A walk in a forest

Let’s go back to my story about the black and white TV set. Representing colors on a TV definitely adds a dimension – but are the colors accurate? A clever guy named Pointer measured a lot of colors found in ‘everyday’ nature, and derived what’s today called ‘Pointers real-world color gamut’. Refer to Figure 1 below (and remember that in fact it’s actually a color volume). It doesn’t mean that colors beyond these don’t exist – think for a moment of lasers – pure wavelengths. It’s just that we don’t really bump into lasers when we go for a walk in a forest… The holy grail of display technology would be the ability to faithfully represent each and every color that is in nature (provided it can be captured correctly by a camera – but that’s a different story).

The color gamuts of early television were defined by the practicality of the phosphors used in the CRT television sets. So we have color gamuts such as ‘Rec.601’ or ‘SMPTE’ for TV.

Then came HDTV and with it a newer color gamut standard (a kind of mix between the EU and US standards for standard definition (SD) television, still driven by CRT phosphor availability). This became the now famous ‘Rec.709’ color gamut – which is the same as RGB used in the graphics industry.

Next came the age of digitization of film projectors. Because of the properties of the film, one could not express all the colors simply by three points. But all digital projectors worked only with three primaries – so a sufficiently wide color gamut was needed to represent most of the colors captured by film – and again, economical, practical etc. to make. Next up was the ‘DCI P3’ color gamut. This is currently the standard for digital cinema projectors.

And recently, with the advent of new technologies such as OLEDs, quantum dots and laser projectors, an even wider color space is being proposed – the so-called Rec.2020 color space.

The chart in Figure 1 shows the three most current color gamuts in the industry today, together with the Pointer color gamut of real-life colors. The Rec.2020 target primary gamut includes most of the Pointer colors – so in theory, such a display would be capable of faithfully producing them, which was probably a driving force in defining this gamut in the first place.

Figure 1 – A (u’,v’) representation of three common color gamuts: Rec.709 for HDTV (the smallest triangle), DCI P3 for digital cinema projectors (the middle triangle) and Rec.2020 for next generation UHDTV sets (widest triangle). The Pointer gamut of real life colors is contained by the wobbly line.

The price of color

That’s it for the theory. Let’s get practical now.

One of the key things we have learned is that not all colors (wavelengths) have equal brightness at the same optical power: 555nm green produces the most lumens per watt, blue and red produce far fewer. So we need to strike a balance in terms of going ‘crazy’ in terms of wide color gamut, and frying the projector. The closer the green is to 555nm, the more lumens it makes. The shorter the red wavelength (the closer to orange) and the longer the blue wavelength (the higher up on the chart), the more lumens the projector will have as well. 

For example, to achieve the DCI P3 color gamut, we need a green wavelength of 547nm or shorter –547nm would be ideal in terms of lumen/watts. The red needs to be 616nm – or longer but preferably not as that decreases the lumens – and blue needs to be between 455-465nm – 465nm thus – to fall within the DCI tolerances. This is all summarized in Table 1 and Figure 2.

Table 1 – A summary of required laser wavelengths and resulting power to make 60.000 lumens with the chosen color gamut and white point.