Did you know there is no such thing as a ‘white’ LED (light-emitting diode)? LEDs only emit a relatively narrow spectrum of light. ‘White’ LEDs are made by starting with a Royal Blue LED and painting over it with a phosphor material that looks yellow. When energized, the blue light causes the phosphor to emit broadband light. Some of the blue still comes out, and it is the mix of blue light and the phosphor emission that appears white.
The photo on the left below shows a white LED (left) and Royal Blue LED (right) viewed under white light illumination. You can see that the LED on the left appears yellow. For the photo on the right the LEDs were illuminated by an external Royal Blue light and photographed through a yellow filter that blocks the reflected blue light. The strong glow coming from the phosphor on the LED on the left is obvious.
(Click image for larger view)
Another way to do this, however, is to incorporate the phosphor in a plastic material and position it apart from the blue LED. This is called a ‘remote phosphor’. This approach is used with some of the LED lights available for general illumination. The blue light from the LED strikes the remote phosphor, is converted to longer wavelengths, and the resulting glow appears white.
Several of the lights available from NIGHTSEA, the GoBe and the Sola, are supplied with a removable remote phosphor. You can use the inherent blue emission of the LED to search for fluorescence, then put the remote phosphor in front of the light when you want white light. Very handy! The phosphor emits in all directions, so the relatively tight beam pattern of the blue light is not preserved. The white light illumination will cover a broader area.