Efficient discrimination
of active lighting in machine vision
Laser illumination is commonly used in machine vision applications, e.g. in obstacle detection or dimensional measurements. For safety and economy reasons, we would usually like to utilize diode lasers whose powers are limited down to a few milliwatts. Compared to much higher powers of ambient illumination typically found in industrial environments, laser lighting may be obscured. Typically, the remedy has been to introduce a special transmissive filter that has a passband at the laser wavelength. Interference filters are especially effective, because they may have very narrow passbands. However, they typically have high attenuation, close to 50% in the passband, and are directionally very sensitive. A new optical filter arrangement is introduced here. Instead of a transmissive filter, a double reflective filter arrangement is presented, in a Maksutov - type mirror telescope design. The following figure shows the relative spectral responsivity of a line scan camera (dot dash) and the combined reflectance of the dielectric mirror - gold mirror telescope lens (continuous line). The laser wavelength 680nm is shown with a dotted line. Note that both high reflectance of the telescope mirrors and high sensitivity of the line camera detector are achieved at the laser wavelength. Excellent attenuation of spurious light is achieved at IR where the detector has high sensitivity and typical light sources maintain high intensity, while moderate attenuation is achieved at wavelengths below 600nm where the detector is less sensitive and typical light sources less intense.
An additional advantage follows from the compact size of such a mirror telescope. Figure 2 shows a size comparison between our 60mm 1:2.5 mirror telescope and a commercial 50mm 1:1.4 multielement CCTV lens. Note that the diameters are irrelevant due to the speed difference, but the length ratio is 1:1.65 in favor of the mirrored design (length 34mm), measured from the first lens surface to the focal plane. The mirror design was successfully applied to a laser rangefinder where previously a conventional design was used. Gold coating was prepared by sputtering, while the dielectric mirror was a commercial component.
Contact information: VTT AUTOMATION
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