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 | 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 input
Page Contents
This page enumerates the settings required to obtain metrics named in standards and Imatest’s recommendations.
- Recommendations
- Syntax
- Calculation Settings
- Unsupported Parts of Standards
- References
Recommendations
Imatest offers two recommendations:
- New user: stripped down settings to get initial results relatively quickly
- Advanced user: Imatest’s recommended settings for better comparison of results, which require extra measurements and settings. In some literature, this is Point Source Rejection Ratio (PSRR).
There are two measurements with IEEE Std 2020-2024 [1]:
- Flare intensity: See IEEE Std 2020-2024 [1] §2.8.1
- Normalized flare: See IEEE Std 2020-2024 [1] §2.8.3; similar to Imatest Recommended Advanced User
Syntax
Items in fixed-width font indicate settings that are determined from the test setup.
Calculation Settings
| Standard/Recommendation | Notes | ||||
|
Imatest Recommended New User |
Imatest Recommended Advanced User |
Flare Intensity ([1] §2.8.1) |
Normalized Flare ([1] §2.8.3) |
||
| Standard/Recommendation Notes | These settings are intended to help a new user get some results quickly. Results may not be comparable from sensor to sensor due to the lack of normalization. | These settings are what Imatest recommends for camera-to-camera comparable stray light flare measurement. | This is inspired by “Imatest Recommended Advanced User”. | ||
| Calculation Type | Transmission | Transmission | Transmission | Transmission | Note: the prerelease version of IEEE-2020 [2], used Attenuation for the flare attenuation metric. |
| Data Transform | Linear (None) | Linear (None) | dB (Voltage) | Linear (None) | |
| Dark Level | 0 | Set based on camera noise | 0 | 0 | IEEE Std 2020-2024 [1] does not specify subtracting off a dark level. |
| Clip To One | Disable | Disable | Enable | Disable | Clip To One performs IEEE Std 2020-2024 [1] eq 2-6. |
| Analysis Channel(s) | Automatic | Each Channel | Luminance | Luminance | IEEE Std 2020-2024 [1] §2.7.1 does not specify which luminance weights to use. IEEE Std 2020-2024 [1] §1.7.6 recommends using the weights of the color space, if given, otherwise to use ITU-R BT.709-6 (sRGB) weights. |
| Normalization Type | None | Direct Reference Image | Lambertian Reference Image | Direct Reference Image | Normalization types other than none require other settings. See the additional settings for Direct Reference Image and Lambertian Reference Image below. |
| Test Image Mask Type | None | Image Processing + Circle (Radius) |
Image Processing or Image Processing + Circle (Radius) |
Image Processing or Image Processing + Circle (Radius) |
IEEE Std 2020-2024 [1] provides three methods for creating the mask:
Test Image Masking requires additional settings for types other than None. See below. |
| Crop LRTB | 0, 0, 0, 0 | Set to exclude any non-imaging rows/columns | Set to exclude any non-imaging rows/columns | Set to exclude any non-imaging rows/columns | IEEE Std 2020-2024 [1] does not mention the cropping of non-imaging pixels, however, this is recommended. |
| Percentile Ranks | use default | Set values as desired | Set values as desired | Set values as desired | IEEE Std 2020-2024 [1] does not mention percentiles as summary metrics. |
| Level Count Thresholds | use default | Set values as desired | Set values as desired | Set values as desired | IEEE Std 2020-2024 [1] does not mention count-based summary metrics. |
| Level Count Calculation | % Of Pixels | % Of Pixels | % Of Pixels | % Of Pixels | IEEE Std 2020-2024 [1] does not mention count-based summary metrics. |
Normalization Settings
Direct Reference Image Normalization Settings
| Standard/Recommendation | Notes | ||
|
Imatest Recommended Advanced User |
Normalized Flare |
||
| Image Filename | Select the reference image | Select the reference image | |
| Normalization Aggregation | Median | Median | |
| Integration Time Ratio | Set based on the test setup | Set based on the test setup | |
| Gain Ratio | Set based on the test setup | Set based on the test setup | |
| Light Level Ratio | Set based on the test setup | Set based on the test setup | |
| ND Measurement Type | Set based on the test setup | Set based on the test setup | |
| ND Density Value | Set based on the test setup | Set based on the test setup | This setting is displayed in the GUI when the ND Measurement Type is Density. |
| ND Transmission Value | Set based on the test setup | Set based on the test setup | This setting is displayed in the GUI when the ND Measurement Type is Transmission. |
