DOCSLIB.ORG

A Performance Comparison Between Classical Schlieren and Background-Oriented Schlieren

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
A Performance Comparison Between Classical Schlieren and Background-Oriented Schlieren 18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016 A Performance Comparison between Classical Schlieren and Background-Oriented Schlieren P. Manovski1,*, J. Wehrmeyer2, K. Scott2, B. Loxton1, H. Quick1, S. Lam1, M. Giacobello1 1: Aerospace Division, Defence Science and Technology Group, Australia 2: Arnold Engineering Development Complex, Arnold Air Force Base, United States of America * Correspondent author: [email protected] Keywords: Background-oriented schlieren, Quantitative or Calibrated schlieren ABSTRACT A performance comparison was made between a classical calibrated schlieren system and a background-oriented schlieren (BOS) system at the Defence Science and Technology Group (DST Group) Transonic Wind Tunnel (TWT) in Melbourne, Australia. Comparisons between these two techniques have been made by others. This study aims to contribute to the findings by conducting a similar investigation in a large-scale facility using the latest high pixel resolution (29 MP), high sensitivity cameras to maximise spatial resolution and the ability to resolve small refraction angles. This study also aims to investigate the effects of the conical perspective of a typical BOS setup compared with the cylindrical view of schlieren. Two configurations were investigated, firstly a standard setup and then a high sensitivity setup. The sensitivity was determined experimentally and compared with theory. The minimum detectability was determined to be of the same order, approximately ±1 arc-second for both systems. The performance of each system was then investigated by imaging the compressible flow around a cone-cylinder model for a range of Mach numbers from 0.6 to 1.2. Qualitatively, a good agreement was obtained. By translating the BOS system in the streamwise direction, at 1.05 Mach, the effect of the BOS system conical perspective was evident, a relatively strong bow shock at the tip of the cone-cylinder model could be made to disappear from the BOS image. A quantitative comparison of the measured line-of-sight refraction angle showed a good agreement near the centre of the BOS lens but away from the centre discrepancies were evident and are likely due to the BOS conical perspective. The schlieren system range was also shown to be limited. 1. Introduction Both classical schlieren and background-oriented schlieren (BOS) techniques measure refractive- index variations and can be used to visualise the line-of-sight quantitative density gradient information. The techniques are widely used to study compressible flow phenomena, such as shock waves, expansion fans, and variable-density wakes and boundary layers in high speed flows. Standard schlieren imaging is used in many wind tunnels, such as the Defence Science and Technology Group (DST Group) Transonic Wind Tunnel (TWT) in Melbourne, Australia and has a long development history [1]. In contrast, BOS is a relatively new technique that relies on digital imaging cameras and computer-based image correlation software [2]. BOS is used in the Arnold Engineering Development Complex (AEDC) 16T (16 foot Transonic) wind tunnel, rather than classical schlieren, because of the latter technique’s requirement of optical access that would block a large percentage of the test section walls. Wall blockage is detrimental to test section flow quality for transonic wind tunnels, whose test section walls are 18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016 ventilated to minimize shock reflection. The DST Group TWT is often used with a half model mounted on the side wall which blocks the optical access required for schlieren. For the DST Group, BOS presents an alternative that is worth investigating. The purpose of this study was to obtain a relative performance comparison between the BOS and a standard z-type schlieren system installed at the DST Group TWT while considering the facility’s geometric constraints. In particular, the aim was to compare the ability of the two techniques in detecting small density gradients. Comparisons between these two techniques have been made by others [3, 4]. This study aims to contribute to the findings by conducting a similar investigation in a large scale wind tunnel facility using the latest high pixel resolution (29 MP), high sensitivity cameras to maximize spatial resolution and the ability to resolve small refraction angles. This study also aims to investigate the effects of the conical perspective of a typical BOS setup compared with the cylindrical view of schlieren. Two configurations were investigated, firstly a standard setup that is typically used for test campaigns in the TWT and then a high sensitivity setup. The sensitivity and minimum detectability were experimentally determined by introducing a wedged optic that produces a very small (known) angular refraction as the schlieren object. The performance of each system was assessed qualitatively and quantitatively by imaging the compressible flow around a cone-cylinder model for a range of Mach numbers from 0.6 to 1.2. 