Actually i'm interested in the UV Diode now.

I came across some UV LED's from DX, 20 pack for $3.80 or something like that so i bought them they're great but what can a LOWER wavelength UV do that a cheaper UV can not? except damage your eyes?...

http://www.dealextreme.com/details.dx/sku.2398

390 - 410nm uv.... how would a lower wavelength (http://www.elfa.se/pdf/75/07500366.pdf) of 370nm vs 390nm what can the 370nm do that the 390nm can not? any ideas? plus i've driven these UV's up to 200ma (by accident) but not only did it handle it, it still lives to tell the tail, i'm betting 100ma is the max it can handle though and 25ma nom or so, almost double the expensive one

 

Culled from http://forum.allaboutcircuits.com/showthread.php?t=13191

 

If you are using the UV light for photochemistry, e.g., exposing PCB's, as a general rule, it is better if the wavelength is a too short side rather than too long.

The reason is that photochemical reactions require that the molecule absorb a photon of light that has sufficient energy for the reaction of interest. If the wavelength is too long, either the photon won't be absorbed or (much more rarely) you won't get the excited state that is needed for the reaction.

If the wavelength is too short, it still must be absorbed by the molecule, but the chance that absorption is near zero at shorter wavelengths is low. So, assume it will be absorbed, but maybe not as well as at the optimal wavelength.

The biggest problems with using light that has too short a wavelength are that it is likely to be absorbed by other materials, thus blocking the molecule that you want to absorb it, and products from the desired reaction may absorb the shorter wavelength (more energetic) light and undergo secondary and undesirable reactions.

As a example to illustrate the above, assume you want to expose a positive resist PCB and you have three sources of light from which to chose: 1) a tungsten lamp; 2) a fluorescent lamp with a maximum output at about 375-390 nm; and 3) a low pressure mercury lamp with an output at 254 nm.

1) The tungsten lamp will not work, because its wavelength is too long and the resist simply won't absorb it. Thus, there will be no chemical change.

2) The fluorescent lamp will work.

3) The mercury lamp won't work, because its UV won't penetrate the glass hold-down plate for positive film. You could use quartz for the hold down plate, but you would still have to find a polymer for the transparency film that will pass the 254 nm light. And even if you get the quartz and UV-transparent film, you still need to worry about unwanted secondary reactions in the exposed area. Maybe the initial products that are soluble in base, eliminate various functional groups or further polymerize to form polymers and other compounds that are not soluble in base.

As for your choice of 370 vs. 390 nm, either will probably work and any difference will be minimal. Check to find out what wavelength is optimal.

John