These fascinating UV beads contain a unique pigment that changes color when exposed to ultraviolet light from the sun or other UV sources. The beads are not affected by visible light—they are chemically developed to only react to UV light and will remain white indoors or when shielded from UV light.
How to activate the UV beads
The electromagnetic radiation needed to affect a change in the color of the UV beads is between 360 and 300 nm in wavelength. This includes the high-energy part of UV Type A (400-320 nm) and the low energy part of UV Type B (320-280 nm). Fluorescent type black lights (both traditional tubes and CFLs) work very well, as do most UV LED lights, but please note that incandescent black lights (the type used to make fluorescent paints glow) will not change the color of the beads nor will UV Type C (280-1 nm).
How UV beads work
The dye molecules in these ultraviolet light-detecting beads consist of two large, planar, conjugated systems that are orthogonal to one another. No resonance occurs between two orthogonal parts of a molecule. Imagine two planes at right angles to one another, connected by a carbon atom. When high
energy UV light excites the central carbon atom, the two smaller planar conjugated parts form one large conjugated planar molecule. Initially neither of the two planar conjugated parts of the molecule is large enough to absorb visible light and the dye remains colorless.
When excited with UV radiation, the resulting larger planar conjugated molecule absorbs certain wavelengths of visible light resulting in a color. The longer the conjugated chain, the longer the wavelength of light absorbed by the molecule. By changing the size of the two conjugated sections of the molecule, different dye colors can be produced. Heat from the surroundings provides the activation energy needed to return the planar form of the molecule back to its lower energy orthogonal colorless structure.
Changing UV bead color back to white/clear
Although UV light is needed to excite the molecule to form the high-energy planar structure, heat from the surroundings provides the activation energy to change the molecule back to its colorless structure. If colored beads are placed in liquid nitrogen, they will not have enough activation energy to return
to the colorless form. The UV detecting beads remain one of the least expensive qualitative UV detectors available today. They can cycle back and forth thousands of times.
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