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A telecentric lens is a special
An object-space telecentric lens has the entrance pupil (the image of the lens's aperture stop, formed by optics before it) at infinity and provides an
An image-space telecentric lens has the exit pupil (the image of the aperture stop formed by optics after it) at infinity and produces images of the same size regardless of the distance between the lens and the
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 being observed and the image sensor from the lens, allowing more precise object size measurements than with a mono-telecentric lens (i.e., the measurements being insensitive to placement errors of the object and the image sensor). A bi-telecentric lens is afocal (a system without focus) as the image of an object at infinity formed by the first part of the lens is
A telecentric lens is a special optical lens
A lens is a transmissive optics, optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a #Compound lenses, compound lens consists of several simple ...
(often an objective lens
In optical engineering, an objective is an optical element that gathers light from an object being observed and focuses the light rays from it to produce a real image of the object. Objectives can be a single lens or mirror, or combinations of ...
or a camera lens
A camera lens, photographic lens or photographic objective is an optical lens (optics), lens or assembly of lenses (compound lens) used in conjunction with a camera body and mechanism to Imaging, make images of objects either on photographic film ...
) that has its entrance or exit pupil
In optics, the exit pupil is a virtual aperture in an optical system. Only ray (optics), 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 optic ...
, or both, at infinity. The size of images produced by a telecentric lens is insensitive to either the distance between an object being imaged and the lens, or the distance between the image plane and the lens, or both, and such an optical property is called ''telecentricity''. Telecentric lenses are used for precision optical two-dimensional
A two-dimensional space is a mathematical space with two dimensions, meaning points have two degrees of freedom: their locations can be locally described with two coordinates or they can move in two independent directions. Common two-dimension ...
measurements, reproduction (e.g., photolithography
Photolithography (also known as optical lithography) is a process used in the manufacturing of integrated circuits. It involves using light to transfer a pattern onto a substrate, typically a silicon wafer.
The process begins with a photosensiti ...
), 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, the aperture of an optical system (including a system consisting of a single lens) is the hole or opening that primarily limits light propagated through the system. More specifically, the entrance pupil as the front side image of ...
at one of the lens's focal points. This allows only rays including the chief ray
In optics, a ray is an idealized geometrical model of light or other electromagnetic radiation, obtained by choosing a curve that is perpendicular to the ''wavefronts'' of the actual light, and that points in the direction of energy flow. Rays ...
s (light rays that pass through the center of the aperture stop), that will be about parallel to the optical axis on the other side of the lens, to pass the optical system for any object point in the field of view
The field of view (FOV) is the angle, angular extent of the observable world that is visual perception, 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 elec ...
. 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 this 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
Magnification is the process of enlarging the apparent size, not physical size, of something. This enlargement is quantified by a size ratio called optical magnification. When this number is less than one, it refers to a reduction in size, so ...
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 (the image of the lens's aperture stop, formed by optics before it) at infinity and provides an orthographic projection
Orthographic projection (also orthogonal projection and analemma) is a means of representing Three-dimensional space, three-dimensional objects in Plane (mathematics), two dimensions. Orthographic projection is a form of parallel projection in ...
instead of the perspective projection
Linear or point-projection perspective () is one of two types of graphical projection perspective in the graphic arts; the other is parallel projection. Linear perspective is an approximate representation, generally on a flat surface, of ...
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
For an interchangeable lens camera, the flange focal distance (FFD) (also known as the flange-to-film distance, flange focal depth, flange back distance (FBD), flange focal length (FFL), back focus or register, depending on the usage and source ...
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
The focal length of an Optics, optical system is a measure of how strongly the system converges or diverges light; it is the Multiplicative inverse, inverse of the system's optical power. A positive focal length indicates that a system Converge ...
and f-number
An f-number is a measure of the light-gathering ability of an optical system such as a camera lens. It is calculated by dividing the system's focal length by the diameter of the entrance pupil ("clear aperture").Smith, Warren ''Modern Optical ...
