# DIY UV-pass visible-cut filter

Hello Physics Stackexchange,

I'm currently working on a project with 1W 365nm UV LEDs, similar to this item. I'm using it to make some markers light up in a dark room. The problem is that aside of shining with the proper UV-A and creating expected lightning effects, the LED gives off a noticeable amount of visible light, which is not acceptable.

My question is, is there a dirt-cheap material that I can use in my project to block all/nearly all the visible light? I don't mind the filter cutting off a lot of the UV too, as I can increase power to the LED quite liberally. I can accept any DIY solutions and have access to a decently equipped workshop. Proper UV-pass camera filters seem far beyond my budget.

This is my first Stack Exchange question, so if there are any issues with it I'm eager to correct them once notified. Thanks!

• Have you tried an internet search using your title? – sammy gerbil Jul 30 '16 at 18:11
• How much you are willing to spend on this filter – hsinghal Jul 30 '16 at 18:17
• @hsinghal I'd say 5USD is the limit; as you can see in the question, money is an issue here. Googling returns only the "cut the blacklight bulb" solution which seems beyond the budget too. – Karzon Jul 31 '16 at 15:47

I tried to search the internet but chances are slim. One of the options is to use Scott's color filters.One of the options is Scott's UG11 filter. I dont think you can get it under \$5 but the price is ~\$25. However in the case of bulk order you can get lower prices or if your application permit you can cut this into parts to decrease the price.

Other option is to buy the phosphors yourself and coat it on the glass there are several references here. For example following phosphors will work for you. LiYF4:Pr 218 nm, YPO4:Pr 232 nm, KYF4:Pr 235 nm. However I must warn you that these phosphors may be highly toxic and reactive. Please read MSDS before using them.

regards

• Very informative, especially the phosphorus part. Thanks a lot, I'm waiting for another project which will allow me to try both your solutions. If you're curious what worked - sorta - for me in the end, read my own answer below. – Karzon Aug 2 '16 at 14:27

After a few days' experiments, I found a don't-try-this-at-home solution: I purchased a straight UV CFL for less than 2USD and broke it, then cut the glass shards and used them as my filter. It allows an amount of deep violet visible light to pass through, yet it's much better compared to the bright white glow of the unfiltered LED.

I found it quite surprising that rubbing the fluorescent coating off the inner side of the tube made it far less effective in filtering away the visible light - I was expecting the black (dark violet actually) enamel on the glass to do the trick. Still, it works.

Future readers: please DO NOT attempt to recreate this duct-tape-engineered filter for your application. Making it exposes you to all sorts of dangers: flying shards of glass, mercury and its vapors, possibly harmful fluorescent coating of the broken glass tube, and barely noticeable strong UV radiation. My setup allowed me to stay safe and decently isolated. I'm not responsible if you disregard the warning.