Stray light (flare) documentation pages

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 calculations | Normalization methods | Light source mask methods | Summary Metrics | Analysis Channels

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

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Page Contents

This page provides an overview of the summary metrics

• Overview
• Level-Based Metrics
• Count-Based Metrics
• Minimum 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:

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:

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.

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.