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How to calculate power rating required for a laser in order to produce a dot at a specific distance after reflection? For example, the laser is 10cm from the mirror, at an angel of 30 degrees it shines onto the mirror and the beam reflects, creating a dot 50cm away, how powerful the laser should be?

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    $\begingroup$ Is the setup in air? vacuum? water? On what surface does the dot appear, wood? cardboard? opaque plastic? Is the sensor a person or camera? What type of laser? more info :) $\endgroup$ – jiggunjer Apr 21 '15 at 16:51
  • $\begingroup$ Green light appears brighter than red light at the same power. IR lasers create invisible dots but you may be able to feel them with your skin. If you mean visible by the unaided eye, how far away is the eye? Small dots are better visible than large dots. A dot may be visible in the dark, but invisible with ambient light, e.g. direct sunlight. How reflective is the mirror? 99%? 90%? 1%? $\endgroup$ – darsie Apr 21 '15 at 19:34
  • $\begingroup$ The set-up is in the air and a person is looking at it right where the laser is reflected. The calculation is general, so assuming a texture like wood, a "common" reflectivity of a mirror, and not a brightly lit room. $\endgroup$ – Yan Apr 23 '15 at 14:31
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A 1 mW red (diode) laser will make a dot visible many meters away, is cheap, widely, legally available, not dangerous and uses little electricity.

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  • $\begingroup$ Although it's not dangerous in the sense of causing long-term damage, shining one into someone's eye (especially a cop) will get you arrested. $\endgroup$ – WhatRoughBeast Apr 21 '15 at 21:33
  • $\begingroup$ Yeah, so it's about as dangerous as a towel you throw at someones face. Or a photo flash from 50 cm distance. $\endgroup$ – darsie Apr 22 '15 at 10:16
  • $\begingroup$ Sure it will, however I would like to have some proof that under normal circumstances *comment 377278_177569, this will be enough. $\endgroup$ – Yan Apr 23 '15 at 14:32
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What you can do is find out the illuminance (lux=lm/m2) of a magnitude 6 star, which is the limit of human vision of light spots and see what luminous flux (lm) creates that illuminance at your certain distance from the spot, assuming that the light disperses uniformly in a semisphere (unless you have better information on the light distribution). From the luminous flux and the specific efficacy (lm/W) of your laser light wavelength you get the power of the laser that is barely visible in your situation.

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