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Section 10 Vignetting Vignetting the Stop Determines Determines the Stop the Size of the Bundle of Rays That Propagates On-Axis an the System for Through Object

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10-1 I and Instrumentation Design Optical OPTI-502 © Copyright 2019 John E. Greivenkamp E. John 2019 © Copyright Section 10 Vignetting 10-2 I and Instrumentation Design Optical OPTI-502 Vignetting Greivenkamp E. John 2019 © Copyright On-Axis The stop determines the size of Ray Aperture the bundle of rays that propagates Bundle through the system for an on-axis object. As the object height increases, z one of the other apertures in the system (such as a lens clear aperture) may limit part or all of Stop the bundle of rays. This is known as vignetting. Vignetted Off-Axis Ray Rays Bundle z Stop Aperture 10-3 I and Instrumentation Design Optical OPTI-502 Ray Bundle – On-Axis Greivenkamp E. John 2018 © Copyright The ray bundle for an on-axis object is a rotationally-symmetric spindle made up of sections of right circular cones. Each cone section is defined by the pupil and the object or image point in that optical space. The individual cone sections match up at the surfaces and elements. Stop Pupil y y y z y = 0 At any z, the cross section of the bundle is circular, and the radius of the bundle is the marginal ray value. The ray bundle is centered on the optical axis. 10-4 I and Instrumentation Design Optical OPTI-502 Ray Bundle – Off Axis Greivenkamp E. John 2019 © Copyright For an off-axis object point, the ray bundle skews, and is comprised of sections of skew circular cones which are still defined by the pupil and object or image point in that optical space. Stop Pupil Chief Ray y y y y y y = 0 z -y Image Plane Since the base of the cone is defined by the circular pupil, the cross section of the ray bundle at any z remains circular with a radius equal to the radius of the axial bundle. The off-axis bundle is centered about the chief ray height. The maximum radial extent of the ray bundle at any z is yyyMAX 10-5 I and Instrumentation Design Optical OPTI-502 Unvignetted Greivenkamp E. John 2019 © Copyright While the stop alone defines the axial ray bundle, vignetting occurs when other apertures in the system, such as a lens clear aperture, block all or part of an off-axis ray bundle. No vignetting occurs when all of the apertures pass the entire ray bundle from the object point. Each aperture radius a must equal or exceed the maximum height of the ray bundle at the aperture. Note that the required apertures size will change if the system FOV (chief ray) is changed. Ray y Bundle Unvignetted: y ay y y a x Aperture 10-6 I and Instrumentation Design Optical OPTI-502 Fully Vignetted Greivenkamp E. John 2019 © Copyright The maximum FOV supported by the system occurs when an aperture completely blocks the ray bundle from the object point. y Ray Bundle y Fully Vignetted: ay y y and a ay x Aperture The second part of this vignetting condition ensures that the aperture passes the marginal ray and is not the system stop. By definition, vignetting cannot occur at the aperture stop or at a pupil. 10-7 I and Instrumentation Design Optical OPTI-502 Half Vignetted Greivenkamp E. John 2019 © Copyright A third vignetting condition is defined when an aperture passes about half of the ray bundle from an object point. y Ray Half Vignetted: Bundle y ay and y a ay x Aperture 10-8 I and Instrumentation Design Optical OPTI-502 Ray Bundles and Aperture Diameter Greivenkamp E. John 2019 © Copyright Because the Field of View of an optical system is symmetric (i.e. ±h or ±), the vignetting conditions apply equally well to the upper and lower ray bundles. Stop Pupil Chief Ray y y z -y -y Image Plane The maximum radial extent of both ray bundles at any z is yyyMAX If a system is used with a rectangular detector, different amounts of vignetting can occur in the horizontal, vertical and diagonal directions. 