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Blacklight Bulb. Yep, that's the secret ingredient! Blacklight bulbs are made of a type of glass called Woods Glass, which is used as a pass filter for UV and IR radiation. Be careful when going out to purchase your blacklight bulb, however: there are many fake blacklight bulbs on the market that are just painted, and you'll be wanting the incandescent blacklight bulbs, not the fluorescent type!
If you want something a bit less crude, i.e. less curved, you can also purchase flat pieces of Wood's Glass relatively cheaply online.
Sandpaper. Those sharp glass edges hurt. If you want smooth edges on your glass, especially for inserting it into an old filter, then sandpaper's the way to go. If you're not planning on making an attachable filter, you can also use tape around the edges to make sure you won't cut yourself.
Old filter. If you don't want to be holding this UV pass filter up to your camera every single time you use it, then you can add in an old disposable filter to the mix. This can be scratched or damaged—doesn't matter. If you're using an old UV filter, ironically enough, then you'll have to remove the filter for sure. Glass cutting utensil is also necessary to cut the glass to fit within precisely. You'll also need glue to fasten the glass in, preferably something that won't expand too much as it dries like hot glue.
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Your first step is the most dangerous: breaking the glass. You need large pieces, so it's better to start by snapping the neck and going from there. Yes, you will end up with curved pieces, but it doesn't matter: as long as it covers the lens. If it doesn't, then you'll have to do some creative cropping in your post processing work. Also, depending on the bulb, there may be poisonous mercury vapors within it. You should check beforehand (a warning label should be on the packaging), but regardless, you should break the actual bulb in a well ventilated environment.
After that, it's up to how professional you want your set up. If you want to make a proper filter out of it, instead of just holding it up to the lens every time you want to take a shot, then you'll need to make yourself a filter. Take that old filter and remove the old one out of it, cutting and sanding as necessary. Once you've got that down, sand and cut the glass to shape so that it will fit snugly within the filter. With this complete, place the glass within the inside of the filter and apply the glue, wiping off any excess and making sure that it dries in a vertical position.
If not, then just tape around the edges of the glass so you won't cut yourself in the process of taking UV photographs.
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Using The UV Pass Filter & Variations
You know that purplish glow you get when you use a blacklight bulb normally? A small amount of visible light will also make its way into your photos using this DIY method. Many UV photographers choose to keep this in their photos, though others prefer to adjust the colors with some photo editing software, or have a custom white balance for their UV photography.
Since blacklight bulbs use Wood's Glass, there will also be a small amount of IR light passing through to your camera lens—so, if you want to be improving the “UVness" of your shots, then adding in some sort of IR blocking filter will help things along. A major source of IR radiation is the Sun - so if you don't shoot during the day and use another blacklight bulb to light the scene, you can further improve performance.
Nor does will this capture the full range of UV radiation—as you might have guessed, only the wavelengths that are closer to that of visible light. Only the most expensive of filters can capture anything close to the full range, and even then, that requires extremely specialized equipment.
Also, many modern lenses block a good deal of UV light already in having built-in UV filters (which will obviously block the UV light you're trying to isolate.) So, if this doesn't work for your camera, try it with a friend's camera. Don't make great demands of sensitivity, either: this is a rough DIY trying to approximate a very specialized and difficult technique.
Otherwise, if all goes well, you'll be exploring a whole new world of light that just isn't visible to the naked eye—all through the lens of your camera.
For the inspiration for this article, in addition to some sample pictures, check out this Instructables article.