Gels & Diffusion – Everything You Should Know – Part 2

In a perfect world, every light source would cast the perfect color onto your scene, but in reality, the color temperatures of the light sources in your scene can have a wide range, and this can cause color balance issues in your footage. In this segment we talk about the basics of color temperature, and how to use color correction gels on your light sources to create a balanced scene. Understanding how varying color temperatures affect your scene, and learning to correct the imbalance will have your scenes looking good.

Color temperature is an important concept that all video producers should have a firm grasp on, so let's tackle a quick overview. Every source of light has a color temperature.  The color temperature of a light source is measured in degrees kelvin, and can range from 1700k for a match, to upwards of 10k for skylight.  The lower temperatures produces an orange color cast, while the upper temperatures produce a blue color cast.  Professional lights are designed to operate at specific color temperatures. A tungsten light operates at 3200K, which produces an orange cast, while daylight balanced lights operate at 5500k or 5600k, which produces a blue cast.  Some LED lights even have adjustable color temperatures that cover that whole range. Your camera must shoot at a specified color temperature, and setting the white balance properly counteracts the color cast so your footage looks natural.

Of course in the real world, you'll often find that a scene has mixed color temperatures, and this can create issues with the color balance in your shots. This is where color correction gels come into play. Color correction gels typically come in increments of eighth, quarter, half, three quarter, full, and even double versions in order to change the temperature of a light source by varying degrees. There are 4 common types... color temperature orange, or C-T-O, color temperature blue, or C-T-B. Plusgreen, and minusgreen.  CTO gels are used to decrease the color temperature of a light source. The most common gel used for this is the ¾ CTO, which converts 5500k light to 3200k. 

In this example, we have a 5500k light on the right, and a 3200k tungsten light on the left.  Our camera white balance is set to 3200, so the 5500k light has a blue cast to it.  Now let's add a ¾ cto filter to the 5500 light.  As you can see, the blue cast is gone, and the lights are balanced.  You can even cover entire windows with these gels to convert outdoor light to 3200k. CTB gels are used to raise the color temperature of a light source.  The most typical gel used here is a FULL CTB, which converts 3200k light to 5500k.
In this example, we have sunlight coming in from the windows, and a tungsten light being used with neutral density filters to fill the right side.  Our camera white balance is set to 5600, so the 3200k light has an orange cast to it.  Now we'll use a FULL CTB filter on the tungsten light.  As you can see, the orange cast is gone, and the lights are balanced.

It's important to know that tungsten lights, by nature, don't produce a lot of light on the blue end of the spectrum, and CTB gels can significantly cut down on light transmission. Our uncorrected tungsten light had an ND-3 gel that we removed before adding the full CTB gel. The CTB gel we used reduced the light by 1.5 stops and only let 36 percent of the light through, so you'll need to take that into account whenever you use gels. 

While professional fluorescent lights have come into their own, consumer fluorescent lighting still has it's own unique challenges, and using the right gels can help combat them. There are many different types of consumer fluorescent bulbs. The color temperature can range anywhere from 3000k to 6500k depending on the bulb.  One thing they all have in common is a spike in the green color range that can cause color issues with your footage. Plusgreen filters are applied to daylight and tungsten sources in order to match the green spike that occurs in fluorescent light.

In this example we have a cool white fluorescent overhead that's around 4100k, and a tungsten light below.  With our white balance set at 4100k, our tungsten light gives an orange cast that doesn't match up.  Now lets use a plusgreen gel to add the green spike, along with a ½ CTB filter to raise the color temperature to 4100k.  Now that our scene is balanced, we can simply remove a touch of green in post production.

One other way to use a Plus Green filter is to bring out the color of plants in the background of a shot. In this example you can see the plants don't exactly jump off the screen.  Now we'll switch on the light to show the difference.  It definitely brings out the green nicely.
Of course, in the previous fluorescent light example, we could also use gels to modify the fluorescent light to match our tungsten light.

Minus green gels are used to remove the green spike found in standard fluorescent bulbs. In the same scene, we can place a minusgreen gel over the fluorescent light, and use a ¼ CTO to take the color temperature down from 4100 to 3100.  As you can see, the scene now looks nicely balanced, with no green spike. Minus green gels can also be placed over a backlight in a green screen setup to reduce the green spill from the wall on your subject.

Great lighting is one of the cornerstones of producing high quality projects.  Knowing how to use gels properly can help hone your lighting skills, and prevent those nasty color temperature issues in your footage. Thanks for watching.