Let's say there is a luminous light bulb that only emits blue light, and you (or a camera if you prefer) look at it from a close distance. Will the maximum brightness perceived not have any "white" to it? Because there is no white light if it's all blue. Probably the brightest it could be would be solid blue? Therefore, blue photons could produce a range of color of black to solid blue depending on the luminous or amount of photons hitting your eye per second? If that's the case then what happens if you raised the intensity of the bulb to an extreme? Is there a cut off point where even if you add more intensity to the bulb, you still see the same solid blue color?

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    $\begingroup$ You should probably narrow your question down to a blue laser emitting a specific frequency of light. There is no such thing as white photons. "white light" is perceived when there is a roughly equal mix of frequencies. $\endgroup$
    – Brandon Enright
    Mar 28, 2013 at 22:13
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    $\begingroup$ @BrandonEnright I am aware of that, I didn't say white photons. $\endgroup$
    – Dan Webster
    Mar 28, 2013 at 22:28
  • $\begingroup$ I think the perception tag might be appropriate here, since the photons will always stay within a narrow spectral region and the "darkness" or "whiteness" of the light is merely determined by the lowest sensitivity and the saturation level of the receptors in our eyes, which depend on the number of photons (which determines the intensity of the light). $\endgroup$
    – Wouter
    Mar 28, 2013 at 23:01
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    $\begingroup$ @Nathan hm, good point. (Flag it!) It's sort of borderline, because there is a fair amount of physics involved in understanding how our retinal cells respond to different wavelengths of light, but the fact that this is really about color perception does suggest it as being off topic. $\endgroup$
    – David Zaslavsky
    Mar 29, 2013 at 0:55
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    $\begingroup$ '"white light" is perceived when there is a roughly equal mix of frequencies.' Not even that. White light is perceived when there is a roughly equal excitation of the three distinct types of color sensitive photocells in the eye, and they each have a broad sensitivity. Which is essentially the same information you need to answer all of these "How do humans see XXX?" questions. We've accepted them in the past because (1) there is some physics in them and (2) there was nowhere else for them to go, but at this time I would favor moving them out. $\endgroup$ Mar 29, 2013 at 3:15

1 Answer 1


The color that human vision perceives for a specific frequency of light (prior to saturation of any cones) is derived from the relative responses of different cones. For example, at about 480nm, the response of S and M cones are roughly equal. However, the human eye is not able to distinguish pure colors (single wavelengths) from combinations of wavelengths which produce the same response in each of the three cone types.

It is possible that a blue light source could appear white to the human eye, because the cones which inform our perception of color are not strictly monochromatic. Rather, they have a range of sensitivity across the spectrum, and all cones are at least somewhat sensitive to blue light. (See this diagram.) If the blue light is so intense that it saturates all cones, S (blue), M (green), and L (red), then it will be perceived as white light. However, in order to saturate the low-sensitivity M and L cones, the intensity of the light would be blindingly high. Prior to this point, but after saturating the S cones, the apparent color of the light would shift toward green before eventually appearing white. For intensities that are not saturating to any cones, the hue of the color blue will remain constant, while it's value (or lightness) may change.


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