...and my high-school buddies said you'll never use this stuff in real life. Hello, my name is Matt and once in a while I deal with the practical application of Physics in a wide range of real-life settings. Most of the time I'm capable of researching the given topic myself. This time, I need a confirmation from someone knowledgeable, someone who paid attention in high-school Physics class :) A very practical scenario / questions: A cosmetic laser used for hair removal emits NIR light 810 - 1300 nm. Protective glasses are worn by the operator. To delineate treatment area, a grid is drawn on skin, using opaque, white grease pencil. The laser is pressed against the skin and fired. Emitted light energy "burns" the hair and follicle. And voila; perfect, hairless skin. As the treatment progresses, the pencil wears off, flakes off and forms a thin, not transparent film on the laser, which limits the amount of light reaching the skin and by reflecting the light back, causes excessive heat buildup inside of the device. If a black-light marker (no wearing off, no flaking off) was used to delineate the treatment area and the skin was illuminated by a black light lamp (protective glasses not necessary), will the grid be visible through the laser - protective glasses? If so, which marker colour will be visible the most? Also, which marker colour(s) will interfere the least with the emitted NIR laser light? Any and all suggestions will be greatly appreciated.