Introduction: Intro to stray light testing and normalized stray light | Outputs from Imatest stray light analysis | History Background: Examples of stray light | Root Causes | Test overview | Test factors | Test Considerations | Glossary Calculations: Metric image | Normalization methods | Light source mask methods | Summary Metrics | Analysis Channels | Saturation Instructions: High-level Imatest analysis instructions (Master and IT) | Computing normalized stray light with Imatest | Motorized Gimbal instructions Settings: Settings list and INI keys/values | Standards and Recommendations | Configuration file inputStray light (flare) documentation pages
Page Contents
This page provides an overview of the summary metrics
- Overview
- Level-Based Metrics
- Count-Based Metrics
- Minimum Measurable Stray Light
- Maximum Measurable Stray Light
Overview
Similar to how MTF50 (SFR50) and MTF area provide a single number for resolution, stray light also has single number measurements for characterizing stray light performance. These summary metrics fall into two categories:
- Level-Based: Metrics that are about the amount (level) of the stray light
- Count-Based: Metrics about the number of pixels (count) that meet a criteria
Level-Based Metrics
Overview
Level-based summary metrics describe the amount (level) of stray light in the image. They include standard summary statistics (e.g., mean, min, max, etc.).
Level-based summary metrics are output in both the output files and in line plots.
Named Metrics
The following named stray light level-based metrics are available:
| Level Metric | Description | Notes |
|---|---|---|
| Minimum | The minimum stray light value within the subset. | |
| Minimum (Non-Zero) | The minimum stray light value within the subset of the non-zero data in the original image. |
|
| Maximum | The maximum stray light value within the subset. |
|
| Mean | The mean stray light value within the subset. |
|
| Median | The median stray light value within the subset. | |
| RMS | The root-mean-square of the stray light values within the subset. | |
| Standard Deviation | The standard deviation of the stray light values within the subset. |
Percentiles
The percentile(s) of the metric image can be computed by entering the desired ranks (e.g., the stray light at the 90th, 95th, and 97.5th percentiles).
Units
Level-based metrics have the same “units” as the metric image from which they are derived.
Count-Based Metrics
Overview
Count-based summary metrics describe the number or percent of pixels that satisfy a criterion.
Count-based summary metrics are output in both the output files and in line plots.
Named Metrics
The following named stray light count-based metrics are available:
| Level Metric | Description | Notes |
|---|---|---|
| Total | The total number of pixels within the subset. | |
| Source | The number of masked-out source pixels within the subset | |
| Saturation | The number of pixels at or above the calculated saturation level | The saturation value is calculated per analysis frame. The image max-based methods are susceptible to errors, particularly when the source is out of the FOV. |
Thresholds
The counts of stray light that are worse than user-provided thresholds can be computed.
For transmission-style calculations, the counts are the number of pixels that satisfy
\(\text{Stray Light Metric Image} \ge \text{threshold}\)
For attenuation-style calculations, the counts are the number of pixels that satisfy
\(\text{Stray Light Metric Image} \le \text{threshold}\)
Note: in all cases, the threshold is inclusive.
Note: the range of stray light will depend on the calculation and the data transforms. Different thresholds may be required if either of these are changed.
Units
There are two options for the units of count-based metrics:
- Count: number of pixels that match the criteria.
- Percent: percentage (0-100) of the pixels in the image that match the criteria.
Overall Metrics
Saturation
Saturation is calculated per analysis frame and for any normalization reference image. The overall saturation is the maximum of all of the aforementioned saturation estimates.
Units
Saturation is measured in digital numbers.
Minimum Measurable Stray Light
Overview
The minimum measurable stray light is the stray light that corresponds to one digital number larger than the dark noise. It represents the smallest stray light value that the test configuration is capable of measuring.
It is useful when comparing results between two different test setups. This value is influenced by:
- The dark noise of the camera
- The exposure settings of the camera
- The normalization factor (for image-based normalization factors, this includes the “brightness” of the source in the reference image)
Adjusting any of the above may change the stray light dynamic range the setup can measure and therefore change the minimum measurable stray light.
The minimum measurable stray light is output in the results files.
Units
The minimum measurable stray light has the same “units” as the metric image from which they are derived.
Maximum Measurable Stray Light
Overview
The maximum measurable stray light is the stray light that corresponds to one digital number less than the saturation. It represents the worst stray light value that the test configuration is capable of measuring.
It is useful when comparing results between two different test setups. This value is influenced by:
- The dark noise of the camera
- The exposure settings of the camera
- The normalization factor (for image-based normalization factors, this includes the “brightness” of the source in the reference image)
- The bit depth of the camera
Adjusting any of the above may change the stray light dynamic range the setup can measure and therefore change the maximum measurable stray light.
The maximum measurable stray light is output in the results files.
Units
The maximum measurable stray light has the same “units” as the metric image from which they are derived.