What is Stray Light?
Different terminology is used in different industries to describe stray light. Common terms are stray light, lens flare, veiling glare, ghost image, image glare, and many more. Most of these terms are not rigorously defined and will be interpreted differently by different readers. A useful introduction to stray light is provided in ISO 9358 – “Optics and optical instruments – Veiling glare of image-forming systems – Definitions and methods of measurement.” This standard is actively maintained (2014) and describes how to measure stray light in a lens assembly.
ISO 9358 includes the following definitions:
Veiling Glare – Unwanted irradiation in the image plane of an optical or electro-optical system, caused by a proportion of the radiation which enters the system through its normal entrance aperture. The radiation may be from inside or outside the field of view.
Veiling Glare Index – Ratio of the irradiance at the center of the image of a small, circular, perfectly black area superimposed on an extended field of uniform radiance, to the irradiance at the same point of the image plane when the black area is removed. VGI is expressed as a percentage unless otherwise specified. VGI occurs when an image is contaminated from surrounding light which lowers contrast – for example, a camera looking into a backlit image, or sunlight coming in at the side of an image and causing glare.
Glare Spread Function – Irradiance distribution in the image plane, produced by a small source object, normalized to unit total flux, in the on-axis image of the small source. GSF occurs when an image has bright light on a dark background – for example, streetlights, headlights, or the sun across a windshield.
Optikos Solutions for Measuring Stray Light
Stray light kits for LensCheck™ and OpTest® Systems for measuring lens VGI
Measure lens GSF with field stop assemblies and automated routines for LensCheck™ and OpTest® Systems
A configurable production camera tester with Meridian® software for measuring camera VGI
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