ChapterV Field stop and aperture stop英文资料.doc

Chapter V Field stop and aperture stop视场光阑与孔径光阑5.1 Field stop and aperture stop 视场光阑与孔径光阑5.2 Filed of view 视场5.3 Vignetting and field lens 渐晕与场镜5.4 Telecentric system 远心系统5.5 Depth of focus and depth of field 焦深与景深5.6 Stray light in lenses 透镜中的杂散光5.1 Field stop and aperture stop一、Field stop and aperture stopAperture stop: The stop close to lens limits the size of the bundles of rays of axis point. It determines the amount of light reaching any given point in the image, therefore controls the brightness of image (image plane).Field stop:Just front of the focal plane limits the angle at which the incident bundles can get through to this plane (image plane). It determines the extent of the object or the field. (Field stop is usually made to coincide with image.)二、Entrance pupil and exit pupil: The image of aperture stop formed by preceding it.: The image of aperture stop formed by following it. The image of this stop, as formed by the lens is found to lie at the position .*Since is inside the focal plane, its image lies in the object space and is virtual and erect, PL called entrance pupil.* lies in the image space, called exit pupil.*, are pairs of conjugated points. Any ray in the object space directed through one of these points will after refraction pass through its conjugate point in the image space.三、Chief ray Any ray in the object space that passes through the center of entrance pupil is called chief ray. After refracting also pass through the center of the exit pupil.*Chief ray is a central ray of oblique rays. preliminary design of an optical instrument, must determine which one actually does the limiting?The entrance pupil of the centric system is the image of the aperture stop formed by all lenses preceding it.The exit pupil is the image formed by all lenses following it.四、Stop between two lenses Chief ray pass through the entrance pupil, the stop and the exit pupil. How to determine aperture stop?五、Two lenses with no stop The periphery of any lens in system is a potential stop.Object at infinity, : entrance pupil. Object at A, : entrance pupilDetermination of the aperture stop The system consists of tow lenses and a stop. is the virtual image of the first ()and a stop formed by (): Virtual image of the stop P formed by .: Virtual image of the stop formed by .: Virtual image of the second lens formed by .For all axial object points lying to the left of X, limits the entrance bundle of rays to the smallest angle. constitutes the entrance pupil. constitutes the exit pupil.For all axial object point lying to the right of Z, is the entrance pupil and is the exit pupil.Nominal aperture stop: The aperture stop of any system may change with a change in the object position.General rule: the aperture stop of the system is determined by that stop or image of stop which subtends the smallest angle as seen from the object point.Telescope and microscope (low power): usually aperture stop is selected at the large-sized objective lens.5.2 Field of view一、Field of view2W: object field and image fieldLook out at landscape through a window. Fig 1: The field of view is limited by the size of window and by the position of the eye. Window is field stop.General speaking, field stop is at image plane. ( such as telescope, microscope) The margin of reticle or film is field stop or frame.二、Field of a plane mirrorThe field of view offered by a plane mirror is very similar to that of a simple window. : observers eye, which constitutes the exit pupil. : The entrance pupil, virtual image of the eye pupil formed by mirror.Chief rays define the field of view.The object B and entrance pupil MN are in the object space. 三、How can you define the field of view of a convex mirror? 四、Field of a positive lensThe pupil of eye is an exit pupil and aperture stop.: the image of eye is an entrance pupil.The field stop in this case is the lens itself.Object is in a position inside the primary focal point . Through each of three points 、 , rays are drawn from to lens.Refracted rays are directed through 、, virtual image is located at .*When converging lens is used as a magnifier, the eye should be placed close to the lens, since this widens the image-fieldangle.For visual visional optical instruments, the eye should be placed the exit pupil of optical system, such as telescopes and microscopes.五、Entrance window and exit windowEntrance window is the image of field stoop formed by all lenses preceding it.Exit window is the image of field stoop formed by all lenses following it. Both entrance and exit window are at infinity.