> Not true, brightness is a function of energy, not a function of frequency. Richard. I'm not sure you are correct there old bean, unless we are failing to distinguish between photon energy with total energy flux density. Two definitions to clarify where I'm leading: Intensity: - the absolute measure of a light wave's power density. Brightness: - the relative intensity as perceived by the average human eye. By energy were you meaning intensity? For any (visible) mixture of wavelengths (spectrum :) the more intense the light, the brighter it will appear. It's not going to be linear though and there will almost certainly be tail off at both upper and lower ends. - Lower - it just becomes invisible - Upper - you never see anything, ever, again. Staring at the sun type damage and all that. Dealing with photons - The energy of light is proportional to its frequency: E = hf, where h is a constant - We also know that frequency is inversely proportional to wavelength hope we can agree with those. For equal (radiometric) intensity sources of monochromatic electromagnetic radiation - light with wavelength around 550 nm wavelength appears the brightest to most observers (Well, the old "standard observer" anyway) ... taken to the extreme UV and IR are about as "unbright" as you can get :o) So there is clearly a relationship between wavelength and brightness (and therefore by definition frequency and brightness). No matter how intense a green light is though, it never looks white. What I don't know is whether - for equal intensity (total energy flux density) a "white" mixture looks brighter white that a pure green looks bright green. Bob
