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640 nm is near the 'bottom' end of the red part of the visible spectrum. Hence, this filter would be used to target the melanin in hair, with very little absorption in blood vessels.
700 nm filter
640 nm filter
This 700 nm filter stops all colours of visible light except the very red wavelengths. This is good for hair removal in darker skin tones where we don’t want to damage the epidermal melanin.
This is the start of the infrared portion of the spectrum.
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