3
$\begingroup$

I was wondering how this lens works. It converts blue laser light into white light and effectively turns a portable laser into a flashlight. The info mentions phosphor coating. I used my Google Fu and found this: Phosphor-Coated LED Converts Blue Light to White.

Could someone please give me a more conceptualized description?

$\endgroup$
1
$\begingroup$

You probably know that isolated atoms have discrete energy levels and therefore have sharp absorption and emission lines. So if you shine your blue laser at an isolated atom there would be no light absorption unless the laser wavelength happened to match an energy level in the atom, and when the atom re-emitted the light it would have the same wavelength.

When you put your atom in a solid two things happen. Firstly the energy levels become broadened into bands, so rather than sharp absorption lines we now have broad ones. Secondly energy absorbed from light can be dissipated by transferring it to lattice vibrations, and this means that the emitted light does not necessarily have the same wavelength as the absorbed light, but instead may have lower energy (i.e. be redder).

We normally distinguish two difference absorption/emission processes in solids, fluorescence and phosphorescence. They're actually not that different except that in phosphorescence the absorbed energy ends up in metastable states that decay and emit energy only slowly. This means the light from phosphorescence is emitted slowly so the object continues glowing after the incident light is turned off. I mention this because your question menations a phosphor, but actually the word phosphor tends to be used loosely and I'd guess your torch actually exploits fluorescence.

Different fluorescent materials will have different emission spectra, so I'd guess your torch contains a mixture of fluorescers balanced to give an overall roughly white light.

$\endgroup$
0
$\begingroup$

It is well known that a standard white light having a chromaticity such as the CIE source C , can be obtained by combining a single pair of complementary monochromatic sources, at wavelengths of 448 nm (blue) and 568.7 nm (yellow) in the right proportions. This mixture has a luminous efficacy of 400 lumens per Watt, the highest obtainable for a source C white light. It can make a great flash light, but is not a good illuminant, because there is no red in the spectrum, so colors are not reproduced properly.

A practical white light source similar to this is obtainable with a unique phosphor consisting of Cerium doped YAG. (Yttrium Aluminum Garnet). In this crystal, the Cerium ion, has a strong and narrow absorption at about 460 nm in the blue, and the YAG phosphor then fluoresces with a broad yellow spectrum. So blue InGaN LEDs can be used to "pump" this phosphor, at that wavelength. Some of the blue photons escape from the phosphor, unabsorbed, and the mix with the broad yellow to give a visual white light. More phosphor concentration, absorbs more of the blue to give a warmer white color, around 2,700 to 3,000 Kelvin color Temperature, while lower phosphor concentrations give more blue residual so the color Temperature is higher, and the lumens per Watt is higher, because of the lower Stokes shift losses. Additional red phosphors need to be added to give better color rendition, but that is not needed for a flash light.

The light you referred too evidently uses a blue InGaN laser to get the blue light, and they could be using the YAG phosphor or some other kind. The Cerium doped YAG phosphor system is patented by Nichia Corp of Japan, possibly discovered by Shuji Nakamura, who invented the blue GaN laser and LED.

It is not clear that the blue laser, gives any advantage over using a blue LED. The yellow phosphor is going to scatter the yellow photons, and also the residual blue photons, so a point source laser, does not necessarily have any advantage; and is more expensive to make.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.