Stray Light (Flare)

Stray Light Normalization

Background Testing Calculation Testing With Imatest Inputs Outputs

 


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Normalization Methods

None

This is the simplest of the normalization methods (don’t normalize). It has two main benefits:

  • Easy to get started (no extra info is needed).
  • Stray light metric will be in DN units (similar intuition to noise levels can be used).

Inputs

  • None

Assumptions

  • The camera response is linear

Normalization Factor

The normalization factor for this method is always 1.

Metric Range

For a normal calculation, stray light will range from 0 (best possible measurement) to the maximum digital number of the image (worst possible measurement).

 

On-Axis Reference Image

The on-axis reference image

  • Is used for computing the point source rejection ratio (PSRR) metric [1].

Inputs

  • On-axis reference image
  • Source mask of the on-axis reference image (or settings to compute source mask)
  • (Optional) Source attenuation type
  • (Optional) Source attenuation value

Assumptions

  • The camera response is linear
  • The image of the source is not saturated in the on-axis reference image

Normalization Factor

  1. Capture an image of the source on-axis.
    1. Note that the image of the source should not be saturated.
    2. If it is, adjust the level of the source (directly or with ND filters) and/or the exposure of the camera until the image of the source is not saturated.
    3. Record the parameters used to generate the reference image.
  2. Create (or apply) the mask of the source to identify the pixels in the image that correspond to the light source.
  3. Take the mean of the source pixels.
  4. The normalization factor is the mean of these source pixels transformed back to an equivalent to the “test images”.

Metric Range

For a normal calculation, stray light will range from 0 (best possible measurement) to 1 (worst possible measurement). Note that due to numerical and algorithmic issues, values slightly higher than 1 are possible.

References

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