2. Experimental equipment 2.1 DST Group Transonic Wind Tunnel The DST Group Transonic Wind Tunnel is a closed-circuit continuous flow wind-tunnel. A schematic of the circuit is shown in Fig. 1. The tunnel is equipped with a Plenum Evacuation System (PES), used to pressurize and depressurize the tunnel circuit as well as improve the flow quality in the test section by removing air through the slotted test section walls, and re-injecting it downstream of the first corner after compression. The tunnels convergent nozzle can be operated in an under-expanded mode to produce supersonic Mach numbers by evacuating the test section to allow expansion of the sonic nozzle flow. The total pressure in the circuit can be evacuated down to 40 kPa or pressurized up to 200 kPa. When the test section is empty the tunnel has a Mach number range of 0.30 to 1.20. The test section is nominally 2700 mm long, 806 mm high and 806 mm wide. The sidewalls are interchangeable and can be either solid or slotted. Each slotted wall consists of six slots per wall giving an overall porosity of 4.97%. The main model support is downstream of the test section, incorporating a pitch and roll mechanism. The pitch mechanism has a position accuracy of ±0.02 deg and the roll drive is accurate to within ±0.1 deg. Further details of the model support system are in [5]. The Mach number is controlled by adjusting the main fan speed and plenum suction via two control valves (one for gross control, the other for fine control) in the PES inlet pipe, thus controlling the rate of air removed from the plenum. The Mach number is monitored by the tunnel control system from the total pressure in the settling chamber and static pressure in the plenum, and is controlled to within ±0.002 for subsonic operations and ±0.01 for supersonic [5]. 18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016 Temperature in the test section, which can be set between 30°C and 40°C, is controlled via a heat exchanger in the settling chamber to within ±2°C. Humidity is reduced to below 2000 ppmv before each test run through a dehumidifier in the PES. Fig. 1 DST Group TWT circuit schematic Fig. 2 Modified z-type schlieren system layout in the DST Group TWT. 2.2 DST Group TWT schlieren system 18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016 A classical schlieren system for flow visualisation is based on the deflection of a light beam as it passes through a change in refractive index caused by a density gradient in the wind tunnel test section [1]. A properly calibrated system can measure the magnitude of the density gradient, normal to the knife edge from the intensity of the light reaching the sensor [1]. The DST Group schlieren system employs a modified version of the z-type Toepler layout and is shown in Fig. 2. The modifications include the use of two plane mirrors (PM1 and PM2) that fold the light path to reduce the space necessary for the standard schlieren system. The system mirrors consist of two 406 mm parabolic schlieren mirrors, with focal lengths SM1 = 2517 mm and SM2 = 2533 mm. The z-type arrangement introduces astigmatism into the system, however, a combination of careful alignment and minimizing the off-axis angles reduces this to a minimum and the resultant effect in the classical setup with a knife edge cut-off is negligible. Further details of the TWT schlieren system can be found in [6]. In this study a classical setup was used with a rectangular light source slit and a vertical knife edge allowing the visualisation of the streamwise density gradients. A micrometer was used to accurately measure the cut-off percentage or unobstructed source height, a. A white LED light source (LED Engin 24 die LZP-DOCW00) was used to provide stable high powered uniform illumination during the test period. The LED was driven by a Garadsoft RT820F-20 controller, capable of providing continuous or pulsed operation, with pulse widths from 3 µs – 200 ms. An achromatic doublet imaging lens of focal length 100 mm was used to provide a field-of-view encompassing the entire schlieren window. The camera used for the schlieren imaging was LaVision Imager LX 29M with camera link interface, and provided a 12 bit greyscale pixel depth. To calculate the sensitivity for the schlieren system, the camera pixel intensities, E, is converted into a contrast or normalised intensity as per, ∆� � − �'() � = = , (1) �'() �'() where �'(), is the background image or reference intensity. The reference intensity is the image intensity of undisturbed (zero density gradient) region of the image.
