Having given an introduction to the concept of colour saturation in another blog, I would now like to take a look at its implementation in products. For the sake of simplicity, in this post I'm talking about "enhancing" colours - meaning increasing the saturation of colours. The effects that can be achieved with increased colour saturation are very desirable in some lighting situations.
There are a variety of colour enhancing LED sources on the market, but as far as I can fairly judge the competitors' products, I have to say that I am not impressed with what I have seen so far. In short, these products often have an unpleasant way of saturating colours while producing a pinkish hue of light. We are familiar with these lights from meat and sausage counters in various supermarket chains, for example. I suspect that this has a lot to do with the outdated colour science tools used to develop these products.
The aim in developing such light sources should be to enhance the colours "just right" so that they look pleasant but not unnatural. To date, the most widely used tools for this task are the CRI for colour fidelity and the Gamut Area Index or GAI for colour saturation. Manufacturers aim for a fairly high GAI value, while not setting the CRI too low. Unfortunately, both of these instruments have some weaknesses. I have already discussed the shortcomings of the CRI in a previous post; however, in today's context, it is the characteristics of the GAI that are most important.
The first difficulty is that the GAI is only a single number that tells us the average colour saturation, but not which colours are saturated. As we will see in a moment, this is simply not sufficient information. The second problem is the use of outdated colour science - in particular, a very inconsistent colour space that overestimates the saturation of some colours, especially in the blue range. Finally, the CRI and GAI were developed independently of each other and do not use the same basis for calculation, meaning that the fundamental trade-off between colour fidelity and colour gamut or saturation cannot be accurately assessed.
The practical consequence of these factors is as follows: The easiest way to increase the GAI while maintaining the CRI is to add many blue wavelengths to a spectrum; this produces light with a pink colour value, which increases the saturation of yellows and blues and drives up the GAI value immoderately. Although the resulting light source receives a high rating according to CRI and GAI, it is not necessarily pleasant in practice. The question of chromaticity is a complex issue that deserves a separate discussion. In some cases, a tint below Planck's effective quantum is indeed pleasant; however, a metric that pushes you in that direction without good reason is problematic. Furthermore, yellow and blue colour enhancements and saturation enhancements are generally not very popular.
For a long time, the GAI was the only tool for evaluating the colour gamut, and its existence was nevertheless a stroke of luck despite these technical shortcomings. But today we have better tools at our disposal, especially those offered by TM-30, which remedy these shortcomings. Rf and Rg (which replace the CRI and GAI respectively) use a uniform calculation method so that the trade-off between colour fidelity and colour gamut can be accurately predicted. They also use a very uniform colour space so that there are no distortions for certain colours. In addition to Rf and Rg, the TM-30 colour field gives us valuable information on specific colour enhancements or enhancements and attenuations.
A first advantage of TM-30 is that there is less incentive to pull the colourfulness of the source towards a pink tone: Rather, we can manipulate the chromaticity and the colour gain independently of each other. In particular, it is possible to design a spectrum with a strong saturation enhancement while remaining on-planck.
Another significant advantage is that the colour field gives us the opportunity to clarify a crucial question: Which colours should we boost and to what extent? For example, consider the two sources in Figure 1. Both have a similar colour fidelity Rf = 80 and colour gamut Rg = 110, but they obviously have very different effects on the colours: one enhances the yellows, the other the reds and greens.
Figure 1: Two sources with the same Rf and Rg values, but very different colour distortions. Note that despite the identical Rg values, the second source has a much lower GAI.
One can now ask whether both sources would be subjectively rated equally well by users. As with everything to do with individual preferences, there is no systematic answer. However, research has shown that in practice we are most sensitive to saturation enhancements of warm tones (such as red, orange, pink and skin tones) - meaning that the second source in Fig. 1 is more likely to match users' preference. Note, however, that the GAI for the first source is much higher (because its spectrum happens to have a large blue peak): Therefore, if we are only guided by the GAI, we might wrongly conclude that the first source is probably "better" at colour enhancement.
Our human preference for saturating reds and warm colours has been known for some time, but not in a very sophisticated form. Recently, however, SORAA has investigated this in more detail in a research collaboration with Penn State University. Using the TM-30, the researchers designed a series of colour-enhancing sources, in which the direction of colour enhancement was finely varied, and asked observers in a restaurant about their preferences (Fig. 2). The results clearly showed that some objects (again, mostly warm tones) were the most important, and more importantly, that colour enhancements with a specific direction and strength were the most popular. This is where the fine granular information of the TM-30 is important. Using the colour field, we can understand colour distortions in great detail and distinguish between different "flavours" of colour enhancement. To illustrate this, Fig. 2 shows three enhanced spectra: all enhance warm colours, but in different ways, and the observers preferred one of the three.
Figure 2: Three sources analysed in the PSU/SORAA study. All have similar Rg values but enhance warm colours in different ways. In a restaurant environment, the source in the centre is the preferred one.
SORAA's new Enhance Snap filter, which transforms SORAA's Vivid high-fidelity LEDs into high-gamut lamps, is largely a result of this study. It was developed to enhance warm colours in the specific way that proved to be optimal. At the same time, the chromaticity of the light is maintained in a Planckian manner. This is invaluable in an installation where colour enhancing light sources are mixed with high fidelity sources: All light sources retain the same chromaticity, but some lamps can locally enhance the colours according to the lighting designer's intention. So taking my example of meat counters in supermarkets: no more pink-red islands of light, but consistent colouring with the same white point and still the desired colour enhancement of the red tones for the counter display. Finally, the SORAA Enhance Filter also retains the perfectly graduated reproduction of white tones that is familiar and appreciated from the SORAA Vivid LED. In my opinion, SORAA Enhance really stands out in the landscape of colour enhancing products.
I hope these explanations have given you a good idea of why trustworthy colour metrics are so important for researchers and manufacturers: We need unbiased tools that allow us to qualify the properties of light in an objective way to help us advance the research itself and the resulting new products. TM-30 is a milestone in this respect.
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