Photoluminescent pigment goes by many names in the market, it is also called as glow in the dark pigment, luminous powder and phosphorescent pigment. One of the most common question we get from our customer is how long can your photoluminescent pigment’s glow last? Is your claim that the pigment can glow for over 12 hours true?  

     To answer this question, we need to firstly understand the measurement unit for luminosity. The international standard unit for measuring luminosity is cd/m² (candela per square meter) and mcd/m² (millicandela per square meter). The former is a thousand time larger than the latter.

     To make things easier to understand, let’s imagine the glow in the dark powder as a battery. In order for it to glow, the pigment needs to “store” the surrounding light first, and then the light is released when the environment is dark. So for the photoluminescent powder to glow, you first need an excitation light source to “charge” the luminous powder for a period of time. The moment the excitation light source stops irradiating, the glow intensity of the luminous powder is the highest, but generally it will not exceed 20cd/m². For reference: the indoor lighting at night is generally 100-1000 cd/m².

     The brightness attenuation of the glow in the dark pigment is not linear. Within one hour after the excitation light source stops irradiating, its luminosity drops drastically. Taking our MHG series pigment for example, usually after 10 minutes, its afterglow intensity level is about 200~1000 mcd/m²; after 1 hour, its afterglow intensity level is about 20~150 mcd/m².

     Does this mean the pigment will stop glowing after one hour? The answer is no. After an hour of rapid decay in the brightness, the decay speed begins to slow down greatly. Usually after 12 hours, the luminosity of Zhejiang Minhui’s glow in the dark powder still maintains at 2 ~ 10 mcd/m².

     The weakest light that the human eye can observe in complete darkness is 0.32 mcd/m², and it usually takes more than 10,000 minutes for the brightness of luminous powder to decay below this value. Thus, the luminous powder can continue to emit light for over 10,000 minutes (166.67 hours) after the excitation light source is removed. This is the reason why we refer to photoluminescent products as a “weak” light source, yet a durable one.

     Some may argue that at this point, the glow is weak to distinguish to human eyes. It is indeed hard to tell if the pigment is still glowing if you are not observing in a completely dark room. However, the glow is still very easy to find and follow in environment like dark rooms, buildings, tunnels and mines. This can be crucial in emergencies when the power goes completely off. The glow can illuminate and indicates the life-saving escape route and the location of fire equipment, therefore, save many lives.