# Stray Light Calculations

 Background Testing Calculation Testing With Imatest Inputs Outputs

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At a high level, the measurement of stray light is very simple: normalize the digital numbers (DNs) of an image, while (optionally) masking out the image of the light source.

## Metric Image Calculations

### Normal

The “normal” stray light metric image calculation is to take the test image and divide it by a normalization factor.

$$\text{stray light} = \frac{\text{image [DN]}}{\text{normalization factor [DN]}}$$

With appropriate normalization factors, this is used to compute the Point Source Transmission (PST) [1] and Point Source Rejection Ratio (PSRR) [2] metrics.

For a “normal” calculation, no stray light is indicated with a value of 0 and the worst possible stray light is indicated with a value of 1.

### Reciprocal

The “reciprocal” stray light metric image calculation is to take a normalization factor and divide it by the test image.

$$\text{stray light} = \frac{\text{normalization factor [DN]}}{\text{image [DN]}}$$

With appropriate normalization factors, this is used to compute the flare attenuation metric proposed within IEEE-P2020 [3].

For a “reciprocal” calculation, no stray light is indicated with a value of infinity and the worst possible stray light is indicated with a value of 1.

# References

[1] E. Fest. 2013. “Stray Light Analysis and Control”. SPIE Press. ISBN: 9780819493255. DOI: https://doi.org/10.1117/3.1000980.

[2] B. Bouce, et. al, 1974. “GUERAP II – USER’S GUIDE”. Perkin-Elmer Corporation. AD-784 874.

[3] IEEE-P2020 Automotive Image Quality Working Group. https://site.ieee.org/sagroups-2020/