Why do we use infrared light in infrared lamps to heat e.g. inflamed skin? Wouldn’t the complementary color of the skin tone be better because we know that the skin absorbs these wave lengths?
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Infrared goes deeper, and making infrared light is cheaper. We don’t want it to all get absorbed in the outer layer of the skin.
And even if we wanted to, actual absorption is not as simple as matching the complementary color. We can make light that looks yellow by mixing red and green light, but it’d not be absorbed by something that only absorbs actual yellow wavelengths.
All matter absorbs infrared light, though different materials absorb different wavelengths better than others. Infrared wavelengths that are absorbed are also “complementary colors” – they’re just not visible. So there’s no reason to restrict yourself to the visible spectrum, or why the visible spectrum should be special. In fact, the visible spectrum is pretty crummy if you want to heat things up. It’s very narrow, spanning only from about 380-750 nm. Infrared spans all the way from 700 to 1,000,000 nm (AKA 1 mm). So in the infrared you have much more bandwidth to play with.
Moreover, it isn’t possible (with current technology) to generate visible light without generating heat. Even LEDs, which are very efficient (much more than e.g. incandescent light bulbs) still generate more heat than light. So, to generate enough visible light to warm your skin and the underlying tissue, you’d also end up generating an enormous amount of heat, likely (directly or indirectly) in the form of infrared radiation.
Consider for instance that small radiant space heaters can easily have a power output of 2000 W, and they will feel pleasantly warm if you hold your hand next to them, but won’t burn you unless you touch them. Now compare that to modern LED light bulbs. A very bright consumer LED bulb, say 2500 lumen, consumes less than 30 W of power. Hold your hand next to that, and you will feel no heat (even though more than half of that 30 W is actually lost ast heat), because there’s just so little energy in that light.
To feel as warm as that 2000-W space heater, you’d need to output at least 2000 W of visible light (assuming perfect absorption), which at LED efficiencies means also generating at least 2000 W of waste heat. Not only that, but 2000 W of visible light would be staggeringly, blindingly bright. In the neighborhood of 200,000 lumen. So it makes a lot more sense to just emit similar amounts of energy as infrared, which is much more efficient and doesn’t blind people.
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