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304 North Cardinal St.
Dorchester Center, MA 02124
In my last post on Direct Green Lasers (DGL) I wrote about electrical power to lumens conversion. In this post, I am going to talk about the color range/space and how it is affected by the green wavelength used. The color chart above is a standard CIE chromaticity chart, often referred to as a “horseshoe plot” due to its shape, that plots all the possible colors in terms of an “x” and “y” chromaticity coordinate. The color wavelengths in nanometers (nm) are labeled on the outside of the horseshoe.
First of all, I am not going to give a deep scientific definition to color space, but rather try and give the reader some practical information to help in understanding the significance of the wavelength spec for green lasers. I apologize in advance to the serious color scientists as I am probably going to butcher some terms.
Humans are not very good at absolute color measurement as the human visual system is adaptive and relativistic. A wide range of wavelengths of light will look “green,” and this is particularly if you don’t see them side by side.
Laser light has very narrow bandwidth which puts it on the edge of the CIE horseshoe so to speak. It turn out that diode LED’s while as pure as lasers are still comparatively very saturated and would plot very near the edge of the horseshoe as well. A big exception to this would be so called “white LEDs” (and some other colors) that are actually made with blue LEDs stimulating phosphors).
Usually 3 primary colors with wavelengths that are considered, red, green, and blue are mixed to form any of the other possible colors (some systems use more than 3 color primaries). The RGB phosphors used in old CRTs were not pure color wavelengths and so these “primaries” where inside the horseshoe rather than on the edge like lasers. The TV standards for broadcasting grew up with these limitations set how what color could be represented. If you plot the 3 primaries used for standard definition television you the SDTV triangle. Also ploted is the newer HDTV standard which defined a slightly larger color space triangle.
The the lasers, I have also plotted the color spaces (triangles) assuming a 640nm red and a 460nm blue and then a triangle for each of 510nm, 525nm, or 532nm for the green. For a given set of RGB wavelengths only colors “inside the triangle” can be represented. Also if you follow an edge of the triangle it shows what color can be reproduced in between two of the colors assuming the 3rd color is off when mixing the the other two primary colors.
One thing immediately obvious is that the laser primaries are way outside the color gamut/triangles for SDTV and HDTV. While this means a wider range of color could be represented, it also poses a problem when using existing standard video and still image standards. For example if you have bright green grass in a video, the video signal will call for nearly 100% green, but if you use a 532nm green laser at 100%, the grass will look like it is glowing green rather than green grass. So if you want the grass to look right, you actually have to desaturate the green by adding red and blue to it to get to a point on or inside the HDTV/SDTV triangle if you want the image to look like it is indended.
If you have a “wide color gamut display” using lasers or LEDs then you need content that matches your gamut to take advantage of it. If you use the commonly available video and photos formats which were coded/compressed for small color gamuts you can’t take advantage of the full color gamut if you want the images to look like they were intended (and not an over-saturated glowing look)
Consider particularly the triangle made by the 510nm green lasers and notice how it cuts off the bright yellows and yellow-greens of the SDTV and HDTV color spaces. There is no way to mix 510nm “green” with 640nm “red” to give a good yellow. You have to have at least a 520nm green to fully represent the yellow within the standards. This a major reason why there is the push to have direct green laser wavelengths of at least 520nm or longer.
You may notice that any of the greens from 520nm to 545nm (much more than 545nm and it starts cutting off some of the blue-green areas) will give a larger color space than HDTV. But if you go back and look at the photopic response curve from part one (copied below) you will see that as the wavelength goes from 510nm to 555nm, the lumens per Watt improves. For example, if the wall plug efficiency (WPE) was the same you would get nearly double the lumens per Watt at 532nm that you would get at 510nm. Since 532nm is the common wavelength targeted by frequency double green lasers I tend to “derate” the WPE of shorter wavelength green lasers by their difference in lumens per Watt. So a 525nm greens efficiency would be multiplied by 542/603=90% to get its effective WPE compared to a 532nm green laser.
One more thing on the CIE chart at the top, you will the “black body curve” in the middle of the chart numbers on it like 6500 or 10,000; these are the so called “color temperatures” of a black body is heated to the given temperature (in Kelvin). A 6500 “white” is a little on the red side (also known as “warmer”) where a 10,000 “white” is a little slightly blue (also known as colder) which to the human looks “whiter than white” (and why some detergents put “bluing agents” in them). “D65” is a common standard “white” that is very close to 6500 but slightly off the black body curve. In the industry it is known that most westerners tend to prefer warmer colors toward D65/6500, whereas people living in Asia seem to prefer the cooler colors such as D93/9300 or even 13,000 Kelvin where the “white” has a clearly blue tint to it (I haven’t seen a study as to why).
It turns out that the target color temperature and the wavelengths of the red, green and blue will set how much of each color in Watts you will want. If for example the color temperature is set for 6500, it requires will need somewhat more red but if you want 10,000, it requires somewhat more blue and green.