Stray light in optical systems

Stray light is a general term for all abnormal transmission light in optical systems, all optical systems are affected by stray light to a certain extent, understanding the source of stray light is a necessary measure to design optical systems, according to the stray light analysis results, take corresponding measures to minimize its impact and optimize performance.

Stray light refers to the light that reaches the focal plane of an optical system after passing through the system that is not required by the design, and it is a general term for all non-normal transmitted light in the optical system. All optical systems are affected to some extent by stray light, and the impact on the performance of the optical system varies depending on the system.

For example,

When observing objects with a telescope, stray light can cause unnecessary reflections and create ghost images.

For radiation measurement satellites or other precise space optical systems with high sensitivity requirements, stray light has a significant impact on the system's accuracy.

In devices such as digital cameras, camcorders, and scientific instruments, stray light can interfere with the performance of optical sensors, leading to inaccurate readings or distorted images.

When infrared cameras are in operation, stray light from the system's own infrared radiation can affect the system.

Stray light in spectrometers and other multi spectral measurement devices can cause cross-talk between spectra.

The sources of stray light in optical systems can be divided into the following categories:

1. External light sources: Stray light from external light sources outside the optical system, which may include ambient light, sunlight, artificial light sources, or light leaking through system apertures or holes. These external light sources enter the optical system, causing glare, contrast reduction, and affecting imaging quality.

   

   The effect of external light sources

2. Scatter light: Due to defects in optical components, surface roughness, or other factors causing light to scatter instead of following the expected path, internal scatter light may be generated within the optical system. Scatter light can cause blocking glare, reduce contrast, and signal-to-noise ratio.
3. Ghost images: Ghost images are caused by internal reflections within the optical system, where light bounces off optical surfaces multiple times before reaching the detector. These reflections can produce faint duplicate images of the main scene, reducing overall image quality.

   

   Optical system ghost image generation schematic

4. Stray radiation: Stray radiation refers to unwanted electromagnetic radiation (such as infrared or ultraviolet light) that can enter the optical system, interfering with the detector or image formation process. This may result in inaccurate color reproduction, reduced image quality, or other adverse effects.
5. Stray reflection and refraction: Unexpected reflections and refractions from stray objects, edges, or surfaces in the optical path can introduce stray light into the system, producing unnecessary glare, flares, or other artifacts that degrade image quality.
6. Radiation from system components themselves: For example, infrared optical systems themselves can generate infrared radiation, or optical structures with driving systems or other electronic systems can produce thermal radiation when operating for prolonged periods, causing noise on the focal plane due to severe stray light from internal components.

How to reduce?

In modern optical design, stray light analysis has become an important part of optical design work. Understanding the sources of stray light is necessary for designing optical systems to minimize its impact and optimize performance. Based on the results of stray light analysis, appropriate baffles (as shown in Figure 1) can be used in the optical system, and an effective aperture structure can be set up internally to suppress stray light;

Fig1  Optical system with baffle and sunshield

Improving the transmittance of optical elements surface (or reflectivity of mirrors) and black coating the edges (as shown in Figure 2); adding light-absorbing threads on the lens barrels and applying light-absorbing paint (as shown in Figure 3); strategies such as temperature control for infrared optical systems can help reduce the impact of stray light and improve overall image quality.