Optical glass does not have to be transparent for all wavelengths used in image processing. This is particularly true for UV and infrared lighting.

Therefore, the wavelength selected will depend on the type of material to be inspected, the colour and the type of inspection to be carried out.

A key indicator for lens selection is the determination of the field of view according to working distance that the lens can control as well as the size of the camera sensor.

The scale for magnifications/reductions is taken from the relationship between the size of the image sensor and the field of view. It has a considerable influence on the pixel resolution, i.e. the sensor can recognize the smallest details. In the case of entocentric lenses, the image scale can be changed by altering the working distance. On the other hand, in telecentric lenses it is not possible to alter the scale of the image since the field of view is fixed.

The working distance refers to the distance between the camera sensor and the part to be inspected. Depending on the field of view, the size of the sensor and the lens used, the working distance will vary.

The C and CS mount have been established for use in matrix cameras. Lens frames with a larger diameter than C-mount lenses are mainly used for linear cameras. CS-mount lenses can be used with 5 mm, intermediate ring in C-mount cameras. The opposite is not possible. The lens mount is closely related to the focal length.

In the case of ethnocentric lenses, the focal length determines the angle of view of the lens. A short focal length means a wide viewing angle, and a long one creates a narrower viewing angle. The focal length and the viewing angle are inversely proportional: by halving the focal length, the viewing angle is doubled. The same focal length creates different viewing angles and therefore different visual fields. Lenses whose focal length can be altered (zoom lenses) are suitable for laboratory use for quick and flexible experimental setups. Lenses with fixed focal lengths are used in industrial applications.

The apertures are used to control the lens illumination pitch. The opening is fixed in the ring, it must always be mechanically correctable. The luminous flux is halved by closing the aperture and doubled by opening it. The smaller the aperture number, the brighter the lens will be, meaning that images can be captured with less exterior lighting.

Depends on the size of the pixel in the sensor and the aperture assembly. Whether the lens is ethnocentric, telecentric or hypercentric, the depth of field depends exclusively on 3 factors: the aperture, the scale of the image and the permissible level of the lack of definition. The depth-of-field interval means that the object to be inspected does not always have to be at a constant distance from the camera during inspection, but can be reliably recognized in one path.

As a matter of principle, all lenses have image errors. The chances that a lens can achieve accurate images depend greatly on its construction, the materials used and the complexity of its optical construction, are reflected in the price of different types of lenses. Image sensors with smaller pixels also require higher resolution lenses. The qualitative characterization of lenses is a complex issue and cannot be represented in just a few parameters. It is physically impossible and it would also be financial madness to try to build an overall high-performance objective'. Therefore, it can be very helpful to look at the individual criteria for rating the lens according to its use:

● Geometric fidelity:
Distortion parameter: specifies the degree to which the object and the test image are mathematically similar in geometry.

• Modulation Transmission Function (MTF):
Resolution parameter: characterizes the fine details that are represented by the lens glass and with which contrast can be better represented.

● Fidelity of brightness:
Vignette setting: describes the level of brightness loss that can be expected at the edges of the image.

● Color fidelity:
Color Image Error Parameters: describes the efficiency of color transmission through the lens and/or what error in color effects appear at the edge of the image.

General information about image errors:
● In general, image errors are less frequent in the center of the image and more frequent at the edges.
● Image errors can be minimized by a medium aperture.
● Lenses with short focal lengths always have more image errors than lenses with long focal lengths due to lens curvature.

The quality of the lenses can only amply demonstrate the use of a wide range of properties and parameters.

Transitions from light from air to glass and from glass to air, even within the lens, are associated with loss of brightness and contrast. Modern lens designs always contain multiple lenses, thereby increasing the loss of brightness and contrast, so measures need to be developed to minimize these losses. This is achieved by using thin, optically transparent layers that are vapor-coated in a vacuum on the lens lens and appear to the human eye as a mist of color on the lens surface. Even the individual layers lead to a considerable increase in the transparency and contrast of the images.

In addition to the visual impression of the ethnocentric perspective that humans are familiar with, telecentric and hypercentric lenses are also used in image processing.

● Application:
Color control, character codes and reading, rotary orientation recognition. Full presence control. For metric measurements they are extremely limited

● Fixed focal length lenses:
They have an ethnocentric perspective that is fixed by focal length and is largely characterized by focal length and brightness data.

● The macro targets:
They have a shorter minimum working distance than standard lenses, which is achieved by increasing the cost of optical construction. They create images that are particularly high quality at close range.

● Perpendicular lenses:
They are a particular design of ethnocentric lenses that can be used where space is limited. As a matter of principle, these lenses are only available with a focal length > 25 mm. The perpendicular lenses always show the inverted images.

Telecentric lenses are especially suitable for precise metric measurements, also for inspection tasks, detecting presence controls and exhaustiveness on large surfaces to be inspected with different levels of height and measurement, as well as geometrically complex parts that cannot be inspected using ethnocentric lenses. Parts with varied surfaces, glossy and optically active materials, such as glass and plastic, are also particularly suitable for this type of inspection.

Telecentricity has nothing to do with depth of field. It describes the changes in image size. In the depth of field on the other hand, the changes in image sharpness are described. In addition, the principle of telecentricity has no influence on the size of the depth of field. The same conditions apply to telecentric and ethnocentric targets.

As a main issue, the optical construction of telecentric lenses is relatively long and requires sufficient space. In each case, precise orientation is required between the lens, which is being tested, and the multi-axis illumination. If this does not occur, there can be no significant problems due to parallel projection.