Load more

Recommended publications
  • Lens Equation – Thin Lens
    h s’ s f h’ Refraction on Spherical Surface θa=α +φ ; φ = β +θb ⎫ an sinθ a= n bsinθ b n⎬aα + n bβ = ( n b− )φ n a anθ a≅ n b θ b ⎭ h h h tanα = ; tanβ = ; tanφ = s + δ s'−δ R − δ h h h n n n− n ≅α ; β ≅; φ ≅ ⇒a+ b = b a s s' R s s' R Refraction on Spherical Surface n n n− n a+ b = b a Magnification s s' R R positive if C on transmission side; negative otherwise y − y′ tanθ = tanθ = na y nb y′ a b = − s s′ s s′ nasinθ a= n bsin θ b y′ na ′ s esl angallFor sm m = = − y n s tanθ≅ sin θ b Refraction on Spherical Surface n n n− n a+ b = b a . A fish is 7.5Example: Fish bowl s s' R cm from the front of the bowl. y′ n′ s Find the location and magnification of the m = = − a fish as seen by the cat.bowl. Ignore effect of y nb s Radius of bowl = 15 cm. na = 1.33 R negative b O n n n− n n n− n n I s’ a+ b =b a b = b a− a s s' R s' R s s n 1 s′ = b = =6 .− 44cm n− n n 0− . 331 . 33 a b a− a − R s −cm15 7 . 5 cm n′ s 1 .( 33 6− . 44cm ) The fish appears closer and larger. m= − a= − ∗ 1= . 14 nb s 1 7 .cm 5 Lenses • A lens is a piece of transparent material shaped such that parallel light rays are refracted towards a point, a focus: – Convergent Lens Positive f » light moving from air into glass will move toward the normal » light moving from glass back into air will move away from the normal » real focus Negative f – Divergent Lens » light moving from air into glass will move toward the normal » light moving from glass back into air will move away from the normal » virtual focus Lens Equation – thin lens n n n− n n n n− n a b+ b = a b+ a = a b 1s s 1' R 1 2s s 2' R2 For air, na=1 and glass, nb=n, and s2=-s1’.
    [Show full text]
  • Location of Cardinal Points from the ABCD Matrix for the General Optical System
    EE482 Fall 2000 Handout #2 Page 1 Location of Cardinal Points from the ABCD Matrix for the General Optical System I II n1 n2 F1 P1 N1 P2 N2 F2 f1 f2 Figure 1 Cardinal points of a system which is characterized by an ABCD matrix between an input plane I and an output plane II. Cardinal Point Measured Function Special Case From To (n1=n2) First Focal Point P1 F1 -n1/(n2C)-1/C First Principle Point I P1 (n1-n2D)/(n2C)(1-D)/C First Nodal Point I N1 (1-D)/C (1-D)/C Second Focal Point P2 F2 -1/C -1/C Second Principle Point II P2 (1-A)/C (1-A)/C Second Nodal Point II N2 (n1-n2A)/(n2C)(1-A)/C Table 1 When analyzing an optical system, we often are first interested in basic properties of the system such as the focal length (or optical power of the system) and the location of the focal planes, etc. In the limit of paraxial optics, any optical system, from a simple lens to a combination of lenses, mirrors and ducts, can be described in terms of six special surfaces, called the cardinal surfaces of the system. In paraxial optics, these surfaces are planes, normal to the optical axis. The point where the plane intersects the optical axis is the cardinal point corresponding to that cardinal plane. The six special planes are: First Focal Plane Second Focal Plane First Principle Plane Second Principle Plane First Nodal Plane Second Nodal Plane Once we know the location of these special planes, relative to the physical location of the lenses in the system, we can describe all of the paraxial imaging properties of the system.
    [Show full text]
  • Laboratory 7: Properties of Lenses and Mirrors
    Laboratory 7: Properties of Lenses and Mirrors Converging and Diverging Lens Focal Lengths: A converging lens is thicker at the center than at the periphery and light from an object at infinity passes through the lens and converges to a real image at the focal point on the other side of the lens. A diverging lens is thinner at the center than at the periphery and light from an object at infinity appears to diverge from a virtual focus point on the same side of the lens as the object. The principal axis of a lens is a line drawn through the center of the lens perpendicular to the face of the lens. The principal focus is a point on the principal axis through which incident rays parallel to the principal axis pass, or appear to pass, after refraction by the lens. There are principle focus points on either side of the lens equidistant from the center (See Figure 1a). Figure 1a: Converging Lens, f>0 Figure 1b: Diverging Lens, f<0 In Fig. 1 the object and image are represented by arrows. Two rays are drawn from the top of the object. One ray is parallel to the principal axis which bends at the lens to pass through the principle focus. The second ray passes through the center of the lens and undeflected. The intersection of these two rays determines the image position. The focal length, f, of a lens is the distance from the optical center of the lens to the principal focus. It is positive for a converging lens, negative for a diverging lens.
