A telecentric lens is a special

optical lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements ...

(often an

objective lens In optical engineering, the objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image. Objectives can be a single lens or mirror, or combinations of several optical elem ...

or a camera lens) that has its entrance or

exit pupil In optics, the exit pupil is a virtual aperture in an optical system. Only rays which pass through this virtual aperture can exit the system. The exit pupil is the image of the aperture stop in the optics that follow it. In a telescope or compou ...

, or both, at infinity. Telecentric lenses are often used for precision optical

two-dimensional In mathematics, a plane is a Euclidean ( flat), two-dimensional surface that extends indefinitely. A plane is the two-dimensional analogue of a point (zero dimensions), a line (one dimension) and three-dimensional space. Planes can arise as ...

measurements or reproduction and other applications that are sensitive to the image magnification or the angle of incidence of light. The simplest way to make a lens telecentric is to put the

aperture stop In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane. An optic ...

at one of the lens's focal points. This makes the chief rays (light rays that pass through the center of the aperture) on the other side of the lens parallel to the optical axis for any point in the

field of view The field of view (FoV) is the extent of the observable world that is seen at any given moment. In the case of optical instruments or sensors it is a solid angle through which a detector is sensitive to electromagnetic radiation. Human ...

. Commercially available telecentric lenses are often ''compound lenses'' that include multiple lens elements, for improved optical performance. Telecentricity is not a property of the lenses inside the compound lens but is established by the location of the aperture stop in the lens. The aperture stop selects the rays that are passed through the lens and the specific selection is what makes a lens telecentric. If a lens is not telecentric, it is either entocentric or hypercentric. Common lenses are usually entocentric. In particular, a single lens without a separate aperture stop is entocentric. For such a lens the chief ray originating at any point off of the optical axis is never parallel to the optical axis, neither in front of nor behind the lens. A non-telecentric lens exhibits varying magnification for objects at different distances from the lens. An entocentric lens has a smaller magnification for objects farther away; objects of the same size appear smaller the farther they are away. A hypercentric lens produces larger images the farther the object is away. A telecentric lens can be ''object-space telecentric'', ''image-space telecentric'', or ''bi-telecentric'' (also ''double-telecentric''). In an object-space telecentric lens the image size does not change with the object distance, and in an image-space telecentric lens the image size does not change with the image-side distance from the lens.

Object-space telecentric lenses

An object-space telecentric lens has the entrance pupil at infinity and provides an orthographic projection instead of the perspective projection in an entocentric lens. Object-space telecentric lenses have a ''working distance''. Objects at this distance are ''in focus'' and imaged sharply onto the image sensor at flange focal distance in the camera. An object that is closer or farther is out of focus and may be blurry, but will be the same size regardless of distance. Telecentric lenses tend to be larger, heavier, and more expensive than normal lenses of similar focal length and

f-number In optics, the f-number of an optical system such as a camera lens is the ratio of the system's focal length to the diameter of the entrance pupil ("clear aperture").Smith, Warren ''Modern Optical Engineering'', 4th Ed., 2007 McGraw-Hill Pro ...

. This is partly due to the extra components needed to achieve telecentricity, and partly because the first element in an object-space telecentric lens must be at least as large as the largest object to be imaged. The front element in an object-space telecentric lens is often much larger than the camera mount. In contrast to entocentric lenses where lenses are made larger to increase the aperture for increased collection of light or shallower depth of field, a larger diameter (but otherwise similar) object-space telecentric lens is not faster than a smaller lens. Because of their intended applications, telecentric lenses often have higher resolution and transmit more light than normal photographic lenses. Commercial object-space telecentric lenses are often characterized by their magnification, working distance and maximum image circle or image sensor size. A truly telecentric lens has no

focus ring In the field of photography, a manual focus camera is one in which the user has to adjust the focus of the lens by hand. Before the advent of autofocus, all cameras had manually adjusted focusing; thus, the term is a retronym. The focus itself m ...

