Overview
ISO 9358:1994 - "Optics and optical instruments - Veiling glare of image‑forming systems - Definitions and methods of measurement" defines how to quantify and measure veiling glare in optical and electro‑optical imaging systems. The standard adopts two complementary measures: the Veiling Glare Index (VGI) and the Glare Spread Function (GSF), provides laboratory measurement techniques (general instrumentation configurations and recommendations), and gives practical guidance for producing accurate, repeatable results-chiefly in the visual spectral range.
Key topics and technical requirements
- Definition of veiling glare: unwanted irradiation in the image plane caused by stray radiation that entered the system via the entrance pupil or input face.
- Two measurement approaches:
- Integral (black‑patch) method - VGI: ratio of irradiance at the centre of a black patch image to the irradiance of the surrounding bright field. VGI is expressed as a percentage.
- Analytical method - GSF / GSFR: spatial distribution of veiling irradiance in the image plane normalized to unit source flux (GSF in m⁻²). The related GSFR (radiant intensity form) is used for afocal systems (units sr⁻¹).
- Measurement configurations: recommended laboratory setups include integrating sphere methods, two‑hemisphere arrangements, integrating‑sphere + collimator, and irradiated rectangular box cavities. Specifications and performance recommendations are given for detectors, apertures and auxiliary optics.
- Specimen classification: test methods depend on specimen class-object at infinity, finite object area, or inaccessible image plane-and the standard specifies suitable adaptations for each class.
- Scope and limitations: techniques are primarily for the visual spectral range; adjustments may be needed for other wavelengths. Definitions and radiometric terms refer to established ISO vocabularies.
Practical applications and users
Who uses ISO 9358:
- Optical designers and lens manufacturers validating stray‑light and contrast performance
- Camera, projector, and telescope manufacturers assessing image contrast degradation
- Electro‑optical system developers (night‑vision, image intensifiers, star sensors) where isolated bright sources or internal scatter are critical
- Test laboratories, metrology facilities and quality control teams performing standardized veiling glare measurements
Why it matters:
- VGI is practical for scenes with broadly uniform radiance (e.g., diffuse daylight).
- GSF is essential when isolated bright sources dominate the scene (e.g., star sensors or sun‑near‑field scenarios) and enables prediction of VGI by convolution with scene radiance.
- Standardized measurement enables objective specification, design optimization and comparative testing.
Related standards
- References radiometric/photometric terminology (see ISO 31‑6 and ISO terminology for optics).
- Prepared under ISO/TC 172 (Optics and optical instruments) - useful to consult related ISO optics standards for complementary test methods and definitions.
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