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
- (Optionally) Applying a data transformation
- (Optionally) Downsample the image
Dark-Level Subtraction
(Optionally) subtract off the user-provided dark level.
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
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.
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.
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 |
|
| Log 10 |
|
| dB Power |
|
| dB Voltage |
|
| Log 2 |
|
| Natural Log |
|
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 2x2 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