. 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
The depth of field (DOF) is the distance between the nearest and the farthest objects that are in acceptably sharp focus (optics), focus in an image captured with a camera. See also the closely related depth of focus.
Factors affecting depth ...
, 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 itsel ...
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
Optical resolution describes the ability of an imaging system to resolve detail, in the object that is being imaged.
An imaging system may have many individual components, including one or more lenses, and/or recording and display components. E ...
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
Metrology is the scientific study of measurement. It establishes a common understanding of Unit of measurement, units, crucial in linking human activities. Modern metrology has its roots in the French Revolution's political motivation to stan ...
applications, where a machine vision
Machine vision is the technology and methods used to provide image, imaging-based automation, automatic inspection and analysis for such applications as automatic inspection, process control, and robot guidance, usually in industry. Machine vision ...
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
A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (corresp ...
sources.
Image-space telecentric lenses
An image-space telecentric lens has the exit pupil (the image of the aperture stop formed by optics after it) at infinity and produces images of the same size regardless of the distance between the lens and the film
A film, also known as a movie or motion picture, is a work of visual art that simulates experiences and otherwise communicates ideas, stories, perceptions, emotions, or atmosphere through the use of moving images that are generally, sinc ...
or image sensor An image sensor or imager is a sensor that detects and conveys information used to form an image. It does so by converting the variable attenuation of light waves (as they refraction, pass through or reflection (physics), reflect off objects) into s ...
. This allows the lens to focus light from an object or sample 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 (light rays that pass through the center of the aperture stop) 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 of light. Interference-based color-selective beam splitter
A beam splitter or beamsplitter is an optical instrument, optical device that splits a beam of light into a transmitted and a reflected beam. It is a crucial part of many optical experimental and measurement systems, such as Interferometry, int ...
s or filters
Filtration is a physical process that separates solid matter and fluid from a mixture.
Filter, filtering, filters or filtration may also refer to:
Science and technology
Computing
* Filter (higher-order function), in functional programming
* Fil ...
but also Fabry–Pérot interferometer
In optics, a Fabry–Pérot interferometer (FPI) or etalon is an optical cavity made from two parallel reflecting surfaces (i.e.: thin mirrors). Optical waves can pass through the optical cavity only when they are in resonance with it. It is ...
s are two examples where image-space telecentricity is used. Another example is minimizing crosstalk
In electronics, crosstalk (XT) is a phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel. Crosstalk is usually caused by undesired capacitive, ...
between pixels in image sensors and maximizing the quantum efficiency
The term quantum efficiency (QE) may apply to incident photon to converted electron (IPCE) ratio of a photosensitive device, or it may refer to the TMR effect of a magnetic tunnel junction.
This article deals with the term as a measurement of ...
of a sensor. The Four Thirds System
The Four Thirds System is a standard created by Olympus Corporation, Olympus and Eastman Kodak for digital single-lens reflex camera (DSLR) design and development. Four Thirds refers to both the size of the image sensor (4/3") as well as the a ...
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
In radiometry, irradiance is the radiant flux ''received'' by a ''surface'' per unit area. The SI unit of irradiance is the watt per square metre (symbol W⋅m−2 or W/m2). The CGS unit erg per square centimetre per second (erg⋅cm−2⋅s−1) ...
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 being observed and the image sensor from the lens, allowing more precise object size measurements than with a mono-telecentric lens (i.e., the measurements being insensitive to placement errors of the object and the image sensor). A bi-telecentric lens is afocal (a system without focus) 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 laser beam is an archetypical example. A perfectly collimated light beam, with no divergence, would not disp ...
by the second part.
Commercial bi-telecentric lenses are often optimized for very low image distortion 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 plane ...
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 optical elements are commonly used in optical lithography (that copies a template of an electrical circuit to print or fabricate onto semiconductor
A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. Its conductivity can be modified by adding impurities (" doping") to its crystal structure. When two regions with different doping level ...
wafers for mass semiconductor device production) because small image distortion and placement errors can be critical for manufactured device functionality.
References
{{reflist Microscope components Photographic lenses Machine vision