| Reference Image Mask Options | Image Processing | IEEE Std 2020-2024 [1] does not specify separate settings for reference and test images. Use the test image masking settings. | |
| Reference Image Level Threshold Type | Otsu’s Method | IEEE Std 2020-2024 [1] does not specify separate settings for reference and test images. Use the test image masking settings. | |
| Reference Image Mask Area Threshold | 1 | IEEE Std 2020-2024 [1] does not specify separate settings for reference and test images. Use the test image masking settings. | |
Lambertian Reference Image Normalization Settings
| Standard/Recommendation | Notes | |
|
Flare Intensity |
||
| Image Filename | Select the reference image | |
| Normalization Aggregation | Mean | IEEE Std 2020-2024 [1] specifies to use an average in the symbols of equation 2-5. |
| Integration Time Ratio | Set based on the test setup | |
| Gain Ratio | Set based on the test setup | |
| Light Measurement Type | Set based on the test setup | |
| Radiance From Lambertian Diffuser | Set based on the test setup | |
| Irradiance at the Camera Location | Set based on the test setup | |
| Luminance From Lambertian Diffuser | Set based on the test setup | |
| Illuminance at the Camera Location | Set based on the test setup | |
| Reference Image Mask Options | IEEE Std 2020-2024 [1] does not specify separate settings for reference and test images. Use the test image masking settings. | |
| Reference Image Level Threshold Type | IEEE Std 2020-2024 [1] does not specify separate settings for reference and test images. Use the test image masking settings. | |
| Reference Image Mask Area Threshold | IEEE Std 2020-2024 [1] does not specify separate settings for reference and test images. Use the test image masking settings. |
Test Image Mask Settings
| Standard/Recommendation | Notes | ||||
|
Imatest Recommended New User |
Imatest Recommended Advanced User |
Flare Intensity |
Normalized Flare |
||
| Radius Type | N/A |
User-Specified |
Focal Length Ratio |
Focal Length Ratio | |
| Mask Radius | N/A | Set via measurement of the on-axis image. |
N/A |
N/A | |
| Camera Pixel Pitch |
N/A | N/A | Set to the pixel pitch in μm of the device under test. | Set to the pixel pitch in μm of the device under test. | This is part of IEEE Std 2020-2024 [1] eq 2-4. |
| Camera Focal Length | N/A | N/A | Set to the focal length in mm of the device under test. | Set to the focal length in mm of the device under test. | This is part of IEEE Std 2020-2024 [1] eq 2-4. |
| Source Focal Length | N/A | N/A | Set to the focal length in mm of the light source. | Set to the focal length in mm of the light source. | This is part of IEEE Std 2020-2024 [1] eq 2-4. |
| Source Diameter | N/A | N/A | Set to the diameter of the light source in mm. | Set to the diameter of the light source in mm. | This is part of IEEE Std 2020-2024 [1] eq 2-4. |
| Level Threshold Type | N/A | Percent Of Max | Percent Of Max | Percent Of Max | |
| Level Threshold % | N/A | 95 | 95 | 95 | See IEEE Std 2020-2024 [1] §2.7.2.1. The 95% threshold is allowed to be tuned. |
| Minimum Source Level | N/A | Set to a value slightly less than the level of the direct image of the source in the image. | Set to a value slightly less than the level of the direct image of the source in the image. | Set to a value slightly less than the level of the direct image of the source in the image. | Although not explicitly part of IEEE Std 2020-2024 [1] §2.7.2.1, the first paragraph of that clause assumes the source’s image is in the FOV. Setting this value helps validate that assumption. |
| Image Close Radius |
N/A | 5 | Set as needed to fill in holes in the mask. | Set as needed to fill in holes in the mask. | In IEEE Std 2020-2024 [1] §2.7.2.1, it says to perform a closing, but does not specify a radius. |
| Mask Area Threshold | N/A | Set to a value slightly larger than the pixel area of the well-exposed source. | 0 | 0 |
This helps filter out saturated ghost images smaller than the direct image. IEEE Std 2020-2024 [1] §2.7.2.1 does not specify a mask area threshold. |
Unsupported Parts of Standards
IEEE Std 2020-2024 [1]
The analysis software does not support:
- Mask creation from a geometric calibration model (§2.7.2.2). Note that IEEE Std 2020-2024 [1] §2.7.2.2 only specifies a simple pinhole model, which is only valid for cameras with minimal distortion.
- Mask creation from well-exposed reference images (§2.7.2.3). Note that this requires two exposures for each capture position.
References
[1] IEEE Std 2020:2024 “IEEE Standard for Automotive System Image Quality” https://standards.ieee.org/ieee/2020/6765/
[2] IEEE P2020/D3 – “IEEE Draft Standard for Automotive System Image Quality” https://ieeexplore.ieee.org/document/9969530