Wood's glass

Wood's glass was commonly used to form the envelope for fluorescent and incandescent ultraviolet bulbs ("black lights").

It is a very deep violet-blue glass, opaque to all visible light rays except longest red and shortest violet.

Due to manufacturing difficulties, Wood's glass is now more commonly used in standalone flat or dome-shaped filters, instead of being the material of the light bulb.

With prolonged exposure to ultraviolet radiation, Wood's glass undergoes solarization, gradually losing transparency for UV.

[citation needed] Photographic filters for ultraviolet photography, notably the Kodak Wratten 18A and 18B, are based on Wood's glass.

Two "black-light" ultraviolet fluorescent tubes. The glass envelopes illustrate the dark blue color of Wood's glass, although these modern tubes actually use another optical filtering material.