Definition
Veiling glare is a phenomenon in optical systems wherein stray or scattered light within an instrument reduces the perceived contrast of the image formed on a detector or the observer’s retina. The unwanted light superimposes a diffuse luminance over the intended image, effectively “veiling” fine details and diminishing visual performance.
Physical Mechanisms
The primary mechanisms that generate veiling glare include:
- Surface scattering – microscopic imperfections or contaminants on lens surfaces, mirrors, or sensor windows scatter incident light in non‑specular directions.
- Bulk scattering – imperfections within transparent media (e.g., glass, plastic, or atmospheric particles) cause forward or backward scattering of light.
- Diffraction – edge diffraction from apertures, stops, or structural components can introduce low‑frequency light that adds a uniform background illumination.
- Internal reflections – multiple reflections between optical surfaces, especially in multi‑element lenses, can redirect light toward the image plane.
Measurement and Quantification
Veiling glare is commonly quantified by a veiling glare factor (VGF) or veiling glare coefficient, which expresses the ratio of stray‑light intensity to the on‑axis image intensity. Standardized measurement procedures, such as those defined by ISO 21254 (photographic lenses) and MIL‑STD‑1500 (military optical equipment), employ calibrated light sources and test targets to isolate the stray‑light contribution.
Impact on Different Domains
- Astronomy and Telescope Design – Veiling glare reduces the contrast of faint celestial objects against the night sky, limiting detection sensitivity. Baffling, anti‑reflective coatings, and careful stray‑light analysis are employed to mitigate the effect.
- Photography and Imaging – In camera lenses, veiling glare manifests as reduced contrast and “washed‑out” areas, particularly when shooting bright light sources near darker subjects. Lens designers use low‑dispersion glass, internal coatings, and complex aperture geometries to control stray light.
- Human Vision – The eye’s optical media (cornea, lens, vitreous humor) can produce veiling glare, especially in conditions of bright ambient illumination or with ocular media opacities (e.g., cataracts). Clinically, veiling glare contributes to decreased visual acuity and contrast sensitivity.
- Display Technology – In projectors and head‑mounted displays, internal reflections within the optical train can cause veiling glare that degrades image uniformity.
Mitigation Strategies
- Application of high‑performance anti‑reflective (AR) coatings on all transmissive surfaces.
- Use of light‑absorbing baffles, traps, and matte black surfaces to intercept stray photons.
- Optimizing lens element shapes and spacing to minimize internal reflections.
- Implementing stray‑light analysis in optical design software (e.g., non‑sequential ray tracing) to predict and reduce veiling glare early in the development process.
Historical Context
The term “veiling glare” entered the optical engineering literature in the mid‑20th century, notably in publications concerning photographic lens performance and military optical equipment. Early studies highlighted the importance of controlling stray light to achieve high‑contrast imaging, leading to systematic approaches for quantifying and reducing veiling glare.
See also
- Stray light
- Contrast ratio
- Optical scattering
- Anti‑reflective coating
- Diffraction (optics)
References
- ISO 21254:2015, “Photographic lenses – Determination of veiling glare.”
- MIL‑STD‑1500A, “Optical Performance Test Methods.”
- B. B. Wilson, Stray Light in Optical Systems, SPIE Press, 2010.
- L. R. F. S. Costa, “Effects of veiling glare on human visual performance,” Journal of Vision, vol. 23, no. 5, 2021.