STRAY LIGHT ANALYSIS (=FALSE LIGHT ANALYSIS)

When designing imaging and illumination optical systems one normally starts assuming ideal optical components. The design is then optimized with respect to the desired imaging or illumination system performance. In reality these idealized conditions are never met. There will be always a certain fraction of straylight (unwanted light) compromising the performance of the optical system:

• Reflection from glas or transparent plastic surfaces or from the image sensor; double reflection from optical surfaces leads to so-called "ghosts" in the image plane
  
• Light is scattered from rough or contaminated optical components and mechanical parts (fittings, housing, apertures etc.)
  
• Diffraction from aperture stops
  
• In the infrared, we also encounter thermal radiation from system components in or near the optical path

Stray light deteriorates the system performance through reduced image quality, higher noise/measurement background, damage of components, glare, etc. The straylight performance often decides whether an optical system is operational or not. In any case, it is a criterion for the quality of an optical instrument.

Computer-based straylight analysis allows for the diagnosis of the straylight performance, before the system is fabricated, and gives valuable hints how to avoid straylight, e.g., by using suitable baffles. The earlier a straylight analysis is performed during the optical design process, the lower is the risk for a deficient design.

A thorough analysis of stray light is the prerequisite for desiging counter measures. In general, there three three approaches to stray light reduction:

1. Re-direct stray light such that it cannot reach the sensor plane anymore— for instance by use of reflecting surfaces like in the Stavroudis baffle shown here.
2. Block stray light such that it cannot enter the optical system. This is achieved by baffle structures.
3. Absorb straylight using black coatings.