Telecentric lenses optimized for white LEDs


Because of the dispersion of white light which consists of all wavelengths in the visible range lenses for such color applications must be specially adapted to this broad spectral range.

Due to the extreme increase in the performance of color sensors with high pixel densities, they are getting a standard in modern industrial machine vision setups. Here, a Bayer pattern divides the incident white light into three different color channels and thus obtains color information. Because of the dispersion of white light which consists of all wavelengths in the visible range lenses for such color applications must be specially adapted to this broad spectral range. In addition, the lenses must be broadband coated to achieve maximum transmission. Sill Optics covers this market with its "Color" series.

Sill Optics Color-Series for Maximum Transmission

In the past machine vision mostly used black and white sensors due to their better resolution. Since in color sensors 4 interconnected pixels determine position and color value the pixel size is 4 times larger than in black-and-white sensors with the same technical specification. The upstream Bayer pattern works like a filter that allows only one color per pixel (red, green or blue) to pass. So only one wavelength per pixel can pass through the Bayer pattern, where the intensity of one color gets measured. The color information of two green, one blue and one red channel is then calculated with each other, resulting in a mixed color.

Technical progress in recent years has made it possible to greatly reduce the size of pixels resulting in small and at the same time very powerful sensors with high pixel densities. As a result, color sensors increasingly replaced black-and-white sensors. The resulting loss of resolution was compensated for by the additional color information.

Telecentric Lenses for Color Machine Vision

Today many systems for process monitoring consist of a white light source that is as homogeneous as possible and illuminates the object either from behind, through the lens or from the side, a telecentric color-corrected imaging lens and a camera with Bayer pattern and color sensor. These setups can also use the full imaging capacity of the sensor and make black-and-white images without the Bayer pattern. In this case the object should ideally be illuminated with a non-white light source (only one wavelength) to achieve maximum resolution with the lens as well.

The resolution of the imaging lens should be at least as good as that of the sensor in the entire spectral range of the light source in order not to reduce the imaging quality of the complete system. The resolution of the whole system is maximum as good as that of the weakest link - the part with the poorest resolution.

The dispersion of glasses can be demonstrated by an experiment with a prism under a white light source. Here the dependence of the material refractive index on the wavelength is shown by the rainbow-like imaging of the spectral colors.

Lenses for Color Applications

Since white light consists of all wavelengths in the visible range (approx. 450 nm to 700 nm) it is important that lenses for color applications are color corrected as well as possible in this spectral range. Otherwise, color shadows will cause blurring. A clever combination of low-refractive crown and high-refractive flint glasses is used to achieve the most achromatic behavior possible for the lens. Longitudinal chromatic aberrations, which lead to a focus shift along the optical axis, and lateral chromatic aberrations, which lead to a shift perpendicular to the optical axis, are eliminated in the optical design.

In addition to a high-quality optical design a low‑absorption broadband coating is also important to ensure the highest possible proportion of transmitted light. All lenses are therefore coated with a complicated layer system of high- and low-refractive-index materials reducing the reflected part from about 4% to about 0.5% when passing from air to glass (or vice versa). For a 5-lens objective this increases the total transmission from 66.5% [= (100% - 4%)10] to a value of 95.1% [= (100% ‑ 0.5%)10].

Applications with White Light Sources

In order to cover the market for color applications with white light sources as well, Sill Optics has subjected its most successful series of telecentric objectives to color correction. The technical specifications were maintained and a largely achromatic behavior with a minimum of color errors in the visible range was achieved. The "Color" version enables high-precision measurement tasks with high-resolution color images. Furthermore, wavelengths in the near-infrared range are covered as well with a slight offset of the working distance.  The series includes our slightly smaller objectives from the original T85 series as well as selected objectives from the T120 series for larger measurement objects with diameters up to 123 mm. Sill Optics color lenses are suitable for all sensor sizes in the range of 0.5" to 1.1" and feature a C Mount connection for attaching the camera as well as a variably adjustable aperture. 

Sill Optics is happy to design a custom lenses specified to your needs. On request key data such as the connection can be modified at any time. If you need a different magnification, wavelength range or free aperture, please contact Sill Optics.