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Telecentric lens by fdhjkl rfghjtkl
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Telecentric lens |
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Normal lenses exhibit varying magnification for objects at different distances from the lens. This causes several problems for machine vision and other applications:
the apparent size of objects changes with distance from the camera
some features or objects may be hidden by objects that are closer to the lens
the apparent shape of objects varies with distance from the center of the field of view (FOV). Objects appearing close to the edges are viewed from an angle, while objects near the centre of the FOV are viewed frontally (circles near the centre of the FOV become egg-shaped when moved towards the periphery).
Telecentric lenses, on the other hand, have the same magnification at all distances. An object-space telecentric lens creates images of the same size for objects at any distance and has constant angle of view across the entire field of view. An object that is too close or too far from the lens may still be out of focus, but the resulting blurry image will be the same size as the correctly-focused image would be.
Because their images have constant magnification and geometry, telecentric lenses are used for metrology applications, when a machine vision system must determine the precise size of objects independently from their position within the FOV and even when their distance is affected by some degree of unknown variations. These lenses are also commonly used in optical lithography, for forming patterns in semiconductor chips.
Object-space telecentric lenses have an entrance pupil infinitely far behind the lens; this is, if you look in the front, the apparent aperture is very far away.
Telecentric lenses tend to be larger, heavier, and more expensive than normal lenses of similar focal length and f-number. This is partly due to the extra components needed to achieve telecentricity, and partly because the object or image lens elements of an object or image-space telecentric lens must be at least as large as the largest object to be photographed or image to be formed. As of 2006[update], these lenses can range in cost from hundreds to thousands of US dollars or euros, depending on quality. Because of their intended applications, telecentric lenses often have higher resolution and transmit more light than normal photographic lenses.
In order to optimize the telecentric effect, these lenses are often used in conjunction with telecentric (or "collimated") illuminators, which produce a parallel light flow, often from LED sources.
An image-space (or image-side) telecentric lens produces images of the same size regardless of the distance between the lens and the film or image sensor. This allows the lens to be focused to different distances without changing the size of the image. Image-space telecentric lenses have an exit pupil infinitely far in front of the lens; that is, if you look in the back of the lens, the apparent aperture is very far away.
At the film or image sensor, all of the chief rays from these lenses hit "straight on", or at zero angle of incidence. This property minimizes any angle-of-incidence dependence of the sensor, or of any beam-splitter prism assembly behind the lens, such as a color separation prism in a three-CCD camera. Many lenses that have been specially optimized for digital SLR cameras are nearly telecentric on the image side, to avoid the vignetting and color crosstalk that occur in color filter array-based digital image sensors with oblique incident rays. The Four Thirds System uses this approach.
Since the ray cones approaching the detector surface have the same angle of incidence and angular subtense everywhere in the image plane, the image is evenly illuminated. This feature is commonly used in photography, and is very useful for radiometric and colour measurement applications where one would need the irradiance to be the same regardless of the field position.
Lenses that are double-telecentric have magnification that is more precisely constant than those that are only object-side telecentric, because the principal ray intecept position on the detector doesn't change. This property allows precise measurement of objects regardless of position.
"Telecentric Lenses: basic information and working principles". Opto Engineering. Retrieved 14 Oct 2008.
Technical description of telecentric effect from Edmund Optics, a manufacturer of telecentric lenses
Another good explanatory page by Donald Simanek
Another article from a machine-vision point of view , from Computer Optics, a manufacturer of telecentric lenses.
A course on Gaussian optics of telecentric systems from Schneider Kreuznach Optical measurement techniques with telecentric lenses
Categories: Photographic lensesHidden categories: Articles containing potentially dated statements from 2006 All articles containing potentially dated statements I am a professional writer from China Agriculture Net, which contains a great deal of information about hip hop dance learn , calvin klein t shirt, welcome to visit!
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