Metameric Failure Test with Two Reference Instruments
W-OLED, CCFL LCD, LED LCD
For this article, I want to do something a little different. I just obtained a new Colorimetry Research CR-300, so I thought that this offered a good opportunity to test something about which I have heard a lot of anecdotal evidence, but had never experienced directly: metameric failure.
What is Metameric Failure?
Metameric colors are two colors with dissimilar spectral distributions that, nonetheless, are perceptually the same. This occurs because the human eye is a tristimulus receptor that can interpret a wide range of spectral distributions in unexpected ways. Thus, metameric colors look the same, but exhibit different spectral responses. This is normal. What is not normal is metameric failure. This occurs when two colors measure the same, but appear different to the human eye. The most common source of metameric failure is differences in color perception across individuals. We simply do not all process color exactly the same, the most extreme example of this being color blindness.
However, there has been a lot of discussion recently among those who regularly engage in color measurements about metameric failure due not to differences in human vision, but in inherent limitations of the instruments we use to measure color. The most disturbing aspect of this for those who rely upon accurate color measurements is that high-end spectroradiometers are no less susceptible to this problem.
This is precisely the thesis I wished to test. In so doing I wanted to answer two questions:
1. Is there instrument metameric failure when measuring W-OLED, CCFL LCD, and LED LCD? I will use the CR-300 to test this hypothesis. The CR-300's exceptional optical resolution of 2nm makes it an ideal instrument for this sort of test. It offers twice the optical resolution of most reference instruments, such as the Photo Research 650/670, Minolta CS2000, and Colorimetry Research's own CR-250. The CR-300 operates, as far as I can tell, exactly the same as the CR-250 and it even looks like a CR-250, except that it is much larger and heavier. For those interested in the operation of this instrument, read the previous article on the CR-250.
2. Are there any significant measurement differences between the CR-300 and the JETI 1211 that might account for instrument metameric failure in one but not the other?
The methodology for this test is quite simple.
The two uncalibrated LCDs did not visually appear to have the same white point. In particular, the LED LCD appeared bluer to my eyes. When I measured them, the measurements bore out my subjective assessment. The CCFL LCD (Samsung LN-32B650) was a nearly perfect D65 (x0.3114, y0.3283, 72.64 nits/ R99.5%, G100.1%, B100.4%), but the LED LCD (Samsung UN-32B600) was too blue (x0.3123, y0.3205, 74.28 nits/R102.7%, G98.5%, B105.3%). This verified that my eyes were sufficiently sensitive to identify relatively small differences in the white point. If differences are present, then I should be able to see it without instrumentation.
After calibrating the LCDs and adjusting the light output, they measured very nearly the same.
Next, I matched the luminance of the W-OLED. It had been previously calibrated to x0.3136, x0.3293, 73.14 nits/R100.4%, G99.9%, B99.8%.
Finally, I visually inspected all three displays simultaneously in both a dark and well-lit room.
To my eyes the white patches looked identical. I saw no difference whatever. Clearly, if instrument metameric failure is a thing, it does not appear with these families of displays.
Next, I repeated all of my measurements. I did this because—although I was not seeing instrument metameric failure—I did see the displays drift somewhat. This is something I have seen many times before. Commercial displays are just not perfectly stable. The CCFL was the worst offender. Interestingly, although the W-OLED loses brightness rapidly when displaying a static test pattern for more than about a minute, once you wake it up out of its luminance dive, its color and luminance remained the most stable of the three.
Finally, I measured the same three displays using the JETI 1211. Here are the results.
Clearly, there was no significant difference between the white point as measured by the CR-300 and the JETI 1211. The specification for both instruments is xy ± 0.0015, which means that they could vary as much as xy0.003 and still be within spec. As it turned out, the largest variance was xy0.0017 and the average variance was xy0.0009, which is phenomenal consistency between two instruments, just what you would expect of a reference device.
There was one area of significant variance, and that was luminance. The JETI's luminance measurement was noticeably and consistently lower than the CR-300. However, since the luminance spec for both is only ± 2%, they could vary as much as 4% and still be within tolerances. In the case of the OLED, they were not even within this generous tolerance. When I remeasured the W-OLED with the CR-100 colorimeter it became clear that the problem was with the JETI. Its luminance calibration is clearly off and needs redone.
There is no doubt that instrument metameric failure is real. Sony went so far as to issue guidance on xy offsets calibrators should use with their BVM OLED broadcast monitors. However, these are RGB OLEDs, not the white OLEDs used in the consumer world. So too, RGB laser projectors seem to have a problem with instrument metameric failure, but these also are rarely used in the consumer display market, which generally uses a combination of a blue laser and a yellow filter to achieve all three primary colors. An RGB LED or quantum dot display could probably also pose problems, but these have been abandoned by the consumer world as well in favor of—like laser projectors—blue LEDs and yellow filters.
Fortunately, this test reveals that the problem of instrument metameric failure does not seem to arise on commercial displays. I did not test a CRT or plasma display as these technologies are now obsolete. I know that many people whose opinion I respect swear that they have seen this phenomenon, but all I can say is that I could not repeat it in a fairly careful test. There is little point engaging in debate about this, because ultimately it is about what you see. If someone sees what they believe is instrument metameric failure, how am I to respond? No, you didn't? That would be a silly thing to say. Clearly, people see what they see. I would only suggest that maybe, just maybe, what they are seeing reflects not a failure of instrumentation, but rather a natural variation in human color perception.