Black Lights & Phosphors

    Kids, did you ever wonder why is it that under a "black light" some white objects appear to be so bright that they glow? Or even how black lights work at all?   The answer to both of these questions involves phosphors. 

    Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

    A black light is a fluorescent lamp that has a modfied phosphor coating on the inside of the tube.   Phosphors are fluorescent powders that, when exposed to ultraviolet (UV) light, absorb the light energy and re-emit some of it as visible light.  This process is called fluorescence.  Both fluorescent and black light bulbs make ultraviolet (UV) light. Phosphors in a fluorescent lamp convert the ultraviolet (UV) light into visible light.  But phosphors in a black light absorb only the harmful UV-B and UV-C rays, letting harmless UV-A rays through.  And the black glass tube blocks the visible light, too, except for a little blue and violet (the common "black light" aura).

    If you walked around all night with a portable black light, you would discover that there are phosphors all around. Natural phosphor molecules are in teeth and fingernails, for example. Fluorescent colored items such as highlighters contain them, as do all glow-in-the-dark items. Get creative by drawing pictures or writing messages with highlighters, then test your results under black light. Some sheets of paper will glow more than others will. Check for the invisible fluorescent strip in some larger bills, which is incorporated to help foil counterfeiters. Some white clothing will glow also. This is because most laundry detergents contain phosphors to make whites appear brighter in sunlight, which contains UV radiation. Dark clothes don't glow because the dark pigments absorb UV.

    Some of the phosphors in laundry detergents stick to laundered clothes. Many detergents today have little or no phosphor in them, so using a black light can be a good test for finding which ones do or do not. In our testing facility, we found phosphorescent granules in Wisk® laundry tablets and in Wisk® he powder.   Even some of the ink used on the detergent packaging was found to fluoresce!  Test other cleaning powders, such as dishwashing powder detergent, to confirm that nothing glows in these products.  If you don't have a portable UV light, black lights also come in both tube and bulb form.

    Chemists have created thousands of phosphoring chemicals: zinc sulfide (ZnS) and strontium aluminates are a couple that toy makers use in glow-in-the-dark products. Your TV screen glows because of phosphors that decay just slowly enough that successive pictures blend into each other.  By the way, there is a specific element called phosphorus (P) that is named from the Greek words phôs (light) and phoros (bearer).  Some phosphorus compounds (such as some phosphates) are used in fluorescent light bulbs and TV screens.


    Kathleen Carrado Gregar, PhD, Argonne National Labs 
    [email protected]
    March 2004