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Stray light (flare) documentation pages

Introduction: Intro to stray light testing and normalized stray lightOutputs from Imatest stray light analysis | History

Background: Examples of stray lightRoot Causes | Test overview | Test factors | Test ConsiderationsGlossary

Calculations: Metric image | Normalization methodsLight 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 input

Page Contents

This page provides an overview of the summary metrics

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 MetricDescriptionNotes
MinimumThe 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.
  • This value is useful for scaling plots or metric images when using dB units.
MaximumThe maximum stray light value within the subset.
  • Maximum (worst) flare (stray light) is used as a summary metric in IEEE-P2020.
MeanThe mean stray light value within the subset.
  • Mean flare (stray light) is used as a summary metric in IEEE-P2020.
MedianThe median stray light value within the subset.
RMSThe root-mean-square of the stray light values within the subset.
Standard DeviationThe 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 MetricDescriptionNotes
TotalThe total number of pixels within the subset.
SourceThe number of masked-out source pixels within the subset
SaturationThe number of pixels at or above the calculated saturation levelThe 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.