# Fairly Broad Spectrum Light Source Options

I tried this sight before to find filtering options and got fantastic results, so let's try again!

I am setting up an experiment that requires light of two different frequencies (445nm and 350nm). The light ultimately needs to be focused on a small area. I can think of two good ways to make this happen:

1. Get a broad spectrum light source that emits over this range - maybe a halogen lamp? - and buy two filters: A high pass that cuts off at ~420nm and a low pass that cuts off at ~380nm. For the low pass, I may also include a high pass in order to make a band pass (or just flat out buy a band pass filter).

I will then simply wrap the filters in a black paper cone to "focus" the light (for this experiment, light spread after hitting my target doesn't seem to important). We would use a power meter to tell us what flux is actually hitting our experiment.

My questions for this approach: What kind of broad spectrum source would you recommend? Can you recommend a retail outlet for this source? Our grant ain't so grand, so afford ability is important. Can you think of a better way to focus the light after it passes through the filters? Again, any retail sources would be appreciated.

1. Get two more highly focused sources. Given our small area focusing requirements, we are looking at lasers (Again, 340nm and 445nm). I have found a couple of retail sources that have such lasers, but they are fairly cost prohibitive. Do you know of any sources that sell such lasers - power sources and all - for <500?

Thank you in advance for any help and recommendations. If a paper is produced from this - which I very much am counting on - I will thank the guy who provides the best answer in it (if that is an incentive).

-
You can calculate the 340 nm emission of a halogen lamp by Plancks formula. (I think there will be a very weak emission!) – Georg Apr 2 '11 at 9:29

I haven't checked exhaustively, but I don't think you'll get a laser for under 500 (dollars?). I would recommend a LED light source. Here is a page with an example; they're more than $500 but less than $1000.

I seem to remember that our lab got a cheap fiber-coupled white light LED source for 300 euros, but I can't remember the brand right now and I can't find it online.

Perhaps "black light" fluorescent tubes would meet your needs - the article on Wikipedia has a table listing typical emission peaks depending on what kind of phosphor is used in the tube. I have no idea where the best place to buy those is, but a Google search seems to show that they're available everywhere, perhaps with varying quality.

As for a better way of focusing, it's hard to say without knowing more about your requirements - apparently you don't need temporally coherent light, since you're considering not using a laser, but do you need spatially coherent light? You say light spread doesn't matter, so I assume not. Do note that by "focusing" with a black paper cone, you'll be throwing a lot of light away; perhaps you might use lenses. Microscope objectives are expensive, but perhaps if you can "borrow" one from a microscope in the area, you could use one to focus.

-
Thanks for the response. I think I may have to wind up borrowing from other groups to get the job done. I have only been able to find one laser source for under 500. It's a Chinese company called CNI. The problem is, my department will only allow us to spend grant money on companies that they already have accounts with. So, due to this bureaucracy, I not only have to find a company with affordable lasers or LEDs, but one that is popular as well! It's funny, but these days I'm learning more about how physics departments run than about physics. – pballjew Apr 3 '11 at 18:04

Can you be more specific about what you are trying to do, especially your power/focusing requirements, and the wavelength sensitivity of your experiment?

350nm LEDs are available from this company: https://dcomponents.com/?keyword=350nm&content=results

445nm LEDs are also widely available, and at this wavelength high powered laser diodes which can be focused tightly can also be obtained: http://www.lasersurplusparts.com/shop/80-1w-445nm-laser-diodes.html

For 350nm laser light, your options are much more limited. HeCd has a line at 325, and N2 lasers emit around 337. Pulsed N2 lasers can be built fairly cheaply and easily but are not very coherent.

-