    [Show full text]
  • 1.0 Measurement of Paraxial Properties of Optical Systems
    1.0 MEASUREMENT OF PARAXIAL PROPERTIES OF OPTICAL SYSTEMS James C. Wyant Optical Sciences Center University of Arizona Tucson, AZ 85721 [email protected] If we wish to completely characterize the paraxial properties of a lens, it is necessary to measure the exact location of its cardinal points, that is, its nodal points, focal points, and principal points. For a lens in air the nodal points and principal points coincide. For a thin lens, the two principal points coincide at the center of the lens, so the only required measurement is the focal length, while for a thick lens two of the three quantities--focal length, two focal points, or two principal points--must be determined. 1.1 Thin Lenses 1.1.1 Measurements Based on Image Equation The simplest measurements of the focal length of a thin lens are based on the image equation 1 1 1 + = (1.1) p q f where p is the object distance from the lens (positive if the object is before the lens), q is the image distance from the lens (positive if the image is after the lens), and f is the focal length of the lens. If the lens to be tested has a positive power, a real image can be formed of a pinhole source, and the distances p and q can be measured directly. When the lens to be tested has a negative power, it should be combined with a positive auxiliary lens having sufficient power so that the combination forms a real image. The focal length can then be determine for the auxiliary lens alone and the combination of lenses.
    [Show full text]
  • Optics Course (Phys 311)
    Optics Course (Phys 311) Geometrical Optics Refraction through Lenses Lecturer: Dr Zeina Hashim Phys Geometrical Optics: Refraction (Lenses) Lesson 2 of 2 311 Slide 1 Objectives covered in this lesson : 1. The refracting power of a thin lens. 2. Thin lens combinations. 3. Refraction through thick lenses. Phys Geometrical Optics: Refraction (Lenses) Lesson 2 of 2 311 Slide 2 The Refracting Power of a Thin Lens: The refracting power of a thin lens is given by: 1 푃 = 푓 Vergence: is the convergence or divergence of rays: 1 1 푉 = and 푉′ = 푝 푖 ∴ 푉 + 푉′ = 푃 A diopter (D): is a unit used to express the power of a spectacle lens, equal to the reciprocal of the focal length in meters. Phys Geometrical Optics: Refraction (Lenses) Lesson 2 of 2 311 Slide 3 The Refracting Power of a Thin Lens: Individual Activity Q: What is the refracting power of a lens in diopters if the lens has a focal length = 20 cm ? Phys Geometrical Optics: Refraction (Lenses) Lesson 2 of 2 311 Slide 4 Thin Lens Combinations: If the optical system is composed of more than one lens (or a combination of lenses and mirrors) which are located so that their optical axes coincide: the final image can be obtained by working in steps: 1. Consider the nearest lens only, find the image of the object through this lens. 2. The image in step 1 is the object for the second (adjacent) optical component: find the image of this object. This can be done both geometrically or numerically 3.
    [Show full text]
  • Chapter 23 the Refraction of Light: Lenses and Optical Instruments
    Chapter 23 The Refraction of Light: Lenses and Optical Instruments Lenses Converging and diverging lenses. Lenses refract light in such a way that an image of the light source is formed. With a converging lens, paraxial rays that are parallel to the principal axis converge to the focal point, F. The focal length, f, is the distance between F and the lens. Two prisms can bend light toward the principal axis acting like a crude converging lens but cannot create a sharp focus. Lenses With a diverging lens, paraxial rays that are parallel to the principal axis appear to originate from the focal point, F. The focal length, f, is the distance between F and the lens. Two prisms can bend light away from the principal axis acting like a crude diverging lens, but the apparent focus is not sharp. Lenses Converging and diverging lens come in a variety of shapes depending on their application. We will assume that the thickness of a lens is small compared with its focal length è Thin Lens Approximation The Formation of Images by Lenses RAY DIAGRAMS. Here are some useful rays in determining the nature of the images formed by converging and diverging lens. Since lenses pass light through them (unlike mirrors) it is useful to draw a focal point on each side of the lens for ray tracing. The Formation of Images by Lenses IMAGE FORMATION BY A CONVERGING LENS do > 2f When the object is placed further than twice the focal length from the lens, the real image is inverted and smaller than the object.
