Telecentric lens

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A telecentric lens is a compound lens with an unusual property concerning its geometry of image-forming rays. There are two main types, each with its own use:

A lens that is object space telecentric is usually employed for machine vision systems in order to achieve dimensional and geometric invariance of images within a range of different distances from the lens and across the whole field of view.
A lens that is image space telecentric is used with image sensors that do not tolerate a wide range of angles of incidence. For example, a 3-CCD color beamsplitter prism assembly works best with a telecentric lens, and many digital image sensors have a minimum of color crosstalk and shading problems when used with telecentric lenses.

A lens is called double telecentric if it is telecentric on both sides.

The technical property that distinguishes telecentric lenses is that the chief rays, that is the rays through the center of the entrance or exit pupil, are all parallel to the optical axis, on one or both sides of the lens, no matter what part of the image space or object space they go through.

[edit] Object space telecentric lenses

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 ellipses 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, these lenses can range 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 illuminators, which produce a parallel light flow, often from LED sources.

[edit] Image space telecentric lenses

An image-space 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.

At the film or image sensor, all of the chief rays 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, such as a color separation prism in a three-CCD camera, behind the lens.

Image space telecentric lenses have an exit pupil infinitely far in front of the lens; this is, if you look in the back, the apparent aperture is very far away.

The Olympus Four Thirds System specifies a very distant exit pupil (nearly image side telecentric lenses), to avoid the vignetting and color crosstalk that occur in Bayer pattern image sensors with oblique incident rays. Many lenses that have been specially optimized for digital SLR cameras are nearly telecentric on the image side, and consequently have very small angles of chief ray angles of incidence.