Overview
ISO 14490-6:2014 - Optics and photonics: Test methods for telescopic systems - Part 6: Test methods for veiling glare index - specifies laboratory procedures to determine the veiling glare index (VGI) of telescopic systems and observational telescopic instruments. The standard adopts the black patch (integral) method (as described in ISO 9358:1994) for terrestrial telescopes and defines the measurement principle, required test arrangement, procedure, presentation of results and repeatability criteria.
Key topics and technical requirements
- Measurement principle: VGI, S = E1 / E2, is the ratio of illuminance of the image of a black surface (E1) within a diffusely illuminated white field to the illuminance of the image of a totally white surface (E2). Results are expressed as a percentage.
- Preferred method: Black patch (integral) method for telescopic systems; glare spread function is referenced for astronomical use to ISO 9358:1994.
- Test arrangement components:
- Integrating sphere (diameter ≥ 15× entrance pupil, min. 1 m), interior reflectance ≥ 85% across 380–780 nm.
- Object-side collimator: achromatic lens, min. f-number 10, aperture > 1.2× objective lens diameter; focal length equals sphere interior diameter; antireflection coatings required.
- Measurement & evaluation unit: limiting stop (8 mm at exit pupil), filter (to match detector to photopic response), image-side collimator, measuring stop (20–50% of black image diameter), radiation detector and indicator.
- Reference surfaces: removable white disk and annular black element with light trap (black trap luminance