    [Show full text]
  • Chapter 33 Lenses and Op Cal Instruments
    Chapter 33 Lenses and Opcal Instruments Units of Chapter 33 • Thin Lenses; Ray Tracing • The Thin Lens Equation; Magnification • Combinations of Lenses • Lensmaker’s Equation • The Human Eye; Corrective Lenses • Magnifying Glass 33-1 Thin Lenses; Ray Tracing Thin lenses are those whose thickness is small compared to their radius of curvature. They may be either converging (a) or diverging (b). 33-1 Thin Lenses; Ray Tracing Parallel rays are brought to a focus by a converging lens. 33-1 Thin Lenses; Ray Tracing A diverging lens makes parallel light diverge; the focal point is that point where the diverging rays would converge if projected back. 33-1 Thin Lenses; Ray Tracing The power of a lens is the inverse of its focal length: 1 P = f Lens power is measured in diopters, D: 1 D = 1 m-1. 33-1 Thin Lenses; Ray Tracing Ray tracing for thin lenses is similar to that for mirrors. We have three key rays: 1. This ray comes in parallel to the axis and exits through the focal point. 2. This ray comes in through the focal point and exits parallel to the axis. 3. This ray goes through the center of the lens and is undeflected. 33-1 Thin Lenses; Ray Tracing 33-1 Thin Lenses; Ray Tracing For a diverging lens, we can use the same three rays; the image is upright and virtual. 33-2 The Thin Lens Equation; Magnification The thin lens equation is similar to the mirror 1 1 1 equation: + = d0 di f 33-2 The Thin Lens Equation; Magnification The sign conventions are slightly different: 1.
    [Show full text]
  • Lecture 32 – Geometric Optics Spring 2013 Semester Matthew Jones Thin Lens Equation
    Physics 42200 Waves & Oscillations Lecture 32 – Geometric Optics Spring 2013 Semester Matthew Jones Thin Lens Equation First surface: Second surface: Add these equations and simplify using 1 and → 0: 1 1 1 1 1 (Thin lens equation) Thick Lenses • Eliminate the intermediate image distance, • Focal points: – Rays passing through the focal point are refracted parallel to the optical axis by both surfaces of the lens – Rays parallel to the optical axis are refracted through the focal point – For a thin lens, we can draw the point where refraction occurs in a common plane – For a thick lens, refraction for the two types of rays can occur at different planes Thick Lens: definitions First focal point (f.f.l.) Primary principal plane First principal point Second focal point (b.f.l.) Secondary principal plane Second principal point Nodal points If media on both sides has the same n, then: N1=H 1 and N2=H 2 Fo Fi H1 H2 N1 N2 - cardinal points Thick Lens: Principal Planes Principal planes can lie outside the lens: Thick Lens: equations Note: in air ( n= 1) 1 1 1 x x = f 2 1 1 1 (n −1)d + = o i effective = ()n −1 − + l l s s f focal length : l o i f R1 R2 nl R1R2 ( − ) ( − ) = − f nl 1 dl = − f nl 1 dl Principal planes: h1 h2 nl R2 nl R1 ≡ yi = − si = − xi = − f Magnification: MT yo so f xo Thick Lens Calculations 1. Calculate focal length 1 1 1 1 1 2. Calculate positions of principal planes 1 1 3.
    [Show full text]
  • Physics 1230: Light and Color Lecture 17: Lenses and Ray Tracing
    Physics 1230: Light and Color Chuck Rogers, [email protected] Ryan Henley, Valyria McFarland, Peter Siegfried physicscourses.colorado.edu/phys1230 Lecture 17: Lenses and ray tracing Online and Written_HW9 due TONIGHT EXAM 2 is next week Thursday in-class 1 Last Time: Refraction all the way through block What was happening in Activity 8? U2L05 3 LastConvex Time: Concaveand concave and convex lenses lenses • Each of the two surfaces has a spherical shape. • Light can penetrate through the lenses and bend at the air-lens interface. 4 We build lenses out of glass with non-parallel sides Glass If slabs aren’t parallel - lens Glass A B C Which ray of light will have changed direction the most upon exiting the glass? We build lenses out of glass with non-parallel sides Put film, Retina here! 7 We build lenses out of glass with non-parallel sides Put film, Retina here! • Light rays bent towards each other… CONVERGING LENS. • The less parallel the two sides, the more the light ray changes direction. • Rays from a single point, converge to a single point on the other side of the lens (and then start diverging again). 8 Converging (convex) lens Light rays coming in parallel focus to a point, called the focal point optical axis Focus f Light focusing properties of converging lens a good light collector or solar oven; can also fry ants with sunlight (but please don’t do that) 10 Light focusing properties of converging lens The “backwards” light collector: create a collimated light beam 11 Ray tracing to understand lenses and images Ray tracing: 1.
