Photography - Digital cameras - Image flare measurement
1Key Takeaways
This document specifies the definition of image flare for digital cameras, test patterns, measurement conditions, and methods, so as to enable the comparison of the results of measurement. The methods of measurement are designed to enable the assessment of the performance of digital cameras by using image data output b…
2Expert Interpretation
This in-depth analysis of the ISO 18844:2017 international standard for measuring glare in digital camera images covers test chart specifications, measurement condition settings, a comparison of three measurement types, and implementation recommendations. It is suitable for camera manufacturers and optical inspection professionals.
Analysis of the core content of the standard
ISO 18844:2017 established for the first time a standardized measurement method for the overall glare of the image sensor system of a digital camera, focusing on solving the following problems:
- Clearly defines Image flare as the signal enhancement caused by non-target light sources in the output image of a digital camera
- Standardizes the reflective/transmissive type, size ratio (exceeding 41±2% of the field of view) and contrast requirements (≥40:1 or ≥3000:1) of the test chart
- Develops three measurement processes (Type A/B/C) to adapt to different camera functions
Key technical requirements
| Elements | Specific requirements | Technical Principle |
|---|---|---|
| Test Environment | Reflective Chart Illumination 1000-2000lx Transmitted Light Source Brightness 318-637cd/m² Unevenness <5% | Simulated daylight source (CCT 5700K±1000K) to avoid ambient light interference |
| Chart Design | Window/Dot Matrix Patterns Black Area ≤5% White Area Reflectivity ≥80% | Glare Calculation by Comparing the Signal Difference Between the Central Black Area and the Peripheral White Area |
| Signal Processing | 8-bit RGB to Luminance Signal: Y′luma=0.299R′+0.587G′+0.114B′ | Complies with the sRGB conversion standard of IEC 61966-2-1 |
Measurement Method Comparison
Type A (Precision Measurement)
Applicable to manual exposure cameras, achieving 0.005% high-precision measurement through 8x exposure compensation:
- First capture a white image to adjust Y′luma=225±5
- At 8x exposure, capture black and white images separately to obtain YB3 and YB2
- Calculate using the formula F=(YB3/H2-YB2/H2)/(YW1/H1)×100%
Type C (Fast Measurement)
Requires an ultra-high contrast chart (≥3000:1). Calculate directly from a single shot: F=YB1/YW1×100%
Implementation Recommendations
- Equipment Selection: It is recommended to use an integrating sphere light source to ensure uniform lighting, and a black velvet background to eliminate stray light
- Error Control: Dust on the lens will significantly increase the measured value. It is recommended to operate in a cleanroom environment.
- Data Report: Must include 12 parameters, including focal length, aperture value, and ISO setting (see Chapter 9 of the standard for details).
Background of the Standard's Evolution
Compared to ISO 9358, which only measures lens stray light, this standard innovatively incorporates the impact of the image processor algorithm into the evaluation system. Major improvements to the 2017 version include:
- Added RAW format data processing specifications
- Clarified multi-zone measurement methods
- Supplemented the Type B measurement process for auto-exposure cameras