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DV101: Making the Invisible Visible: Understanding Ultraviolet and Looking at Black “Light”

In recent columns I’ve discussed how infrared radiation affects digital sensors and showed how to combat IR contamination with specialty filters. This month we’ll jump elsewhere on the electromagnetic spectrum and talk about ultraviolet radiation, UV. It gets its name from the fact that the UV spectrum consists of electromagnetic waves with frequencies higher than those we identify as the color violet. These frequencies are invisible to humans but visible to a number of insects, birds and fish.

Ultraviolet radiation is what comes from the sun and burns our skin, giving us that awful sunburn in the summer when we forget to apply sunblock. The wavelength of UV rays is shorter than the violet end of the visible spectrum but longer than the X-ray, covering a range from about 10 nm (ten one billionths of a meter) to 400 nm, just to the edge of light. The very short end, from 10 nm to 200 nm, covers vacuum UV, extreme UV, hydrogen Lyman-alpha UV and far UV, none of which are of real interest to us as they have no photographic applications.

From 100 to 280 nm is the range of ultraviolet C. This short-wave range of radiation is extremely dangerous to living cells. In fact, artificial lamps that emit UVC are used as germicidal instruments to sterilize medical equipment. A specific frequency of short-wave UVC at 253.7 nm is often used to purify air, water and food because bursts of it kill 99.9 percent of all pathogens.

It is UVB, the range between 280 and 315 nm, that causes the pigments in our skin to tan or burn. (UVB exposure also induces the production of vitamin D in the skin.) Exposure to excess UVB radiation can cause some forms of skin cancer, blindness and cataracts. The lamps in a tanning bed emit UVB, which will darken the pigments in your skin. The goggles you have to wear protect your eyes from UVB damage.

Actress Grace DeSilva with UV makeup partially exposed to UVA and partially not.

It’s UVA, the range of UV from 315 to 400 nm—the long-wave range that’s the closest to visible radiation—that’s of interest to us here. UVA, affectionately known as “black light,” is the range we can utilize in photography for nifty effects.

Black “light” is, of course, a misnomer. We know that “light” defines only the range of the electromagnetic spectrum that is visible to human beings. (Visible light falls between about 380 and 760 nm.) Anything beyond that, whether infrared or ultraviolet, is radiation, not light.

While human eyes can’t perceive UV directly, UV radiation can reflect off certain objects and create visible light, which is called “fluorescence.” It’s this fascinating attribute of UVA that makes such cool effects.

Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. Fluorescence (which may be caused by X-rays, UV or light in the visible spectrum) is the most remarkable when the absorbed radiation is in the UV region, and thus invisible, and the emitted light is in the visible region.

Some chemical elements are sensitive to UV radiation. These elements, when a photon of UV radiation comes in contact with them, absorb some of the UV energy and emit radiation that has a longer wavelength (lower energy) than the absorbed radiation—the emission in this case is in the visible range of the electromagnetic spectrum. In other words, when these objects are struck by UV radiation, they fluoresce, or emit visible light. Materials that are not sensitive to UV radiation will appear black under UV.

Fluorescence in Action

Angstrom UV fixture

Recently I directed a small spot, working with cinematographer Ashley Barron, that utilized black light effects. During prep, we spent an afternoon at Wildfire FX in Torrance, Calif., testing out various materials and makeup with makeup artist Desiree Falcon and stand-in actress Laura Manchester, who volunteered to be our guinea pig for the day.

It had been a number of years since I had worked with black lights. The testing was certainly eye-opening, especially working with a digital sensor on my camera as opposed to film. It’s fascinating to see how how different materials fluoresce under black light. Materials that look like they’re identical under normal light can react very, very differently under black light. The only way to really know what will work and what won’t is to test. Test, test, test.

When Laura showed up for the test, she was wearing a white sweater over a white tank top. In normal light, the white values were nearly identical. Under the black light, the sweater had no UV sensitivity at all, but the tank glowed so brilliantly that it was nearly blinding. There’s no way to know until you experiment.

On the day of the shoot, we had the actress, Grace DeSilva, bring in several clothing options for us to test under black light to see which responded best. The results were surprising. Certain materials I thought for sure would fluoresce went black, and others that I would have otherwise dismissed shined brightly!

A shot from the monitor showing the effects of the UVA light on Grace’s makeup.

Additionally, there’s no way to know how vibrantly certainly materials and colors will fluoresce until you test them. Making things even more complicated, digital sensors see vibrant, saturated colors differently—especially with a Bayer pattern sensor like the EPIC, which we used on this project. Certain colors that were crazy vibrant to the eye would appear flat and underexposed on the monitor. Again, the only way to know for sure is to test, test, test and test again!

Most black light fixtures, even the most efficient, emit visible light, and this short-wavelength violet light can look fairly ugly on skin. Incorporating white light can help clean up skin tones. In our case, we avoided white light but incorporated some heavily saturated gels in the background and as edge lights to add more color depth and help eliminate the sickly blue skin tones from the black light.

Another confusing factor of black light is that you can’t think of your intensities like you can with traditional lighting. Although black light fixtures are rated in wattage just like typical fixtures, the wattage on a black light refers only to power consumption, not “light” output. In fact, the output of the fixture itself has significantly less to do with the overall effect than does the material that is fluorescing under the UVA radiation.

We used Kryolan makeup from Wildfire during the spot. Kryolan offers makeup specifically designed to fluoresce brilliantly under black light.

The final look, with speckles of UV makeup all over Grace’s skin. Note how the reflection of visible light from her eyelashes actually lights the skin around the lashes as well.

Motion picture film stocks are sensitive to UV radiation, so when shooting film, it is necessary to incorporate a UV haze filter. Although that might be counterintuitive, the filter is necessary to keep the image from “fogging” from too much UV light. With digital sensors, this is not an issue and the UV filter doesn’t have an effect. I would have liked to have tested out a Hot Mirror IR filter, however, as most UV fixtures also emit a large amount of IR light, but that will have to be an experiment for another time.

A spot meter is really the only way to measure exposure off of a fluorescing object when dealing with film—but in the digital world, we can see the image right on the screen and use waveform and histograms to help moderate exposure.

On our shoot, we generally set the exposure to the effect and lit the backgrounds to match with that exposure. For the most part it didn’t matter how far the UV fixture was from the subject, only that it was as near to the lens axis as possible for the best effect. As Grace turned away from the UVA beam, the effect would disappear very quickly.

There are lots of fun things you can do with black light materials. For example, several companies manufacture “invisible” black light paints that cannot be seen under normal light but fluoresce brilliantly under UV light.

For this project, it was fun to dabble a bit with black light. If you decide to play in the UV world, be sure to test as much as you can before shooting. It’s the only way to really know what the effect will be on the set.