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The shortcomings of using only CCT in film & photography lighting

Many lighting professionals will obsess of color temperature, commonly abbreviated CCT, because it is one of the most prominent color specification for lighting products, and one that is also most obvious at first glance.

Color temperature is a single number that only gives us information about where on the blue-yellow axis a particular color point lies. In fact, it gives us no information about the green-magenta axis. This information is typically much harder to find in product specification sheets, but for film & photography lighting, can be even more important than color temperature.

Chromaticity is two-dimensional

Almost all lighting color is described as having a particular color point. This point lies in a two-dimensional space commonly referred to as a chromaticity chart, with the "CIE 1931" being the most commonly referenced.

These charts are on a coordinate plane, with an x and y axis. Similar to a geographical map that uses longitude and latitude, each color point has a specific pair of color coordinates. For example, in the center of the chart is the white point D65, which has the (x, y) coordinates of (0.3127, 0.3290).

When referring to this white point, (0.3127, 0.3290) sure doesn't roll off the tongue and is a lot of digits to remember! This is why color temperature is more commonly used, because it's a bit more intuitive and easier to communicate. For example, instead of (0.3127, 0.3290), we can simply refer to this point as having a color temperature of approximately 6500K.

Color temperature, however, limits us to a range of colors that is confined to the black body curve. A light source, however, can have a color anywhere in the chromaticity chart. It can be very close to the black body curve, or it can be very far from it, but because of the way color temperature is calculated, it could still have the same color temperature rating despite having a very different color.

Geographical map analogy

Let's go back to our geographical map example for an analogy. Although the most precise way to describe to a friend where you live might be to provide longitude and latitude coordinates, this certainly isn't the most intuitive or convenient.

Instead, as a normal person, you might use nearby freeway exit numbers (especially in New Jersey) or landmarks to describe where you live. This is similar to why color temperature is used in favor of chromaticity coordinates - we've all developed a rough idea that 2700K is roughly an incandescent bulb color.

But the problem with color temperature is that it's a bit like asking "what is the nearest freeway exit to your house" without asking how far it is from the freeway.

A proactive and helpful friend might say "it's just a half mile from exit 50" but rarely do you hear a lighting manufacturer say "our color point is within 0.002 Duv of 3000K" - they'll simply just give you the color temperature value.

The result? Lighting manufacturers can be technically compliant with their color temperature specifications even with wild green and magenta shifts in their lights, because color temperature does not explain anything about the green-magenta axis of a color point.

Correlated color temperature and iso-CCT lines

Ever wonder why color temperature is abbreviated "CCT"? The answer is that the full acronym is Correlated Color Temperature. Correlated refers to the fact the color temperature value is simply the most comparable, or correlated, value to a particular color point. It certainly doesn't say how or to what extent it is correlated.

Another graphical way to understand this is by looking at iso-CCT lines, which are lines in a chromaticity chart that show color points that all fall under a particular CCT value.

In the chart above, the 4000K iso-CCT line is shown in red. All color points along this line are considered 4000K CCT - including those near the top of the chart with a serious green-yellow hue, as well as near the bottom with a clear magenta tinge.

What should we do?

Color temperature is a useful metric and it's here to stay. The metric itself is not flawed - it simply lacks an additional dimension of necessary information.

Our recommendation is to ask lighting manufacturers for information on Duv values or tolerances, as this sets limits on how much green or magenta shift can occur before a light source is considered nonconforming. For example, see below for Waveform Lighting's FilmGrade specification:

A Duv of 0.000 indicates that a light source is exactly on the black body curve. A positive value (e.g. 0.001) indicates that it is above the black body curve, or green, and a negative value (e.g. -0.001) indicates that it is below or magenta.

We've determined through some rough experimentation that Duv changes of 0.008 equates to a roughly one-quarter plus/minus green correction (CC075).

Waveform Lighting's FilmGrade products are specified to a Duv range of 0.0008, which is less than a 1/32 plus/minus green correction range. You can rest assured that when you purchase a 3200K product from us, you will get 3200K with Duv within 0.0000 +/-0.0008, every time.

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