According to Business Wire, in 2003: Ten percent of Americans spend on average $300 per year to tan indoors. This equates to a total US market for tanning of over $9 billion per year, and does not include the $3 billion tanning products market. Enhancements in technology and consumer acceptance are fueling the industry's growth. A suntan is the result of injury to the epidermis, the top layer of your skin.
the most striking example of fluorescence occurs when the absorbed radiation is in the ultraviolet region of the spectrum and thus invisible to the human eye while the emitted light is in the visible region which gives the fluorescent substance a distinct color that can only be seen when exposed to uv light
Lumen measurement example : Consider two light sources, both with 1 W of radiant flux and one source emits a green light at 555 nm and other emit blue light at 480 nm. As the curve shows, the green light appears more bright the blue light even when the total energy of two lights is same. Blue light at 480 nm wavelength will have 68 lumens and green light with 555 nm will measure 683 lumens.