How To Properly Choose A Color Filter

Back when theaters were lit almost exclusively with Tungsten-Halogen lamps, choosing gel colors was easy! In today’s venues, there is a healthy mix of fixture-types – especially as white LEDs find their way on stage. Hopefully this post will help you figure out the Rosco gels you need – no matter what light fixture you’re using.

Color filters create colored light by subtracting certain wavelengths from a beam of light. For example, a red filter absorbs all other wavelengths of light and only allows the red wavelengths to pass. Because the process is subtractive, it is important to choose your color filters with your source in mind. For example, conventional halogen lamps, fluorescent lamps, arc lamps, and white LEDs all emit different amounts of light across the spectrum. Some, like a halogen lamp, will emit at least some light at all of the wavelengths. Others, like fluorescent lamps, arc lamps and many white LEDs have various gaps and spikes in their “spectral distribution”.

Note the smooth curve of the incandescent vs the spiky/gappy curve of the cool white LED

Here’s a fun experiment: get out your Roscolux or Supergel swatch book and locate R39 Skelton Exotic Sangria. Then get two flashlights – one traditional incandescent, and the other a modern cool-white LED. When you shine the traditional flashlight through the R39 swatch, and then do the same with a cool LED flashlight, you will clearly see the difference caused by the difference in the light sources. The R39 on the traditional flashlight is warmer & redder; whereas on the cool LED flashlight, that same gel appears cooler or bluer – closer to a purple. If you look at the incandescent and LED SPD curves above, you’ll notice how much more energy is shown in the red end of the spectrum for the incandescent source. You’ll also notice the big spike in the blue end of the spectrum for LED. That difference in emitted energy results in the differences seen with the gel.

R39 with traditional incandescent (L) and cool-white LED (R)R39 with traditional incandescent (L) and cool-white LED (R)

Use the above flashlight experiment to test as many of the other design elements as possible – model or renderings of the set, costume swatches, etc. Short of that – at least light up your hand to see how the colors look on a skin tone.

Overlaying the SED curve over each source helps explain the resultsOverlaying the SED curve over each source helps explain the results

Once you have a good sense of the transmission output of the source, you can use the spectral energy distribution (SED) curve inside your Rosco swatch book to get a sense of how well that filter will work with the light source you’re using. The curve represents the percentage of each wavelength is transmitted through the filter. For example, R39 transmits approximately 50% of the violet and blue energy of the spectrum, blocks all of the green and yellow, and transmits from 50-80% of the orange and red energy.

ColorFilter2bAll of our color data is available on Rosco myColor, which also provides larger, easier-to-read SED graphics. MyColor also allows you to view, sort, and compare all of our filters online. You can learn more about myColor in this previous Spectrum blog post. It’s worth noting that no screen representation of a color can accurately reproduce exactly what you will experience with an actual color filter on an actual source – no matter how advanced the display. Use myColor to research and collect a palette of colors first. Then use the flashlight+swatch book technique above to see how those colors work on costumes, paint samples, etc. as you and your production team design the show. Good luck and happy coloring!

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Wendy Luedtke

About Wendy Luedtke

Wendy Luedtke served as Rosco's Product Manager for Color and Lighting Products from the mid-2000's to the mid-2010's. As a seasoned lighting designer with theatre, live event, fashion and architectural experience, an Adjunct Instructor at New York University's Tisch School of the Arts, and the Chair of the Color Committee for the Illuminating Engineering Society, she brought her passion for colored light to each Spectrum blog that she wrote.