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Illumination

One field of our ongoing research is illumination, specifically passive, and emergency light solutions.


Glow in the Dark (GITD)

The "Glow" is a Property Called Luminance

Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted or reflected from a particular area, and falls within a given solid angle.

The Phenomenon is Called Photoluminescence

Photoluminescence is light emission from any form of matter after the absorption of photons (electromagnetic radiation). It is one of many forms of luminescence (light emission) and is initiated by photoexcitation (excitation by photons), hence the prefix photo-. Following excitation, various relaxation processes typically occur in which other photons are re-radiated.

Measuring the Glow

Afterglowing characteristics according to German norm DIN 67510-1: Photoluminescent pigments and products

DIN 67510 requires charging the completely discharged sample of photoluminescent material with a xenon lamp, 1,000 lux illumination (92.9 ft-candles) on the sample, for 5 minutes. Luminance, measured in mcd/m2 (millicandela per square meter) is measured 10 minutes and 60 minutes after the charging lamp is turned off. Time to discharge, measured in minutes, is the time required for the sample luminance to decrease to 0.32 mcd/m2. For the dark adapted human eye, a luminance of 0.32 mcd/m2 is 10 times the minimum luminance visible.

Part 1 of DIN 67510 lays down the methods of measuring the afterglow pigments and products and in addition the way of representing or characterizing the measured values.[1]

        Luminance mcd/m2
Class   After                              Time to
        2 min   10 min   30 min   60 min   0.32 mcd/m2
G       3,000   650     190       80
F       2,300   520     155       70
E       1,800   400     120       55
D       1,100   260      85       35       2,000 min
C         690   140      45       20       1,800 min
B         210    50      15        7         900 min
A         108    23       7        3         450 min

The characterization is carried out by means of symbols such as: 28.0 / 3.6 - 520 / w - k [2]

  • 28.0 = luminance in mcd/m2, 10 min. after the end of excitation
  • 3.6 = luminance in mcd/m2, 60 min. after the end of excitation
  • 520 = time in minutes from the end of stimulation after which the luminance has fallen to 0.32 mcd/m2
  • w = perceived color at stimulation (white)
  • k = perceived color of the afterglow (green)

Why Green?

GITD pigments can be produced in many colours. The human eye is however most sensitive to green light, meaning that green GITD appears brighter and so is generally favoured by manufacturers. This is also why night vision goggles typically colour things in green.

State-of-the-Art Materials

In 2015,[3] the raw glow pigment of choice is "Strontium Aluminate"; Alkaline Rare Earth Metal Silicate-Aluminate Oxide Europium Doped.[4][5]

Name                     Ultra Green V10       GreenGlow
Particle size            55-85 microns         100-150 microns
Luminance after 10 min   945 mcd/m2            817 mcd/m2
Luminance after 60 min   126 mcd/m2            139 mcd/m2

The brightest materials (since we are looking for the brightest, we are focusing on green; being the brightest color GITD) are hovering at a value of around 1 cd/m2 (+/- 20%) after 10 min of exposure. (Example: LCD displays have a luminance of 200 to 300 cd/m2. For displaying movies, a brighter luminance rating such as 500 cd/m2 is desirable.)

(Published luminance values are often the very high numbers immediately after charging, while for realistic applications, the value after 10 minutes is the most comparable.)

Materials that are significantly less bright at values of less than 0.7 cd/m2 (+/- 20%) are useful for marking objects. Both the latest and the previous generation share afterglow times of more than a day.

Surface Area

Since luminance is measured per area; more area means more emittance and higher resulting illumination. Ultimately, it's a matter of surface area, and while most contenders are available in comparably high grades of photoluminescence, the largest emitter wins, providing a higher amount of illumination in total. Thus, a brighter emitter can be smaller in size.

Starting at around 5 cm2 (facing in one direction) a latest generation GITD item at full brightness becomes bright enough to illuminate for example a map portion or a keyhole.


Resources:

References:

  1. PSPA Material Classification 
  2. Explaining DIN 67510-1 
  3. Update March 2017: There are no revolutionary new materials on the market since 2015, only minute improvements and variations in e.g. grain size have occured. 
  4. Ultra Green v10 Glow in the Dark Powder 
  5. Green Glow in the Dark Powder and Introducing GID Powder