Each disc is covered in a transparent plastic coating (either blue, green, yellow or red) which lets daylight filter through.

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Generally LED bulbs use either a combination red, green and blue LEDs to create white light, or they use a phosphor coating (more common) in combination with a blue LED.

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These can be made to emit white light by coating the device with a phosphor compound that absorbs part of the blue light and re-emits it as yellow.

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