Together with the cameras and lighting, lenses are the most important components in image processing. Many tasks cannot be performed, or can only be inadequately performed without the right lens. A good lens and good image quality are the prerequisites for successful evaluation of the images.

In industrial image processing, a distinction is made between three main lens groups: endocentric lenses, telecentric lenses and hyper, or pericentric lenses. Essentially, these lenses differ in their perspective and the ray path which forms the image. Endocentric lenses are the most common type of lens. Just as in the human eye, the light rays diverge relative to the lens. This type of lens includes zoom lenses to cover various focal lengths, as well as lenses with a fixed focal length. Telecentric lenses are used primarily for measurement tasks. The rays from the object are parallel to the optical axis, so that there is no perspective distortion in the image and distances can be measured in the image without error. With hypercentric or pericentric lenses, the rays converge relative to the lens (as opposed to endocentric lenses) and therefore enable a 360° view of the outer surface of a cylinder.

The right lens

Selection of the right lens for an application is not easy. With regard to this there are several helpful facts which should be known, for example in association with endocentric, fixed focal length lenses. The object which is to be imaged, the structural geometry as well as the camera - which has usually already been selected - make certain demands on the lens. The size of the object and its structural geometry determine the required field of view and the necessary working distance of the lens. Most manufacturers state the field of view in relation to the size of the sensor in the form of a distance or an angle of vision. By means of the angle of vision, the field of view can be calculated for various working distances. The working distance is the distance between the object and the front surface of the lens. Most lenses can be focussed from a defined minimum working distance up to "infinity". The minimum working distance can be reduced with the aid of extension rings. However this impairs the quality of the image. In order to depict small structures the lens, as well as the camera, must also be able to resolve these structures. For lenses, the resolution is normally stated in the form of an MTF curve, which is stated in relation to the working distance and the aperture. According to the evaluation software, a contrast of 10% is considered as resolved. If a certain depth is to be sharply depicted in the image, the lens must have a certain depth of field. As sharpness is a very subjective term, the details of depth of field a value is often defined (e.g. a certain resolution) which is still considered to be sharp and which limits the depth of field. The depth of field depends on the magnification of the lens, the working distance and the aperture which is set. The larger the aperture number, the larger the area which is sharp - however more light is also required. With small aperture numbers, the depth of field is smaller, however more light passes through the lens to the sensor and the exposure time is shorter.

Camera and wavelength

With an existing camera, care must be taken that the diameter of the image circle of the lens covers the camera sensor, otherwise vignetting will occur. Also, the thread of the lens must match that of the camera. In industrial image processing the most common thread is still the C-mount, even though larger lens mounts are now available for larger sensors. Finally, the wavelength range for which the lens is to be used must be considered. Most lenses are calculated for the visible range from approx. 400 to 700 nm and also have an anti-reflection coating for this range. If the UV or the IR range is to be used, lenses which are specially calculated and coated for this range are required, e.g. VIS-NIR lenses (up to approx. 1,000 nm) or SWIR lenses (up to approx. 1,700 nm).
In industrial image processing, fixed focal length endocentric lenses are usually used for inspection tasks, in which an general view of a particular object is required. Detailed images can be obtained with longer focal lengths or lenses with a higher resolution. Many lenses are of small or very small size and low weight, so that they can be easily used in production facilities or structures where only limited space is available. In the case of lenses which are used in the NIR range, stray (visible) ambient light can be suppressed with a filter and the object illuminated in the near infra-red range. Among other things, SWIR lenses are used to check that bank notes are genuine, or to check the filling level of opaque plastic bottles. These lenses are able to look through materials such as plastic or silicon, which are not transparent with visible light. The selection of the right lens is certainly not easy and takes a little time. However, if this time is invested in the selection of the lens, time and money can be saved in the evaluation of the image.

A wide selection

Edmund Optics supplies compact lenses with fixed focal lengths for use with visible light. Further developments of this basic series are the compact VIS-NIR lenses for a wavelength range from 400 to 1,000 nm as well as high-resolution lenses, which achieve an image resolution of 145 line pairs/mm with current 5MP -2/3" sensors. In addition, a simplified version of the basic series is available with a smaller and lighter housing and a fixed aperture - the compact fixed aperture lenses. The range is supplemented with two new developments: SWIR fixed focal length lenses, which are specially designed for the SWIR range from 900 to 1,700 nm, and hypercentric lenses which enable a 360° view of the outer surface of a cylinder or the inner surface of small tubes. Cameras, filters, tripods, lighting systems as well as test images are available as accessories.