六、Aperture angle and field angleAperture angle: Aperture angle in the object space. On-axis point subtend to the margin of entrance pupil.: Aperture angle in the image space. On-axis image point subtend to the margin of exit pupil.Field angle: The center of entrance pupil subtended to the margin of entrance window.: The center of exit pupil subtended to the margin of exit window.5.3 Vignetting and Field lens一、VignettingAt small filed of view, the highest and lowest rays passing through the lens, strike the top and bottom of,.At greater field of view, entrance upper and lower rays are often limited by the front and rear lens openings. The failure of an oblique beam is known as vignetting.The brightness of an image will be gradually decreased darken, when the field of view increased.Coefficient of vignetting Chief ray (principal ray): passes through the middle of aperture stop from off-axis object point or the ray that enters the lens midway between the highest and lowest rays of oblique beam. 二、Field lens(1) In order to reduce the size of following optical system, field lens is usually at the image plane. (See chapter I)Free from There are only field curvature and distortion caused by . (2)Field lens can be used to change the position of exit pupil to satisfy pupil connection.Exit pupil of preceding system should be coincide with entrance pupil of following system.I Galilean telescope Aperture stop: rim of objective so is entrance pupilField stop: eye piece so is exit windowII Kepler telescopereticle is field stop. Entrance window and exit window are at infinity. Field angle:Exit pupil is close to .if eye iris is less than exit pupil, iris is aperture stop. III Microscope with low magnificationAperture stop is the rim of objective so is entrance pupil.Field stop is the rim of reticle.Entrance window coincides with object plane.Exit window coincide with image plane (infinity).5.4 Telecentric systemI object space telecentric systemII image space telecentric systemIII double telecentric system*In most cases, the aperture stop is located somewhere inside the lens, such as photographic cameras.*eyepieces condenser are exceptions. 一、Telecentric system1) Telecentric system in the object space: The aperture stop is at the rear focal plane .The entering principal ray is parallel to axis. 2) Telecentric system in the image space: The aperture stop is at the front focus .The entering beam come from infinity, and exiting principal ray is parallel to axis. 3)Telecentric system at the both ends:The aperture stop is at . and are coincided. The system usually used as inverting system and relay lens.二、 Contour projectorContour or profile projector is a instrument for projecting a magnified image of a small mechanical parts to check its dimensions. The optical system is invariable telecentric, the virtual of a telecentric system is that the size of the projected image does not alter, if the image is slightly out of focus. The diameter of the lens aperture must be greater than the diameter of the object.Condenser lens near the lamp, providing collimated light, must also be larger than the largest object to be projected.Final image is generally projected on a ground glass screen, sometimes equipped with Fresnel lens to brighten up the outer parts of screen. 5.5 Depth of focus and depth of field 一、conceptAssuming the lens is free from all aberrations.1) There will be a ceitain plane in the object space that is precisely conjugate to the film plane in the image space. This is known as the focused plane in the object.2) The observers eyes have a limited acuity. There will be a small amount of tolerable blur in a photograph that observer will be unable to distinguish it from sharp image.a point object may be imaged as a small circle of confusion on the film.The depth of focus of lens is the distance along the of sharp definition to the plane where the image of a point source just reaches this circle of confusion.The depth of field of lens is the corresponding distance of an object point from the focused plane in object space to the plane where its image reaches the permissible circle of confusion in the film plane.二、calculation1. Depth of focus is obviously equal to the product of the diameter of the permissible circle of confusion on the film and the F-Number F of the lens.The distance is the same whether the image is just within or just beyond the film plane. how large?When a picture is viewed by an observer, most people are incapable of distinguishing between a perfect point and a small circle of confusion that subtends about 2 arc-min.2. The depth of fieldThe diagram is based on the fact that everything lying in the focused plane will be imaged sharply on the film. is the object of (circle of confusion in image space) or the diameter of the acceptable circle of confusion in the focused plane. and are the far and near object point which is imaged and forms a circle of confusion .is far depth of field.is near depth of field.; , is larger than.() If , entrance pupil equal to circle of confusion in object space. becomes infinity.(， )if 2a is reduced, the depth of