lighttight

Lighttight refers to the property of blocking all or nearly all light from passing through a material, a container, or an enclosure. A lighttight space is designed to be impermeable to light, typically through opaque materials and secure closures at seams and openings. In practice, the goal is to prevent any stray light from reaching sensitive contents.

The term is widely used in photography, film, and chemistry to protect light-sensitive media such as photographic film, paper, emulsions, and reagents. Lighttight enclosures are also used in electronics, astronomy, and biology research where controlled illumination is required or unwanted light could affect experiments.

Achieving lighttightness involves selecting opaque materials, avoiding transparent fasteners, and using continuous seals such as gaskets around doors or lids; joints are designed to minimize gaps; interior finishes are non-reflective to reduce light scattering.

Lighttightness can be tested by exposing the interior to a bright light and measuring any light transmission with a photodetector or light meter. Practical checks include placing a light source outside and inspecting for glow or colored light inside, often in a darkened room.

The term is often hyphenated as light-tight or lightproof; in some contexts "lighttight" emphasizes the functional requirement of preventing light intrusion rather than absolute proof against all wavelengths or intensities.

See also: lightproof, light-sealed containers.