How to Test a UV Filter for Effectiveness

Not all UV filters are created equal. In fact many offer very little UV suppression. Here’s a simple test you can perform to gauge the effectiveness of any filter in suppressing UV radiation.

1. Locate or purchase a fluorescent “Black Light” in the form of a tube or compact fluorescent bulb.
2. Turn the light on in a darkened room and allow it to warm up several minutes.
3. Locate a piece of white fabric or other item that glows or ”fluoresces” under the light.
4. Place the filter in question between the light and the fluorescent object.
5. If the item no longer glows, the filter is an effective UV suppressor.

How it Works

It’s not the purple light you can see that causes the object to glow, it’s the UV radiation you can’t see. That’s why simply looking through the filter does not reveal any differences.

However, if the filter does block UV radiation, it won’t reach the object to cause it to fluoresce.

You’ll discover that many filters, such as polarizers, neutral density and skylight filters are effective in suppressing UV. However, these all modify the light in other ways as well. The purpose of a UV filter is to achieve this suppression without modifying the light further. The suppression of any blue cast and increase in contrast and clarity is not the result of modifying the color temperature of the visible light, but of suppressing invisible radiation that interacts with visible light, just as the invisible radiation produces visible light in objects that fluoresce.