What is it that makes natural sunlight difficult to replicate with most artificial lights?
In: Physics
The sun pumps out EM radiation with wavelengths all up and down the visible spectrum (and well beyond it, everything from radio waves with a wavelength measured in kilometers to gamma rays with a wavelength below 10 picometers). So when you see things illuminated by daylight, you are perceiving all possible wavelengths of light that it can reflect.
Artificial lights are of course much lower energy than the sun and emit light in a far narrower band of wavelengths, so when we use them, typically our surroundings will reflect relatively fewer wavelengths of light back to our eye.
The sun shines because it’s around 6000 K, i.e., glowing white hot (it’s incandescent). Anything glowing at a lower temperature will have a more yellow or red light. The highest melting point of any substance is less than 4000 K so it’s hard for us to create a lasting light bulb matching the sun. Our incandescent bulbs use tungsten and even the hottest of those, which use halogen to extend their life, are only around 3000 K. At that temperature the tungsten evaporates until the filament is too thin and breaks.
So we need to use tricks to make the light match the sun. One simple approach is to put a blue filter over an incandescent bulb, blocking some of its red and yellow light, but that’s wasting light. LEDs produce light in a very narrow range of colours and fluorescent lights are similar. Both use coloured phosphors which are excited by the coloured light and glow in a wider range of colours. It’s still difficult (and therefore expensive) to use enough different phosphors to exactly match the colour of the sun.
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