular with irregular film can be A digital scanner is “analog” in For example, it can be set to below). shape and extent. However, minimized via a large depth one sense: its sensor has its the layout of the pixels has From MTF to Scanner own MTF. This arises because a profound effect on the ** If the ITU’s result at 80 lp/mm of 17% is compared with Kodak’s published figure of about 50% for the same Resolution each digital sample has to be frequency, the difference may seem large. However the MTF difference can be explained via two factors: first, geometric MTF curve and its created by looking through a camera film stocks have advanced in performance in the years since the ITU test (limiting resolution remains aliasing, as will be seen later. We need to find the number “window” at the continuous similar, but modulation depth at given spatial frequencies has increased appreciably; to see this, the published data for the 5207 camera stock shown in Figure 1 should be compared with that of the 5274 stock used in the of pixels required on the information on the film. The And Yet Another MTF! ITU’s test). The second factor is the way the film was exposed (through a good-quality practical production lens, scanner’s sensors to read 80 “window” is of course an through a scientific diffraction-limited lens, or using no lens at all?). In any case, the two sets of data merely illustrate possible targets for a film scanner to aim for, and should not necessarily be compared directly. lp/mm on the film and describe individual pixel, but because Between the film and the the information in the popular the pixel is required to measure sensor is the scanner‘s own ** Example: “4K” means 4096 pixels of horizontal resolution, “2K” means 2048 horizontal pixels, etc. Vertical “K” notation**, i.e. quoting just one level for the whole of lens, which has its own MTF, pixel count is not stated, because it can be calculated from the aspect ratio, since pixels in film scanning are usually square. only the horizontal axis. its window, it must average too. However, since this lens

6 www.dft-film.com 7 standing the test of time Sensor with 50% fill factor

Scanity

Figure 3

Scanner Pixel Arrangements, at fN and does not decay • a much higher signal MTF and Aliasing very rapidly back towards frequency – close to fS – zero frequency. This is a will produce a much lower- The left side of Figure 3 shows consequence of the particular frequency alias. some pixels (Photosites) in a sensor layout, where the shape scanner sensor, and on the of both MTF and alias curves The concern here is that from right the resulting geometric is governed by the Photosite fill fN onwards the alias amplitude MTF and alias responses factor3, in this case 50%. is the same as that of the with no prefiltering, i.e. input signal frequency that causes it. frequencies are allowed to Considering the effect of the While the 50% fill factor layout extend beyond the Nyquist 50% fill factor on aliasing: gives a high geometric MTF, limit fN to sampling frequency it also produces high alias fS and beyond. The Nyquist • a signal frequency below fN amplitudes. Most seriously, limit is a function of the pixel will theoretically produce the amplitude remains high pitch: the smaller the pitch, the no alias in alias frequencies close to higher the Nyquist frequency. zero, which are much more • a signal frequency not The geometric MTF (solid visible than aliases at high far above fN will “wrap curve) of this layout is quite frequencies around“ as shown to high (90%) at fN, but the produce a high frequency undesired alias is also 90% alias

8 www.dft-film.com 9 Convolution of standing the test of time MTFs Sensor with 100% fill factor