to adjust the position of the focal plane. Some commercial telecentric lenses, however, do feature a focus ring. This can be used to slightly adjust the working distance and magnification while losing a little bit of telecentricity. Sometimes, manufacturers specify a sensor resolution or pixel size to describe the optical quality of the lens and the maximum optical resolution it can achieve due to the lens's aberrations. Because their images have constant magnification and constant viewing angle across the field of view, object-space telecentric lenses are used for metrology applications, where a

machine vision Machine vision (MV) is the technology and methods used to provide imaging-based automatic inspection and analysis for such applications as automatic inspection, process control, and robot guidance, usually in industry. Machine vision refers to ...

system must determine the precise size and shape of objects independently from their exact distance and position within the field of view. In order to optimize the telecentric effect when objects are illuminated from behind, an additional image-space telecentric lens can be used as a ''telecentric (or collimated) illuminator'', which produces a parallel light flow, often from LED sources.

Image-space telecentric lenses

An image-space telecentric lens has the exit pupil at infinity and produces images of the same size regardless of the distance between the lens and the film or

image sensor An image sensor or imager is a sensor that detects and conveys information used to make an image. It does so by converting the variable attenuation of light waves (as they pass through or reflect off objects) into signals, small bursts of c ...

. This allows the lens to be focused to different distances without changing the size of the image. An image-space telecentric lens is a reversed object-space telecentric lens, and vice versa. Since the chief rays after an image-space telecentric lens are always parallel to the optical axis, these lenses are often used in applications that are sensitive to the angle of incidence. Interference-based color-selective beam splitters or

filters Filter, filtering or filters may refer to: Science and technology Computing * Filter (higher-order function), in functional programming * Filter (software), a computer program to process a data stream * Filter (video), a software component tha ...

but also Fabry–Pérot interferometers are two examples where image-space telecentricity is used. Another example is minimizing crosstalk between pixels in image sensors and maximizing the quantum efficiency of a sensor. The

Four Thirds System The Four Thirds System is a standard created by Olympus and Eastman Kodak for digital single-lens reflex camera (DSLR) and mirrorless camera design and development. The system provides a standard that, with digital cameras and lenses availabl ...

initially required image-space telecentric lenses, but with the improvement of sensors, the angle of incidence requirement has been relaxed. Since every pixel is illuminated at the same angle by an image-space telecentric lens, they are also used for

radiometric Radiometry is a set of techniques for measuring electromagnetic radiation, including visible light. Radiometric techniques in optics characterize the distribution of the radiation's power in space, as opposed to photometric techniques, which ch ...

and color measurement applications, where one would need the irradiance to be the same regardless of the field position.

Bi-telecentric lenses

In a bi-telecentric (or double-telecentric) lens, both entrance and exit pupil are at infinity. The magnification is constant despite variations of both the distance of the object and the image sensor from the lens, allowing for more precise measurements than with a mono-telecentric lens. A bi-telecentric lens is afocal as the image of an object at infinity formed by the first part of the lens is

collimated A collimated beam of light or other electromagnetic radiation has parallel rays, and therefore will spread minimally as it propagates. A perfectly collimated light beam, with no divergence, would not disperse with distance. However, diffraction p ...

by the second part. Commercial bi-telecentric lenses are often optimized for very low

image distortion In geometric optics, distortion is a deviation from rectilinear projection; a projection in which straight lines in a scene remain straight in an image. It is a form of optical aberration. Radial distortion Although distortion can be irreg ...

and

field curvature Petzval field curvature, named for Joseph Petzval, describes the optical aberration in which a flat object normal to the optical axis (or a non-flat object past the hyperfocal distance) cannot be brought properly into focus on a flat image pl ...

for accurate measurements across the entire field of view at great resolution. These lenses often comprise more than 10 elements. Large and heavy bi-telecentric lenses with many elements are commonly used in optical lithography copying a template onto

semiconductor A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...

wafers A wafer is a crisp, often sweet, very thin, flat, light and dry biscuit, often used to decorate ice cream, and also used as a garnish on some sweet dishes. Wafers can also be made into cookies with cream flavoring sandwiched between them. They ...

References

{{reflist Optical microscope components Photographic lenses Machine vision