    [Show full text]
  • A Simple and Inexpensive Schlieren Optical System Using a Fresnel Lens by Ted Kinsman Associate Professor RIT Photographic Sciences Dept
    A simple and inexpensive Schlieren Optical system using a Fresnel Lens By Ted Kinsman Associate Professor RIT Photographic Sciences Dept. January, 2020 [email protected] Schlieren imaging is a fairly standard optical technique for visualizing heat, sound, or pressure differences in air. Technically, a schlieren system is able to see clear disturbances in air due to slight differences in the speed of light in the air. The technique can be sensitive enough to see the heat rising off a human hand at room temperature. High quality schlieren systems typically use large optical mirrors that are expensive and beyond the budget of most optics classes. Here I will present the simipilest design for a system that any student can build, align, and experiment with. This simple design uses a plastic Fresnel lens. A Fresnel lens is a flat lens that is most commonly found in overhead projectors. Historical Fresnel lens were used for light gathering applications in which an image was not dependent on the quality of the lens. The first Fresnel lens was designed for a light house. In recent years, the Fresnel optics have improved so it was time to see if a Fresnel lens could be using in a schlieren system. The real interest in a Fresnel lens is that the lens has a large clear aperture – this means that relatively large objects can be tested in this area. High quality lenses with a 250 mm clear aperture are very expensive. The ideal lens for a simple schlieren system is between �/4 and �/8. The �-number of the lens is simply the ratio of the focal length of the lens divided by the clear aperture of the lens.
    [Show full text]
  • Physics 2310 Lab #5: Thin Lenses and Concave Mirrors Dr
    Physics 2310 Lab #5: Thin Lenses and Concave Mirrors Dr. Michael Pierce (Univ. of Wyoming) Purpose: The purpose of this lab is to introduce students to some of the properties of thin lenses and mirrors. The primary goals are to understand the relationship between image distance, object distance, and image scale. Theoretical Basis: You may be familiar with simple lenses and how they form images. The lens of your eye or a camera collects and focuses light and forms an image on retina or a piece of film or digital detector. A positive, convex glass lens works in much the same way to produce a real image. In contrast, a negative, concave lens can only produce virtual images. See the lecture notes of your textbook for a more complete description. Let’s explore the properties of some simple lenses. Begin by noting the equipment at each lab station. You will find a light source, a selection of lenses and mirrors, a white screen for viewing the images, and some sort of split-screen thing. We’ll come back to it later. We will use the scale on the bench to measure the location of each component and compute the object and image distances. Note that there is a small offset between the light source support and the actual location of the illuminated screen that serves as the object. Your lab TA will give you the value of this correction. Note that the light source “object” has a mm scale and two arrows at right angles so that you can determine the size of the image and its orientation on the screen when imaged by the lens.
    [Show full text]
  • 201 & 202-03 Imaging with a Thin Lens
    OPTI-201/202 Geometrical Geometrical Optics OPTI-201/202 and Instrumental 3-1 © Copyright2018JohnE.Greivenkamp Section 3 Imaging With A Thin Lens OPTI-201/202 Geometrical Geometrical Optics OPTI-201/202 and Instrumental 3-2 Object at Infinity © Copyright2018JohnE.Greivenkamp An object at infinity produces a set of collimated set of rays entering the optical system. Consider the rays from a finite object located on the axis. When the object becomes more distant the rays become more parallel. z When the object goes to infinity, the rays become parallel or collimated. An image at infinity is also represented by collimated rays. z OPTI-201/202 Geometrical Geometrical Optics OPTI-201/202 and Instrumental 3-3 Parallel Rays © Copyright2018JohnE.Greivenkamp Parallel rays are used to represent either an object at infinity or an image at infinity. z One way to think of this is that parallel lines intersect at a point at infinity. Without a lens, these rays could represent either an object at negative infinity or an image at positive infinity. With a lens, it is easy to tell an object from an image: z z But an image at positive infinity can serve as an object at negative infinity: z Infinity is infinity and it does not matter if we are discussing positive or negative infinity. OPTI-201/202 Geometrical Geometrical Optics OPTI-201/202 and Instrumental 3-4 On Axis and Off Axis – Object at Infinity © Copyright2018JohnE.Greivenkamp An on-axis object is aligned with the optical axis and the image will be on the optical axis: z For an off-axis object, collimated light still enters the optical system, but at an angle with respect to the optical axis.
    [Show full text]