I've heard that red light is dimmer compared to green light emitted with same power and to if you want to have same intensity of light you have to crank up power of red light. From equation for power $P=\frac{W}{t}$ you can see that to increase power you have to increase energy or decease time, but we can't increase energy because that would change color of the light according to this equation $E=hf$. E.g. in red laser, how can you increase power in it to have the same color?

I feel like missing some important information on this topic.

  • $\begingroup$ From the question I have asked a second ago, The brain interprets colour via frequency, The equation E=hf is the energy of an individual photon, increase the number of photons and you have a more intense version of that colour. $\endgroup$ Dec 4, 2021 at 14:23

1 Answer 1


Lasers are always issued with a maximum output power that they can achieve. But typically, most can operate at almost any power level below that.

So, in your case, if you have a green and a red laser of a certain power, if you want to match their brightness, you will have to tune the red laser to between 3 to 4x the power of the green laser. Or the other way around, have the red laser operate at maximum power and dim the green laser to be 1/4 to 1/3 of the red's power.

Its just that humans are more sensitive to green light than to red light and hence why the same power in red appears much dimmer.

EDIT after question edit: Yes, you are definitely missing a big part of the picture. Photon energy ($E=\hbar \omega$) has nothing to do with the total amount of energy in a light beam. See this differently, the total energy is equal to the total amount of photons of energy $E$. If your source gives off photons of a certain energy, increasing its output power is just increasing the number of photons emitted, not changing their intrinsic frequency.

  • $\begingroup$ I'm really sorry for not specifying my question. I updated it now, so it's clearer. $\endgroup$
    – cover
    Dec 4, 2021 at 13:35
  • $\begingroup$ RE: " most can operate at almost any power level below that."...Many kinds of lasers will have degraded performance when operated at too low a power. Wider line-width or poor SMSR, for example. Of course you can take a laser and put an attenuator in front of it to get a lower power level without these problems. $\endgroup$
    – The Photon
    Dec 4, 2021 at 17:35
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    $\begingroup$ @ThePhoton I would not think these are things important for OP, who is probably just thinking in simple laser diodes, and the question should not go so deeply into laser sources. True, diodes have a threshold that is definitely non 0, but for the level of the question, those things do not matter. $\endgroup$ Dec 5, 2021 at 13:22

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