Figure 5

as much as 11K digital • at lower frequencies and to that which is useful, i.e. resolution. An 11K scanner so reduced in amplitude by scanning at 4K maximum. would be extremely expensive the pixel fill factor as to be and slow in operation, and invisible. The most important is the could have poor performance speed advantage. It is now in other parameters, such as Scanning a first-generation possible to construct a 4K signal-to-noise ratio. In the OCN is also the extreme scanner that can run at up to Figure 4 majority of projects, the extra case. What the ITU tests also 15 frames/second. Although scanner pixels would not be showed (Figure 2) was that this is achieved partly by the after just one film generation adoption of some special In Figure 4, the sensor now exactly 100% is not possible, in determining the effective capturing any additional image (the answer print), the MTF techniques – see below – has 100% fill factor (touching since some degree of the total MTF between scene details information compared to a fell to zero well before 106 such a speed would not be Photosites). The geometric surface area of the sensor and the digital data captured lower resolution machine. lp/mm and even at 80 lp/ possible at a higher resolution MTF (solid curve) of this layout has to be taken up with non- by the film scanner. Nyquist However, further examination mm was only about 4%; of, say, 6K or 8K, because of is now lower (63%) at fN than light-sensing functions (e.g. frequency in the scanner of practical evidence indicates in fact, 20% modulation the limited electron charge with the 50% fill factor layout, transfer of electron charges corresponds to approximately that provided the sensor level was maintained only to integration capability of the but we can see too that the from Photosites to the output 80 lp/mm on the film. layout is optimally chosen about 50 lp/mm; this second much smaller sensor Photosite undesired alias amplitude is amplifier), but in a good design (close to 100% fill factor), generation‘s information area (one quarter the area, also lower at fN and, most close to 100% is the aim and So, is 4K the Optimum all resolution up to the 80 lp/ content could therefore be assuming 8K versus 4K and importantly, decays very can be achieved. Scanning Resolution? mm for OCN published by film captured adequately with far equal fill factors). Conversely, rapidly towards zero amplitude What the above sections stock manufacturers can be less scanning resolution than attempting to run an 8K at zero frequency. In effect, Figure 5 shows that the MTF initially suggested was that if adequately captured with a 4K 4K; probably a 2.5K scanner scanner at the same speed as the greater window integration of the film in conjunction with a film scanner were designed sensor architecture, because would suffice. at 4K would severely degrade effect of the 100% fill factor the camera lens, the MTF of to capture all resolution up any aliases arising from the signal-to-noise ratio, is giving us a “free” optical the scanner‘s projection lens to the ISO 12233 limiting unfiltered signal frequencies Benefits of the 4K Design detracting from the theoretical low pass filter, producing a and the geometric MTF created resolution, alias-free, in the above this will either be: Decision resolution benefit in the overall valuable sharp cutoff of the by the layout of the pixels in most extreme cases, such a subjective assessment. most visible low-frequency the scanner‘s sensors are all scanner could be calculated • at very high frequencies There are several benefits aliases. convolved (multiplied) together and low amplitude and to limiting the information Another advantage of the 4K in the scanner’s sensors are all to require a digital resolution therefore not visible, or captured from 35mm film decision is the reduction in In practice, a fill factor of convolved (multiplied) together much higher than 4K, perhaps

10 www.dft-film.com 11 SCANITY standing the test of time Light Path hardware costs. This arises Scanity – A Realization of complicate matters, the camera not only from the sensors these Design negative has an orange mask themselves, but also from the (for color gamut enhancement elimination of high-bitrate Principles purposes) that attenuates the downconversion hardware, incident light reaching the A full description of the because few post-production blue recording layer. At the Scanity and Scanity HDR 4K workflows can handle 6K or other end of the spectrum, film scanner is given in other 8K native scan data (those transmission of the red papers, but in this paper that can handle it have to component of the illumination some of its features will now accommodate enormous data must be maintained to long be briefly covered to explain volumes - up to 300 MB per wavelengths to read the the practical realization of the frame or more, compared to modulation in the red recording principles described above. 75 MB for an equivalent single layer, while not allowing frame at 4K*). harmful high-energy infrared to Film Illumination Source impose excessive heat on the Interim Summary of the The prior discussion film. Other film stocks, such Scanning Requirements considered scanning resolution as print, optimally require a Figure 6 in isolation. However, the different spectral distribution in Assume first generation 35 through an integration sphere, pixels in Scanity’s linear lines in the TDI structure, so relationship between resolution the illumination. mm OCN film, exposed via the light passes through the array sensors (Figure 7); the the same row of film “pixels“ capability and illumination a high quality production Two techniques in the film via a very narrow slit and same horizontal row of film is sensed 96 times as one is very direct. Scanning camera lens, and design for illumination method deal with onto the sensors via a color “pixels” is tracked as it moves TDI line after another is resolutions have become this challenging but practical these challenges. The first is beam splitter. through the gate by shifting activated. At the same time, higher with the result that for case (leading to a 4K scanning that the illumination comes the sensor‘s clocking from one the integrated electron charges a given illumination level, the resolution design decision). from the combination of Sensor Architecture sensor line to the next and from prior lines are added to size of electron charge created discrete clusters of spectrally- turning on the LEDs each time the current TDI line, causing Minimize scanner lens MTF in smaller and smaller pixel The second technique extends separate red, green and blue for a brief burst of less than an accumulation of charge loss with small-aperture low- areas diminishes in inverse the total integration period LEDs (Figure 6). This allows one scan line‘s time duration. volume. By the time the last aberration optics with large square proportion. Since considerably, yet without accurate tailoring of the overall TDI line has completed its depth of field. quantum efficiency does not slowing down the scanning spectral power distribution There are in fact 96 such sensing, the accumulation of appreciably change, the only speed (frames per second). to the density spectra of Employ close-to-100% fill options then are to increase This apparent contradiction is the emulsion layers, with a factor pixel layout in the the effective charge integration made possible by the concept minimum of stray energy at sensors for best separation time of the pixels (more on of multiple timed charge unwanted wavelengths. A ratio between wanted signal that later) or to increase the integration periods in the further refinement is that there recovery and aliases. illumination intensity. scanner‘s image sensors. are in fact two sets of red

