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I believe that white light is white because it contains every wavelength, as white by itself has no frequency by itself.

Can somebody explain why really bright pure red, green, or blue LEDs seem white to us and to cameras?

Also, can a bright light ever be pure(eg is it possible to emit a pure red/pure blue/pure green bright light only with wavelengths in the range of that light), or will the inherent brightness of a light force other wavelengths into the light?

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  • $\begingroup$ there are the pure single frequencu colors of the rainbow, and you can get them using a crystal hyperphysics.phy-astr.gsu.edu/hbase/ems3.html#c2 . Then there is the way our brains perceive color, which is complex. Have a look at this answer of mine to a similar question physics.stackexchange.com/questions/421217/… $\endgroup$
    – anna v
    Aug 11, 2018 at 7:47
  • $\begingroup$ so is it purely perceptual, or are there always at least a few other wavelengths being emitted from a bright light source no matter how pure it is? $\endgroup$
    – Dmytro
    Aug 11, 2018 at 7:51
  • $\begingroup$ within a frequency width, due to quantum mechanical uncertainty, the spectrum can be considered "pure". The complex color perception has , as shown in the plot, combinations of frequencies. But in the lab one can always get a "pure" color. $\endgroup$
    – anna v
    Aug 11, 2018 at 7:56
  • $\begingroup$ So brightness in itself has no impact on wavelengths being emitted? $\endgroup$
    – Dmytro
    Aug 11, 2018 at 8:02
  • $\begingroup$ brightness affects the perception, if too bright it "burns" the cones in the retina of the eye, (or the chemicals on a film) and then only brightness is left. $\endgroup$
    – anna v
    Aug 11, 2018 at 12:06

2 Answers 2

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The photoreceptors of the eye respond a little bit to light frequencies different from their peak sensitivity. If you look at the spectral sensitivity of the eye you will see that the response for the red and green receptors cover most of the visible spectrum (blue is a bit narrow). Camera sensors will be somewhat similar but with different curves and sensitivity.

Image from Wikipedia article on spectral sensitivity Image from Wikipedia

This means that if you have a sufficiently strong light at some frequency, you will activate the receptors maximally. It might seem that you would get far more response from the most fitting receptor, but neuron firing rate has a ceiling: beyond a certain point they just fire maximally. Same thing for camera sensors, that saturate. At this point you just get the same output from all of them.

Note that this model predicts that sufficiently deep blue light might not look white to the human eye.

Super-intense spectrally pure light is essentially a description of a laser. There is nothing inherent in light that would force it to spread out spectrally. In a material such as air there will be effects due to light-matter interactions - it might scatter from atoms and particles producing a line broadening doppler effects, it might ionize atoms so they absorb and re-radiate light as spectral lines, or heat up matter to radiate broadband blackbody radiation.

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    $\begingroup$ I don't want to take the time to write an answer myself, so I'm just going to suggest a little more emphasis on the notion of saturation here. $\endgroup$
    – dmckee --- ex-moderator kitten
    Aug 11, 2018 at 15:36
  • $\begingroup$ oh so you are saying that if a light is bright enough, it will disproportionally activate other receptors too and eventually overwhelm all of them, making us see white. $\endgroup$
    – Dmytro
    Aug 12, 2018 at 19:50
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Also there and many types of white, off-white, cold-white, warm white. So white is not perfect. Warm white is perceived by most people/scientists in experiments to have a very small amount of red. Cold white is perceived as having a little bit of blue. In a color camera there are many pixels and each one has is own color filter due to highly precise manufacturing techniques. The camera has been balanced at the factory so that a nice white is produced. Also your computer monitor is also balanced at the factory to make a nice white on your screen.

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