Page Contents

This page provides a description of the fundamental calculations that Imatest uses to generate stray light metric images.

Metric Image Calculations

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. At a deeper level, it is a four-step process of:

(Optionally) Subtracting the dark level
Clipping the image
Applying the normalization factor
- Transmission
- Attenuation
(Optionally) Applying a data transformation
(Optionally) Downsample the image

Dark-Level Subtraction

(Optionally) subtract off the user-provided dark level.

\(\text{image [DN]} = \text{image [DN]}-\text{dark level [DN]}\)

Clipping

Clipping helps address three cases:

The possibility of negative values from dark-level subtraction
Attenuation normalization transforming 0 in the image into infinity
Logarithmic data transforms (undefined at 0)

Any image values less than the user-provided dark level are clipped to

\(\text{clipped image [DN]} = \text{max}\left(\text{image [DN]}, \text{clipping value[DN]}\right)\)

Where the clipping value should be:

1: attenuation calculation
1: logarithmic data transform
0: otherwise

Normalization Factor Application

Transmission

The transmission stray light metric image calculation takes the test image and divides it by a normalization factor.

\(\text{stray light metric image} = \frac{\text{clipped image [DN]}}{\text{normalization factor [DN]}} \)

With appropriate normalization factors, this is used to compute:

Point Source Transmission (PST) [1]
Point Source Rejection Ratio (PSRR) [2]
IEEE Std 2020-2024 [3] Normalized Flare
IEEE Std 2020-2024 [3] Illuminance/Irradiance Flare

For a “transmission” calculation, no stray light is indicated with a value of zero, and the worst possible stray light is indicated with a value of one.

Attenuation

The attenuation stray light metric image calculation takes a normalization factor and divides it by the test image.

\(\text{stray light metric image} = \frac{\text{normalization factor [DN]}}{\text{clipped image [DN]}}\)

With appropriate normalization factors, this is used to compute the flare attenuation metric proposed within IEEE-P2020 pre-release [4].

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

Note: zero is a valid image value (corresponding to no measurable stray light from the test configuration). When using the attenuation calculation, these zeros get transformed to infinity, which in turn will reduce the meaningfulness of some summary metrics (e.g., mean, max). To compensate for this, it is recommended to clip the image values to one.

Data Transforms

There are many data transforms that can be applied to produce the metric image.

Name	Transform
Linear	\(y=x\)
Log 10	\(y=\log_{10}(x)\)
dB Power	\(y=10\cdot\log_{10}(x)\)
dB Voltage	\(y=20\cdot\log_{10}(x)\)
Log 2	\(y=\log_{2}(x)\)
Natural Log	\(y=\ln(x)\)

Note: If using any of the logarithmic data transforms (any except linear), it is recommended to clip the image values to one.

Downsampling

The metric image may be downsampled. The current downsampling options are

Name	Description	Additional Inputs
Full Resolution	Use the full-resolution image	N/A
Scale Factor	Apply a downsampling scale factor	Scale Factor: the amount to downsample the image, e.g., 2 will do a 2×2 downsampling.
Output Image Size	Select the desired output image size	Desired Image Size (rows and columns)
Output Image Rows	Select the desired number of rows in the output image. The aspect ratio of the downsampled and original image will be kept approximately constant.	Desired Number of Rows
Output Image Columns	Select the desired number of columns in the output image. The aspect ratio of the downsampled and original image will be kept approximately constant.	Desired Number of Columns

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 Std 2020-2024. “IEEE Standard for Automotive System Image Quality”. DOI: 10.1109/IEEESTD.2024.10935839

[4] IEEE P2020/D3, Jun 2022 – IEEE Draft Standard for Automotive System Image Quality https://ieeexplore.ieee.org/document/9969530

Stray Light (Flare) Documentation

Stray Light Calculations

Stray light (flare) documentation pages