Technical guide: Telecentric lenses with coaxial illumination


General explanations

A large selection of existing telecentric lenses is available with an integrated coaxial illumination.

A light source is coupled into the beam path by a beam splitter and is collimated by the front lens group of the telecentric lens. That results in a collimated front lighting. The illuminated area equals the clear aperture of the front lens diameter for the defined working distance.

The advantages of collimated front illumination towards a diffuse light source are a better detection of surface textures and a homogeneous lighting of structures with certain depth.

The advantages of integration of coaxial illumination into the imaging lens are less installation space and lower costs.

Light source type

We offer two standard versions regarding the light source type:

Lenses with extension /LED include a red high power LED (623 nm, 2.5 V, 350 mA) that is connected with open cable ends (cable length 300 mm). LED can be controlled directly in continuous and flashed mode.

Lenses with extension /CCS provide a port for Ø 8 mm fiber- or spot connection (e.g. CCS spot). In this case there is no light source included.

Of course, it is possible to request other wavelengths and customized interfaces.

Illumination coupling type

The type of beam coupling strongly influences the illumination quality and the illuminance.

It is necessary to distinguish polarized from non polarized beam splitting.

The benefit of a non polarized beam coupling is a relatively small luminosity for sufficient lighting of reflecting objects. The incident angle of the lighting beam path to the splitter surface and the wavelength dependence are less critical than for the use of polarized beam splitters.

A polarized beam coupling eliminates the central back reflection from the front lens, which occurs within using a non polarized beam splitter. Especially for applications with high necessary light power, the back reflection of non polarized beam splitters will create a spotlight in the center of the image.

On the other hand, the intensity distribution becomes uneven for a high incident angle for reflective targets with polarized beam splitters. Especially for a large front lens diameter the illumination becomes inhomogeneous.

Use of retardation plates for polarized beam splitting

Retardation plates, which rotate the linear polarization plane (half wave) or rather change the type of polarization (quarter wave) offer a way to compensate the problems of a polarized beam splitter and reflective target. The results are an increasing light intensity and a higher illumination homogeneity.

To minimize the costs for a retardation plate for large object field diameters, the installation of the plate has been achieved for many lenses by a lateral slot in the lens body. For lenses with small object field diameters (Ø ≤ 30 mm), the retardation plate can be installed as a header.

From experience, the choice of the retardation plate for reaching a high lighting homogeneity does not depend on the exact wavelength.
The influence of wavelength and angle dependence of the polarized beam splitters is much higher. A polarized beam coupling is not recommended for applications with wideband lighting (e.g. white light source) and high illumination homogeneity requirements.

For the choice of the optimum beam introduction, it is necessary to consider if the polarization information contains important information for the measuring purpose (i.e. testing the mechanical stress inside an element). Therefore it is important to avoid polarization influence through the imaging lens.

Standard version

Each lens listed in the catalogue contains a polarized beam splitter and without any retardation plate. This is the ideal setting for lighting diffuse targets.

Non polarizing beam splitter, which can be exchanged by the user, and a suitable retardation plate are offered as an accessory.

Decision guide

Recommendation: Each possible case of the modular setup should be checked for critical and new projects. The parameters of the measurement setup (requested resolution, illuminance, object surface condition, necessary illumination homogeneity) have an important influence on the best configuration.