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”.
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.
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.
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.