10-9 I and Instrumentation Design Optical OPTI-502 Vignetting Summary Greivenkamp E. John 2019 © Copyright The required clear aperture radius at a given z: Unvignetted: ay y Fully Vignetted:ay y and ay Half Vignetted:ay and ay The vignetting conditions are used in two different manners: - For a given set of apertures, the FOV that the system will support with a prescribed amount of vignetting can be determined. A different chief ray defines each FOV. - For a given FOV and vignetting condition, the required aperture diameters can be determined. A system with vignetting will have an image that has full irradiance or brightness out to a radius corresponding to the unvignetted FOV limit. The irradiance will then begin to fall off, going to about half at the half-vignetted FOV, and decreasing to zero at the fully-vignetted FOV. This fully-vignetted FOV is the absolute maximum possible. This description ignores the obliquity factors of radiative transfer, such as the cosine fourth law. 10-10 I and Instrumentation Design Optical OPTI-502 Example System – Ray Bundle Extent Greivenkamp E. John 2019 © Copyright L1 L2 Stop yy y y y z y y yy EP XP ay y Marginal Ray 10-11 I and Instrumentation Design Optical OPTI-502 Vignetting Example – Page 1 Greivenkamp E. John 2019 © Copyright An object is located 100 mm to the left of a 50 mm focal length thin lens. The object has a height of 10 mm above the optical axis of the lens. The lens diameter is 20 mm, and the lens serves as the system stop. An aperture is placed 50 mm to the right of the lens. What is the required diameter of the aperture so that the system operates without vignetting? First consider the imaging: f = 50 mm h = 10 mm Image z Object h' z = -100 mm z' Stop at Lens DSTOP = 20 mm zmm100 111 zmm 100 f 50mm zzf' Lens is operating at 1:1 conjugates hmm 10 10-12 I and Instrumentation Design Optical OPTI-502 Vignetting Example – Page 2 Greivenkamp E. John 2019 © Copyright Draw the Marginal and Chief Rays and evaluate ray heights at the aperture: h = 10 mm 50 mm Aperture Image yA y z Object A h′ = -10 mm Stop z = -100 mm z′ = 100 mm aSTOP = 10 mm At the aperture: ya/2 5 mm ASTOP For no vignetting: ayymm10 yhA /2 5 mm APERTURE A A DmmAPERTURE 20 10-13 I and Instrumentation Design Optical OPTI-502 Vignetting Example – Page 3 Greivenkamp E. John 2019 © Copyright The ray bundle with no vignetting: D = 20 mm h = 10 mm 50 mm APERTURE Image z Object Stop h = -10mm z = -100 mm z′ = 100 mm The full ray bundle is passed by the aperture. 10-14 I and Instrumentation Design Optical OPTI-502 Vignetting Example – Page 4 Greivenkamp E. John 2019 © Copyright Given this aperture diameter of 20 mm, what is the object height that will be imaged with half vignetting? The marginal ray does not change, but the chief ray changes with object height. Start with an arbitrary object height. The chief ray goes through the center of the stop. h = ? D = 20 mm 50 mm APERTURE Image u u z Object yA Stop h = -h z = -100 mm z′ = 100 mm h hh At the aperture: ymmumm50 50 u A zz z For half vignetting: aymmAPERTURE A 10 hh Equating: ymmmmmm50 50 10 A zmm100 hmm 20 Half Vignetted Object Height 10-15 I and Instrumentation Design Optical OPTI-502 Vignetting Example – Page 5 Greivenkamp E. John 2019 © Copyright The ray bundle at half vignetting h = 20 mm D = 20 mm 50 mm APERTURE Image u z Object yA Stop h = -20mm z = -100 mm z′ = 100 mm aymmAPERTURE A 10 Half of the ray bundle is blocked by the aperture. The chief ray goes through the edge of the aperture and the center of the stop. 10-16 I and Instrumentation Design Optical OPTI-502 Example System – Pupils and Vignetting – Page 1 Greivenkamp E. John 2019 © Copyright The following reverse telephoto objective is comprised of two thin lenses in air. The system stop is located between the two lenses. The system operates at f/4. The object is at infinity. The maximum image size is +/- 30 mm. Stop Image f = -200 mm f = 100 mm 1 2 Plane (+/- 30 mm) 40 mm 40 mm Determine the folowing: - Entrance pupil and exit pupil locations and sizes. - System focal length and back focal distance. - Stop diameter. - Angular field of view (in object space). - Required diameters for the two lenses for the system to be unvignetted over the specified maximum image size. 10-17 I and Instrumentation Design Optical OPTI-502 Example System – Pupils and Vignetting – Page 2 Greivenkamp E. John 2019 © Copyright First set up the raytrace sheet. Trace a potential Chief Ray starting at the center of the Stop. The Pupils are located where this ray crosses the axis in object space and image space. Object EP L1 Stop L2 XP Image Surface 0123456 f -200 - 100 - 0.005 - -0.01 t -33.33 40 40 -66.67 Potential Chief Ray: y 0 -4.00 0 4.00 0 u .12 0.1* 0.1* 0.06 Entrance Pupil: Located 33.33 mm to the Right of L1 Exit Pupil: Located 66.67 mm to the Left of L2 Both Pupils are virtual. * Arbitrary 10-18 I and Instrumentation Design Optical OPTI-502 Example System – Pupils and Vignetting – Page 3 Greivenkamp E. John 2019 © Copyright In order to determine the Focal Length and the Back Focal Distance, trace a potential Marginal Ray.
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