Color Qualities

When I evaluate the color quality of a light, some of the main things I look at are:
• Overall color quality
• Color temperature
• How continuous is the spectrum
• How well the light’s spectrum matches the standard
• Hue shift (green/magenta)
• Specific wavelengths and colors that could be problematic as well as spikes and valleys.

Of course there are also qualities such as brightness, flicker, spread, softness, and throw, but these aren’t color dependent and so simpler meters make more sense. (Some spectrometers and color meters do measure a few of these characteristics, but not all.)

While a larger discussion of these characteristics are beyond the scope of this article, here is a brief description of each.

Overall color quality

Various color perception / color indices were created that summarize how well a light renders color. These indices work by comparing specific color samples illuminated by the test light, then comparing them to the same color samples lit under a light standard. The result is a numbered rank with 100 being the best. These indices include CRI (Ra), CRI (Re) (also known as extended CRI), TLCI, CQS, and TM-30-15. (TM-30-15 Rg can go above 100; in that case the closer the value is to 100 the better the light.) We discuss these further in our article “Understanding the Complete LED Database: Testing Procedures.”

These indices are a great starting point when assessing a light. However, since they are only evaluating a few colors out of millions, and since the final rank is an average of the values of those few colors, these measuring tools can sometimes be VERY misleading. That is why it is crucial to understand what your are looking at so you can find any potential values that could be skewing the final numbers.

Color temperature

Correlated color temperature (CCT) involves evaluating the spectrum as a whole and plotting the resulting “white point” on a chromaticity diagram which also shows the black body curve (technically referred to as the Planckian Locus). The result is a number with units of Kelvin (K).

If your light has spikes or valleys (blue, green, magenta, etc), you may still feel like the light has a specific shade of color even when the camera has been white balanced. What you are seeing is real and can greatly affect your image. An example is LEDs, which currently all contain a massive blue spike, which gives all LEDs (especially daylight balanced LEDs) a blue “feel” even when white balanced. We’ll discuss this more in the section “Problematic Colors / Wavelengths.”

Continuous spectrum

Generally how even is the spectrum? Are there sudden spikes or deep valleys? Perhaps certain wavelengths are missing? There are gels on the market that attempt to remove spikes – such as Minus Green and Plus Green – however since gels affect wavelength regions and not only specific wavelengths, they have the problem of reducing too many wavelengths as they tone down spikes.

When dealing with lighting technology that is prone to major spikes (such as HMI and fluorescents) something to pay attention to is the optical resolution of your spectrometer. Certain spikes are so narrow that some spectrometers with lower optical resolution will not represent them correctly or may miss them completely. We noticed this with low quality HMIs and fluorescent lights when we were comparing different spectrometers. (See Accuracy section below.)

Spectrum Matching The Standard

While the color temperature involved plotting the light as a whole onto the black body curve, a lot of important aspects of the light were ignored in the process since that calculation involved averaging. So I like to see how well each wavelength matches the standard at that color temperature.

Some lights closely match the standard, while others match in only some of the regions. It’s important to notice which regions are not matching well in case your scene has colors then involve those wavelengths. An example is the cyan region with LED fixtures.

Something important to realize when looking at color spectra is the y-axis — the values going vertically — is a relative scale that goes to 1.0. The wavelength that has the highest intensity is set to 1.0, and the relative intensities of all the other wavelengths are based on that.

A lot that can be learned by comparing the spectra of two lights both before and after normalization. (Normalization means that the most intense wavelength from both lights is still set to 1.0 even if one light isn’t as bright as the other.)

When two lights are normalized, the emphasis is on how well the spectra match. When normalization is turned off, the emphasis is on how the intensity of each light contributes to lighting the subject.

You can also use this feature to see how much a gel affects a light, in which case you are comparing the light before and after adding the gel.

In this example of adding Lee Full CTO (green trace) to an Aputure 120d LED light (violet trace), you can see how much the color temperature orange gel affects the spectra. In the normalized graph you can see how the entire spectra was rebalanced. In the not normalized graph you can see how much the CTO gel decreases the intensity of ALL wavelengths. The longer wavelengths (orange and red) are barely filtered while the blues and green wavelengths are greatly filtered. This is why gelling a light decreases its brightness.

As a side note about adding Full CTO: when a color temperature orange (such as Lee CTO #204) gel is added to a light (in this case the Aputure 120d LED), it isn’t that “orange is added” to the light, it is that the entire spectrum is rebalanced, changing the relative intensities for ALL of the wavelengths so that the light better matches the standard for tungsten balanced light. (In fact, as you saw in the non-normalized graph, the opposite of “orange added” happens: everything but orange is subtracted.) If CTO only “added orange,” then only the orange region of the spectrum would change. But this would create a huge spike or hump depending on how specific the gel was. You can see this when an Orange gel (such as Lee Orange #105) is added to the same light.

Hue shift (duv)

Hue shift specifically looks at green or magenta shifts in your light. Spectrometers report this as duv, which is short for “delta uv.” (In science, “delta” refers to “change.”) The light’s “white point” is plotted on a chromaticity diagram. Then a line is drawn perpendicular to the blackbody curve to the light’s white point. The distance from the white point to the black body curve along this perpendicular line is the amount hue shift.

• Negative duv: there is a general magenta shift
• Positive duv: there is a general green shift

Notice I said “general”; green and magenta spikes can affect this value too but brings in a different issue. Correcting duv from general shifts may improve your green/magenta values AND your color quality. However if the shift is caused by green or magenta spikes, attempting to correct them can hurt your overall color quality because you are also affecting other wavelengths around the spike. (“Collateral damage.”) So you should understand if the hue shift is from a general trend or a spike/valley.

Problematic Colors/wavelengths

Some lights struggle to reproduce certain colors, or have spikes and valleys. As we saw above low quality HMI lights have spikes everywhere. Low quality fluorescents are even worse. The absolute worst are sodium vapor lamps.

As a result, specific colors can be very bad even if the rest of the colors from that light are great. With LED lights, saturated red (R9) and saturated blue (R12) are difficult to reproduce although LED manufacturers are finding ways to improve those values using special phosphors. I also look to see how big the blue spike and how deep the cyan valley is with LEDs. The green spikes from low quality HMIs can really ruin skin tones since it is hard to filter spikes.