The Luminosity of a Watch5:02 am
The Visibility Factor
The smooth coating on the indexes and hands of watch dials has been known to save lives in addition to just reading time in the dark.
The need for a sophisticated and advanced way of telling time in the dark has been constantly sought after by master watchmakers and thus the study of luminescent pigments goes back right to the ancient times.
In the early days of civilization, as the sun went down and the sundial was unusable for the night, a graduated candle was used to tell the time. The precision of this method of course suffered due to the inconsistent rate of candle burning.
Accounts from over 1,000 years ago mention the discovery of photoluminescent objects and paint in China and Japan. The Chinese would ground up rare photoluminescent jades to make these into beads and wine cups. The Japanese, on the other hand, made this paint by combining sea-shells with volcanic material (due to the formation of Zinc sulphide along with the presence of impurities like copper for a suitable chemical reaction). By the end of the 19th century, Swiss watchmakers were using these very same natural techniques to create luminescent paint to treat their dials. Thus began the use of these “miracle materials” into various watchmaking parts which helped improve readability in the dark.
The luminous watches today use Super-LumiNova, LumiNova, LumiBrite, etc. The luminosity of a watch depends on the pigments that have compounds which after ‘charging’ from a source of light, emit a glow for several hours thereafter.
How It Works
The property of luminosity in the pigments used in watches occurs due to the phenomenon of “phosphorescence” which is a form of “photoluminescence”.
Photoluminescence takes place whena substance absorbs photons (electromagnetic radiations) and then re-radiates the photons. This period of first absorbing and then emitting the photons can get over within 10 nanoseconds or under special circumstances be delayed to several minutes or even hours .
Phosphorescence is a special form of photoluminescence where the energy absorbed from photons undergoes an intersystem crossing into a higher state of multiplicity usually a triplet state. Once the energy is trapped, transition from a triplet state to a lower singlet state cannot take place immediately, thus making the transition back to a singlet state longer, lasting minutes or hours.
Types of Luminescence
Originally radium paint was used but due to its radioactive side effects it was quickly taken out from all manufacturing units.
Electroluminescence was another method used in the 80s, but it was shot down rather quickly due to its battery life consumption and requirement of pressing.
The current favourites in the industry today are:
Super-LumiNova is a strontium aluminate based non-radioactive and nontoxic photoluminescent or afterglow pigment used for illuminating markings on watch dials, hands and bezels, etc. It was invented in 1993 by Nemoto & Co Ltd, and is considered one of the safest and most popularly used luminescence materials in the industry. The material works on the principle of “phosphorescence” and is charged through exposure to light. It is non-radioactive and slowly decreases in brightness until recharged again. Watches with a marking of “L-Swiss Made” indicate the use of Super-LumiNova on the dial.
A radioactive isotope of hydrogen, tritium filled luminous tubes entered the market in the 70s. The tiny glass vials were made of RC Tritec and others. Tritium can grow without recharge and reduces its luminosity over a very long period of time (over 5 yrs). The side effect of tritium is that it has a relatively low brightness which can be difficult to read in certain conditions and in case of an immediate change of light (from light to dark). Super-LumiNova is brighter than tritium but does not have the longevity that tritium offers. Tritium emits a total of 25 milli-curies (mCi). Tritium gas tubes were used as a light source to backlight liquid crystal display (LCD). Tritium poses no health risk to the wearer or to the workers who assemble the watches. Tritium’s radioactive decay produces only weak beta particles that are contained completely within the sealed glass tubes. Even if exposed, the beta particles do not possess enough energy to penetrate the outer layer of human skin.
Tritium filled tubes which come in different colours like orange, blue, green or red and are coated with phosphor to create the glow are called trasers. The Tritium gas is sealed within the glass tube, so though they are radioactive, they do not emit any radiation. Trasers typically glow for 10-20 years. Ball and Luminox are two Swiss watch companies that use Trasers extensively in their watches.
Charging up your watch lume
The dominant wavelength of the illuminating light, the strength (illuminance) of the illumination and the length of time for which the watch is illuminated are the three main factors that affect the charging up of your watch lume.
The luminous pigment in watches is charged by the UV content of the light and mostly LED lamps are a good bet.
A general rule of thumb is that the longer the time of exposure to light, the higher the initial brightness of the lume. However, there is a limit for each level of illumination. This limit is reached more quickly at higher strengths of illumination.
Here are our recommendations for watches with high luminosity:
By Disha Birdie
Sr. Features Writer, The Watch Guide
A budding writer and watch enthusiast with a passion for discovering Swiss watchmaking know-how.