Blacklight Basics: The Truth About “UV Filters”

by Wildfire on February 16, 2012

How a Wildfire 400W Long Throw UV fixture (left) stacks up against top of the line 1200W to 2000W moving light fixtures equipped with a "UV Filter". (Click to enlarge.)

Moving light fixtures often have a “UV Filter”, presumably to create a UV effect. But do they really work? Is it possible to replace a dedicated UV fixture with a moving light fixture equipped with a “UV Filter”?

In April 2009 we tackled this issue in an article published in Lighting & Sound America entitled “The Truth About UV Filters: Many moving lights feature UV filters, but are they the real thing?” (Click here for a PDF copy of the original article, which includes technical details.)

Since this is still very much a relevant issue, we decided to devote this blog post to the issue of so-called “UV Filters” in today’s moving fixtures, and why they aren’t what they’re cracked up to be.

We get the occasional call from a distressed customer fretting that our “paints are not working.” The first thing we ask is what kind of fixture they’re using. In nearly every case, the fixture is to blame.

A good, solid UV effect is dependent both on a fluorescent material (usually paint) and good quality UV light (“black light”).

Wildfire Long Throw fixtures use a mercury-halide lamp, which is specially designed to produce maximum UV. Since it also produces a lot of visible light as well, it is filtered by an optical grade dyed borosilicate filter glass, which filters out most visible light, leaving only UV with a peak wavelength of 365nm.

Most moving light fixtures use a high-output discharge type lamp, which produces a broad spectrum of light, which is in turn filtered through a “UV Filter” that allows mostly UV to pass through. It is then passed through another glass or plastic lens before leaving the fixture.

Notice how the presence of an additional lens filters out the UV filtered by the UV filter! (Click to enlarge.)

There are two critical flaws with this design that keep these moving fixtures from being effective at producing good quality UV effects…

One, the lamp is not optimized to produce maximum UV. This makes sense because the fixture is designed to perform many functions, most of them in the visible light range. That means compromises have to be made when accommodating UV.

The second problem is the existence of the second lens. Glass and plastic lenses filter out UV light! So the net result is a purple wash of light close to the 400nm range (the dividing point between visible violet and UV light)–not an effective form of black light.

This results in a very poor effect. The reason is because certain UV paint pigments will not respond to 400nm light. Others will weakly fluoresce, and the rest will shift toward purple. White, for example, tends to be more “purple” than true white at 400nm. Red will shift toward pink because of the presence of purple light.

Our measurements determined that Wildfire’s 400 W Long Throw Flood resulted in more than 24 times the UV irradiance compared to a top-of-the-line 2000W fixture equipped with a “UV Filter”. (See the above graph, and the original article for details.)

That’s a pretty significant difference!

So far, there is not a viable solution that will replace a dedicated UV fixture. And Wildfire has the most powerful UV fixtures available… which are about to get even more powerful! (Stay tuned for details on this exciting new development.)

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