[Education] Light Color Metrics

Understanding color metrics and how to use them for production

When working with any light source in motion picture production, a technician should understand its properties from two perspectives: Radiometry, or the measurement of the light itself as spectral energy; and Colorimetry, or the measurement of the color response for people and cameras unique to that light energy.

The two most gaffer-relevant color meter readings are:

1. Spectral Similarity Index (SSI), which provides a radiometric measurement, and
2. TM-30, which provides a colorimetric measurement.

To explain these two measurements and how to make use of them, please look at these two different versions of 3200K light.

The first spectrum of 3200K light (rather, the "color" of 3200K) comes from the amaran 200X S light fixture. The amount this light matches actual ideal 3200K blackbody light spectrum—as seen by most cameras—is indicated by a reading of SSI[P3200] 89. In other words, only 11% of this spectrum deviates from actual 3200K spectrum.

This direct spectral comparison with SSI cannot be "tricked" or "gamed", since it compares the actual spectral fingerprint, not the perception and interpretation of this spectral fingerprint.

But, how does this knowledge actually apply to anyone? Well, TM-30 uses 99 color patches and advanced color science models to predict how this spectral difference of SSI[P3200] 89 can lead to color shifts resulting from that light. Although TM-30 metric models human color response, not camera color response, the general predicted color trends tend to reasonably correlate with camera responses. This correlation can provide a reasonable confidence within the "Color Vector Graphic" (CVG) below to predict object color shift response:

The Black Circle in that TM-30 graphic is the baseline color. The "pie slice" regions 1-16 refer to a general color region, and the Red Circle shows what happens to each color, as indicated by the arrow: inward / outward shifts indicate desaturation and saturation, and left / right movement (relative to the point) indicates the hue shifts.

Since all colors have minimal shifts, as expected given the high blackbody SSI score, the amaran 200X S fixture output will be fine for most, if not all situations needing 3200K blackbody light.

The second example is an RGB LED display tile from a virtual production stage. As we can see, it's very "spiky"—the amount this light matches actual ideal blackbody spectrum at 3200K, as seen by most cameras, is indicated by a reading of SSI[P3200] 24. In other words, 76% of this spectrum deviates from actual 3200K spectrum.

The TM-30 CVG shows what this light does to object colors. Skintone (section 4) and orange objects (section 3) heavily shift in color. Skintones desaturate and hue shift to orange. Orange objects oversaturate and hue shift to red. Green and Cyan objects oversaturate, Blue objects hue shift to cyan, etc.