Telecentric lens
Encyclopedia
A telecentric lens is a compound lens
which has its entrance
or exit
pupil at infinity; in the prior case, this produces an orthographic view
of the subject. This means that the chief rays (oblique rays
which pass through the center of the aperture stop) are parallel to the optical axis in front of or behind the system, respectively. The simplest way to make a lens telecentric is to put the aperture stop at one of the lens's focal point
s.
An entrance pupil at infinity makes the lens object-space telecentric. Such lenses are used in machine vision
systems because image magnification is independent of the object's distance or position in the field of view.
An exit pupil at infinity makes the lens image-space telecentric. Such lenses are used with image sensor
s that do not tolerate a wide range of angles of incidence. For example, a three-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.
If both pupils are at infinity, the lens is double telecentric.
Most lenses exhibit varying magnification
for objects at different distances from the lens. This causes several problems for machine vision and other applications:
Telecentric lenses, on the other hand, provide an orthographic projection
, providing the same magnification at all distances. 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 of the lens, the image of the 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. , these lenses can range in cost from hundreds to thousands of US dollars or euro
s, 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.
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.
Lens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...
which has its entrance
Entrance pupil
In an optical system, the entrance pupil is the optical image of the physical aperture stop, as 'seen' through the front of the lens system. The corresponding image of the aperture as seen through the back of the lens system is called the exit pupil...
or exit
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 compound microscope, this image is the image of the...
pupil at infinity; in the prior case, this produces an orthographic view
Orthographic projection
Orthographic projection is a means of representing a three-dimensional object in two dimensions. It is a form of parallel projection, where all the projection lines are orthogonal to the projection plane, resulting in every plane of the scene appearing in affine transformation on the viewing surface...
of the subject. This means that the chief rays (oblique rays
Ray (optics)
In optics, a ray is an idealized narrow beam of light. Rays are used to model the propagation of light through an optical system, by dividing the real light field up into discrete rays that can be computationally propagated through the system by the techniques of ray tracing. This allows even very...
which pass through the center of the aperture stop) are parallel to the optical axis in front of or behind the system, respectively. The simplest way to make a lens telecentric is to put the aperture stop at one of the lens's focal point
Focus (optics)
In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...
s.
An entrance pupil at infinity makes the lens object-space telecentric. Such lenses are used in machine vision
Machine vision
Machine vision is the process of applying a range of technologies and methods to provide imaging-based automatic inspection, process control and robot guidance in industrial applications. While the scope of MV is broad and a comprehensive definition is difficult to distil, a "generally accepted...
systems because image magnification is independent of the object's distance or position in the field of view.
An exit pupil at infinity makes the lens image-space telecentric. Such lenses are used with image sensor
Image sensor
An image sensor is a device that converts an optical image into an electronic signal. It is used mostly in digital cameras and other imaging devices...
s that do not tolerate a wide range of angles of incidence. For example, a three-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.
If both pupils are at infinity, the lens is double telecentric.
Object-space telecentric lenses
Magnification
Magnification is the process of enlarging something only in appearance, not in physical size. This enlargement is quantified by a calculated number also called "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 cameraCameraA camera is a device that records and stores images. These images may be still photographs or moving images such as videos or movies. The term camera comes from the camera obscura , an early mechanism for projecting images...
- 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, provide an orthographic projection
Orthographic projection
Orthographic projection is a means of representing a three-dimensional object in two dimensions. It is a form of parallel projection, where all the projection lines are orthogonal to the projection plane, resulting in every plane of the scene appearing in affine transformation on the viewing surface...
, providing the same magnification at all distances. 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
Metrology
Metrology is the science of measurement. Metrology includes all theoretical and practical aspects of measurement. The word comes from Greek μέτρον , "measure" + "λόγος" , amongst others meaning "speech, oration, discourse, quote, study, calculation, reason"...
applications, when a machine vision
Machine vision
Machine vision is the process of applying a range of technologies and methods to provide imaging-based automatic inspection, process control and robot guidance in industrial applications. While the scope of MV is broad and a comprehensive definition is difficult to distil, a "generally accepted...
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
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
chips.
Object-space telecentric lenses have an entrance pupil
Entrance pupil
In an optical system, the entrance pupil is the optical image of the physical aperture stop, as 'seen' through the front of the lens system. The corresponding image of the aperture as seen through the back of the lens system is called the exit pupil...
infinitely far behind the lens; this is, if you look in the front of the lens, the image of the aperture is very far away.
Telecentric lenses tend to be larger, heavier, and more expensive than normal lenses of similar focal length
Focal length
The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus...
and f-number
F-number
In optics, the f-number of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the f-number is the focal length divided by the "effective" aperture diameter...
. 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. , these lenses can range in cost from hundreds to thousands of US dollars or euro
Euro
The euro is the official currency of the eurozone: 17 of the 27 member states of the European Union. It is also the currency used by the Institutions of the European Union. The eurozone consists of Austria, Belgium, Cyprus, Estonia, Finland, France, Germany, Greece, Ireland, Italy, Luxembourg,...
s, depending on quality. Because of their intended applications, telecentric lenses often have higher resolution
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 a lens and recording and display components...
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
Light-emitting diode
A light-emitting diode is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting...
sources.
Image-space telecentric lenses
An image-space (or image-side) telecentric lens produces images of the same size regardless of the distance between the lens and the filmPhotographic film
Photographic film is a sheet of plastic coated with an emulsion containing light-sensitive silver halide salts with variable crystal sizes that determine the sensitivity, contrast and resolution of the film...
or image sensor
Image sensor
An image sensor is a device that converts an optical image into an electronic signal. It is used mostly in digital cameras and other imaging devices...
. 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
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 compound microscope, this image is the image of the...
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
3CCD
A three-CCD camera is a camera whose imaging system uses three separate charge-coupled devices , each one taking a separate measurement of the primary colors, red, green, or blue light. Light coming into the lens is split by a trichroic prism assembly, which directs the appropriate wavelength...
camera. Many lenses that have been specially optimized for digital SLR cameras are nearly telecentric on the image side, to avoid the vignetting
Vignetting
In photography and optics, vignetting is a reduction of an image's brightness or saturation at the periphery compared to the image center. The word vignette, from the same root as vine, originally referred to a decorative border in a book. Later, the word came to be used for a photographic...
and color crosstalk that occur in color filter array
Color filter array
In photography, a color filter array , or color filter mosaic , is a mosaic of tiny color filters placed over the pixel sensors of an image sensor to capture color information....
-based digital image sensors with oblique incident rays. The Four Thirds System
Four Thirds System
The Four Thirds system is a standard created by Olympus and Kodak for digital single-lens reflex camera design and development.The system provides a standard that, with digital cameras and lenses available from multiple manufacturers, allows for the interchange of lenses and bodies from different...
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
Radiometry
In optics, radiometry is a set of techniques for measuring electromagnetic radiation, including visible light. Radiometric techniques characterize the distribution of the radiation's power in space, as opposed to photometric techniques, which characterize the light's interaction with the human eye...
and colour measurement applications where one would need the irradiance
Irradiance
Irradiance is the power of electromagnetic radiation per unit area incident on a surface. Radiant emittance or radiant exitance is the power per unit area radiated by a surface. The SI units for all of these quantities are watts per square meter , while the cgs units are ergs per square centimeter...
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.
External links
- 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