Some of the first phosphors to be developed were inorganic zinc sulphide compounds. (German submarine commanders during World War I reportedly rubbed these compounds on their hands to read documents during blackout conditions.) These phosphors typically absorb energy from deep blue and ultraviolet light and emit it as yellow-green light.This is useful in that the peak spectral distribution of the emitted light roughly coincides with the peak spectral sensitivity of the human visual system under isotopic (low-level) lighting conditions (which is around 510 nanometers).
Zinc sulphide occurs in crystalline form, but is not photoluminescent by itself. This requires the addition of activator ions to the crystals, such as copper atoms. These ions absorb the excitation energy of the ultraviolet or visible light and later release it as visible light.
The copper-activated zinc sulphide crystals(identified with the chemical symbol ZnS:Cu) are typically ground into a fine powder with a grain size of 3 to 15 micrometers to avoid light trapping and light piping effects. This must be done carefully however, because the crystals can darken (or gray) if subjected to heavy pressure or excessive mechanical stress, due to disruption of the crystalline structure.