The density spectra of a LEDs of slightly different center TDI (Time Delay Integration) colour film‘s emulsion layers wavelengths; the appropriate architecture makes use of require broadband illumination set is used according to the multiple repeated lines of of sufficient intensity. To type of film stock. After passing Figure 7 - TDI Sensor

12 13 charge is sufficient to multiply components, while avoiding Instead, SCANITY uses virtual the voltage at the CCD output the need for wastefully perforation registration: amplifier approximately 50 excessive pixel resolution instead of subjecting the times. This gives an enormous (above 4K). film’s perforations to repeated improvement in effective insertion of mechanical pins, sensitivity, and therefore signal- When the maximum 4K a completely touch-free optical to-noise ratio, but without scanning resolution is not method using a perforation slowing down the transport required (lower resolution print camera is used. The detected speed or applying excess heat films, previews, etc.), further error is fed to a digital energy to the film. Scanity advantage can be taken of the position shifter (1/64th of a makes TDI work successfully TDI structure to allow charge pixel corresponding to about by precise synchronization binning across multiple pixels. 0.1micron position correction) between sensor line clocking, This allows the scanning speed to reduce the residual error in LED cycling, and the to be increased considerably real time to a level comparable longitudinal positioning of for the same sensitivity. For to mechanical registration. the film in the gate by the example, 2K scanning can transport servo. run at up to 25 frames/ With the film flow through second, while in the extreme, the transport thus free from Accommodation to Different if a resolution of just 0.25K mechanical speed constraints, Film Formats is sufficient (e.g. previewing a continuous motion transport camera rolls), the transport can is possible. Proven over many Compared to area arrays, run at up to 96 frames/second! years in the Spirit DataCine® Scanity linear array TDI and other forerunners of sensors have the advantage Positional stabilization of Scanity, continuous film of adjusting automatically to Film motion lifts the speed of this the different frame heights of film scanner to a multiple of All the efforts described so far various film image formats, the speed limit of start-stop would be ineffective unless the maintaining the same motion machines resolution for all of them. This film is positioned with great This paper analyzed the Associated requirements method in this scanner‘s principle works independently consistency in the gate, frame Conclusion extent of that information for maximizing the image image sensor architecture of the multiple TDI line after frame. with reference to both image performance potential of 4K of boosting sensitivity and structure in the sensors. To support continuing The most obvious way to do future improvements in recovery and avoidance of scanning were also described. noise performance was also In the resolution and aliasing this would be by means of postproduction and projection visible aliasing in order to These included scanning described. This method allows discussion, the pixel fill factor mechanical pin registration of of film-based content, it find the limit to the necessary optics, image sensor pixel fill dramatically raising film emerged as an important the film perforations. However, is recommended that the scanning resolution in practical factor, illumination source and scanning speed to as high as issue. Scantity uses a factor this is stressful to the film maximum useful image applications. A limit of 4K positional stability of the film. 15 frames per second while approaching 100% to obtain with repeated playing and is information contained in resolution in the scanning still retaining full 4K resolution. The related design aspects the most favorable ratio guaranteed to prevent any incoming 35mm film material device was found to be of a new film scanner between wanted modulation possibility of high scanning should be captured at the appropriate, leaving open the embodying these principles and unwanted alias speeds. scanning stage. possibility of extremely high speed operation. were described. An innovative Scanity-whitepaper-190116 burbank |california91506USA dft’s policy is oneofcontinuousimprovements andwereserve therighttochange thespecification atanytimewithoutprior notice 711 southmainstreet t :+18188617419 www.dft-film.com e :[email protected] borsigstrasse 13 t :+4961518503500 darmstadt | germany | 64291 t :-+914423764432 saligramam, chennai-600093|India 28, arunachalamroad,