Various types of objectives, as used for optical instruments like telescopes, photographic objectives and laser scanners, are available with telecentric designs. The telecentricity of an objective essentially means that it has an orthographic perspective as for an observer at an infinite distance. This implies that the principal rays are horizontal to the left and right of the optical system, and the entrance and exit pupil is at infinity. Telecentricity is achieved by using an appropriately placed optical aperture, such as a diaphragm. In some cases this is not even necessary due to the already given restrictions on the incoming rays.
One distinguishes between telecentricity on the object and image side, or on both sides; these cases are explained in the following section.
In practice, the telecentricity condition (e.g. formulated via horizontal main rays) is never perfectly fulfilled. Therefore, telecentric lenses may have an additional specification regarding the telecentric angle, which indicates the deviation from perfect telecentricity.
Object-space Telecentric Lenses
A lens or objective can be made telecentric for the object space by inserting a diaphragm (aperture) in the back focal plane, as shown in Figure 1 for a simple single-lens configuration. (The same principle can easily be applied to a multiple-lens objective – it also has a back focal plane.) Only some of the rays, shown in blue, can pass through the diaphragm and contribute to the image.
Now consider an object at a greater distance (see Figure 2), observed without adjusting the focus. Here, the transmitted rays no longer meet on the original image plane; if the image sensor stays on the original plane, the image will be somewhat blurred. However, the center of the blurred area remains the same as before. This is because the diaphragm selects only those rays that are approximately horizontal in front of the lens. Under these conditions, the position of the object only affects the sharpness of the image, not its position; the blur is symmetrical about the constant center. Note, however, that this only works if the diaphragm is placed in the focal plane of the lens.
Although the aperture cannot affect the position of a true focus point as in Figure 1, it can affect the size and center of the blurred region in the defocused situation. The influence of an aperture stop of a photographic lens on the depth of field is explained in the same way.
If we define the magnification by the size on the selected image plane (independent of the true position of the image plane), the magnification turns out to be independent of the object distance. If the object is moved closer or further away from the lens, only the image sharpness is lost, but the apparent size remains unchanged. The blur effect can be reduced (i.e. the depth of field is increased) by reducing the aperture diameter, but this would reduce the brightness of the image.
Note that the magnification would of course change if the focus adjustment of the lens is changed. Therefore, one will not normally use such a lens with an autofocus device.
The entrance and exit pupil of the lens is obtained by imaging the diaphragm to the left. Since the diaphragm is in the focal plane, the entrance pupil is at infinity. This again emphasizes the orthographic perspective obtained: it is as for an observer at infinite distance, where changes in the longitudinal position of the object do not matter.
In contrast, most other (non-telecentric) lenses or objectives (just like the human eye) have their entrance pupil closer to the center of the optical entrance, and thus see objects as if from a reference point in that plane (entocentric perspective). The result is that more distant objects appear smaller than closer ones. Also, a small near object can completely hide a larger but more distant object. This is not the case with a telecentric system.
The explained principle of operation has some limitations:
- It requires that the image plane has some distance from the stop. Therefore, the device must not be focused to infinity, but to a closer plane. Therefore, one cannot get a large depth of field by using the hyperfocal distance; instead, one must limit the aperture size.
- The field of view – defined here not as an angular range, but in terms of distances – is at most as large as the entrance diameter of the lens. Therefore, object-space telecentric lenses often have to be relatively large and are correspondingly heavy and expensive. For the Photography of larger objects, telecentric lenses are usually not usable – except for very large versions, which can be realized with a Fresnel lens at the entrance.
An important application of object-space telecentric lenses is machine vision, where the orthographic perspective (with low distortion and no parallax error) can greatly simplify image processing. There are also telecentric measuring telescopes, which allow the measurement of true object sizes without significant sensitivity to their distance.
Some telecentric lenses contain additional devices for coaxial illumination. Essentially, this means that collimated light is sent through the optics toward the imaged object. In other cases, collimated light is used that comes from the opposite direction, i.e. towards the imaging system, when examining transparent objects (e.g. for inspection of optical glasses). Note that the camera does not then image the light source; instead, the light source simply results in homogeneous illumination of the image plane.
With this kind of directed illumination, sometimes called telecentric illumination, the most accurate telecentric imaging is possible when it comes to capturing edges and surface structures precisely. The exposure time can then be relatively short, since most of the incoming light is in a direction that can be used by the camera.
Note that telecentric illumination does not necessarily require a telecentric lens; a collimated beam can be obtained simply by using a relatively small light source (e.g. a small light emitting diode) and a collimating lens.
Microscope objectives are also sometimes made to be object-space telecentric. This avoids the apparent size changes of out-of-focus planes in thick specimens. There are also optical profilometers with telecentric optics.
Image-space Telecentric Lenses
Telecentricity can also be implemented in image space by placing an appropriate aperture in the front focal plane. This is shown in Figure 2, again with a simplified setup containing a single lens. Due to the introduced stop, the light rays are essentially horizontal after the lens when the aperture is small.
The vertical position of the reached pixel is then independent of the chosen longitudinal position of an image sensor; it is just that the pixel will be a bit blurred if the sensor is out of focus. Of course, the depth of field can be increased by simply reducing the aperture diameter, but this results in a loss of image brightness.
Figure 2 assumes parallel incoming rays from a distant object, but the same configuration would work for closer objects, just with the focus position shifted slightly to the right.
For an application, it may not be important that the image size is independent of the sensor position, but rather that the light always reaches the sensor with approximately normal incidence – not only in the center, but also in the peripheral regions. This is especially important for image sensors with microlens arrays. Therefore, image-space telecentric lenses are used in some digital cameras.
Another application of telecentric optics are scanning lenses for laser scanners. Telecentric scanning lenses can send a laser beam to different points on a target surface, but each time with approximately the same direction of propagation. There is usually no need to use a diaphragm; the entrance stop is effectively created by the scanning device, e.g. a polygon mirror wheel or a acousto-optic deflector. Such scanning applications include laser materials processing, including special areas such as microlithography.
You can also make bi-telecentric systems (or double-telecentric systems), which means telecentricity in both object and image space. However, this is only possible with afocal optical systems. Figure 3 shows an example. The main ray (passing through the center of the aperture stop) is horizontal to the left of the first lens and to the right of the second lens. Note that this can only work if the distance between the lenses is the sum of their focal lengths.
Bi-telecentric imaging systems can be used, for example, where both orthographic perspective and near-normal incidence of light on an image sensor are required.
Telecentric Zoom Lenses
Most telecentric lenses have a fixed focal length and magnification. However, there are also telecentric zoom lenses where the focal length, and thus the field of view and magnification, are adjustable within a certain range. To maintain telecentricity, the zoom mechanism must not only adjust the focal length, but also keep the stop in the focal plane. In some cases, the telecentric condition cannot be maintained over the entire zoom range.
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