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Optics Edge Blackening


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You may or may not be aware that the  edges on some lenses are blackened. Edge-blackened optics are designed to increase contrast and reduce noise in imaging and electro-optical systems. Blackening the lens edges prevents stray light from reflecting back into the optical path. This increases overall image quality. Manufacturers typically blacken the outer edges of the objective lens and eyepiece, since they get subjected to the largest amount of stray light.

edge black reduce scatter

Stray Light
When integrating lenses into a multi-element system, a common limiting factor in overall performance is stray light. Stray light is energy outside of the clear aperture of an optical system that scatters off of the edges of an optical or mechanical component (Figure 1) and reaches the sensor in the form of noise (rather than signal). One successful method commonly used to reduce stray light within a system involves blackening the edges of the optics in the system.

Measuring BRDF
Scattered signals are typically quantified as scattered light power per unit solid angle, or inverse steradians. Scatter results are made more meaningful by normalizing the results by the intensity of light incident on the scatter source. This function is commonly referred to as the BRDF (Bi-Directional Reflectance Distribution Function). A simplified equation of BRDF is the intensity ratio of reflected to incident light per solid angle. These measurements are taken by projecting a laser onto a surface at a fixed angle from the normal. A detector is then rotated about the point of scatter to record the change in irradiance as a function of angle.

Such a test between one surface coated with edge-blackening ink, and the other uncoated ground glass to simulate the edge of an optic. The plot indicates an order of magnitude improvement in stray light reduction by edge-blackening. This improvement potentially save the need for expensive mechanical system baffling. With the detector close to -90° from normal, the percent of incident light recorded is near zero due to the high angle of incidence. At -5° the percent of incident light recorded is also 0%, due to the detector eclipsing the source. At +5° there is a spike due to the specular reflection. The percent of incident light recorded then approaches zero as the angle of the detector again approaches 90° to the normal.