Documentation – Pilot v22.2

Stray Light (Flare) Analysis

Stray Light (Flare) AnalysisStray Light OutputsStray Light SettingsStray Light Configuration File

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Table of Contents

Intro to Stray Light (Flare)

What is stray light and how is it measured?

Stray light, also known as flare, is any light that reaches the detector (i.e., the image sensor) other than through the designed optical path (definition from IEEE-P2020 pre-release standard). Stray light can be thought of as systematic, scene-dependent optical noise. Depending on the mechanism causing the stray light, it can produce phantom objects within the scene (i.e., ghosts/ghosting), reduce contrast over portions of the image (e.g., vailing glare), and effectually reduce system dynamic range. These factors can adversely affect the application performance of the camera system in a variety of scenarios.

The stray light of a camera can be measured by capturing images of a bright light source positioned at different angles in (or outside of) the camera’s field of view and then processing those captured images into stray light metric images where each pixel in the image is remapped to represent a stray light metric. The light source object itself is excluded, or masked out in the images, as it is not considered to be stray light. Various information and summary statistics can be derived from the resulting stray light metric images.

The primary metric for stray light analysis in version 22.2 of Imatest is Point Source Rejection Ratio (PSRR). The PSRR metric utilizes an on-axis reference image of the light source (specifically, the level within the imaged light source object) to normalize the data of the images under test, providing PSRR stray light metric images as the primary output. PSRR has decades of heritage/use within the aerospace and defense industry as a metric for testing and characterizing the stray light of electro optical systems for airborne and space applications, including telescopes and satellite imagers.

Figure 1: A GIF of a series of color-mapped stray light metric image plots is one possible output from Imatest 22.2 Stray Light (Flare) analysis. Here, the plotted metric is Point Source Rejection Ratio (PSRR). Each pixel in the input image has been normalized to represent PSRR. The light source object in the image is masked out because it is not stray light. Various stray light artifacts can be seen at different light source positions/angles, including when outside the camera’s field of view. These stray light artifacts are quantified via the PSRR metric scaling of the data.

How to use Imatest to test for Stray Light 

Procedure for Batch/Auto Analysis in Imatest Master

  1. Starting with the “Select Images” step of the Workflow, select the image(s) to analyze for stray light

    1. To import the images, click “SELECT IMAGES” to hand-select a series of image files, or click “SELECT FOLDERS” to select a directory of images. Alternatively, select a config file as input or a directory containing a config file. 

      1. See the Stray Light Configuration File page for information on the config file, including the required JSON schema and formatting.

      2. Note: Currently, if selecting images as input, more than one image must be selected.

      3. Note: Users cannot select a mix of image files and a config files as input. 

      4. Note: If using a config file as input, certain meta information defined within will be associated with each of the images to analyze (e.g., position or angle of the light source). Analysis plots will be enabled as an optional output when using a config file. 
  2. Click “Next” to move on to the “Select Analysis” step of the Workflow and select the Stray Light analysis module (Analysis options > Stray Light / Flare > Stray Light)

    Starting in version 22.2, users of Imatest Master can find Stray Light analysis under the “Stray Light / Flare” section of the analysis options screen

  3. Configure the Stray Light analysis settings by clicking the gear icon.

    1. See the Stray Light Settings page for information on settings

  4. Click “ANALYZE” to run the analysis

    1. Wait while Imatest calculates results and generates output figures/files

  5. Examine the resulting figures and output files

    1. See the Stray Light Outputs page for information on the various outputs