The dimmest glow discernible by a dark-adapted human eye is 3.2 nCd/mm² or 3.2 nanocandela per square millimeter.

The ISO standards:
A luminous dial with 11 numerals glow with an intensity of 36nCd.
Its hands must be at least 10nCd.

In photoluminescence, the energy is supplied by electromagnetic radiation (e.g. light). A photoluminescent material absorbs light for a significant period of time, then emits light with a lower frequency than that of the absorbed light.

In the 19th century, scientists paid particular attention to the phosphorescent properties of zinc sulfide. If copper or cobalt are included in the crystal lattice of the zinc sulfide, it can glow for hours after the exposure has ended.

The main drawback in using photoluminescent material in watches and clocks is that the luminescence diminishes rapidly and totally disappears after a few hours.

Radioluminescence can glow without any exposure to light and it is produced by nuclear radiation. Gamma and X-rays or Alpha and Beta particles are used to excite the electrons in a radioluminescent compound such as zinc sulfide. This type of radioluminescent material was used in watches for about 80 years.

Autoluminescent zine sulfide will continue to emit clearly visible light for several years.

Although radioluminescent material can produce bright and long lasting light, it is harmful. Therefore, it is prohibited from using in watches or clocks.

Tritium is a radioactive isotope of hydrogen. It replaced the radium after World War II

The radioactivity is composed entirely of Beta particles that are nearly completely absorbed by the watch crystal or glass covering the dial. The tritium used in watches today is in compliance with the ISO 3157 and NIHS 97-10, which define the acceptable minimum levels for the amount of luminescence required to see the watch dial in the dark. Depending on the quality of the radioluminescent compound, it can conserve its ability to luminescence for several years. The quality also influences the luminous intensity, which also depends on the surface and thickness of the deposit. The pigments of the natural yellow tint yield the best results.

Although Tritium is being used, it is harmful. All application or repair on watches or clocks must be handled with care - Do not touch the luminous dials and hands with your hands.

Super-LumiNova was first synthesized in the 1960s. This is a non-radioactive luminous material.

If the strontium aluminate is enriched with two rare earth metals, europium and dysprosium, and exposed to sunlight for a few minutes, the compound remains noctilucent for many hours.

Sunlight activated Super-LumiNova initially glows much more brightly than tritium. After about 2 hours, its luminosity declines to the same intensity as tritium. For the next 6 hours, its brightness continues to wane until the value of 7nCd/mm².

If the material is activated by artificial light, the brightness is only 1/10 as bright as sunlight activated one. The brightness will reach the threshold of visibility for the unaided eye.

If the markers or hands have been coated with enough Super-LumiNova, they will remain legible up to 12 hours.

LumiNova can be greenish, bluish or greenish-blue. The color is determined when the strontium aluminate is enriched. When the LumiNova is viewed in daylight, the colors is almost limitless, it can be white, black, ... The colors are achieved by dying the crystals of the luminous material with a variety of coloring agents which are organic, fluorescent or non-fading colorants.

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