Sony PVM-1741 (green tint)

[Joseph Owens]

Here is a note that was posted to the TIG earlier this month:

Why We Sometimes See Different Colours
Richard Kirk
6th January 2011

Several people have asked me recently why white on two displays of different types – usually a CRT and an OLED monitor – look different even though they have the same CIE XYZ when measured with a spectrometer. Here is my answer for OLEDs and CRTs: the same aruments can probably be applied to other displays.

* Comparing an OLED and a CRT…

The OLED red is a single broad peak, while the CRT red is a complex spectrum with two sharp peaks.

All video monitors used to agree ten years ago because all quality monitors were CRTs with the same primaries. If you went back 30-odd years and found NTSC primaries with the orangey-red phosphor then you might have seen two CRTs with whites that did not match. If digital projectors were the same size as CRTs then we might have noticed the whites did not match five years ago, but they aren’t and nobody did.

Today we have flat-panel monitors which look respectable next to a BVM CRT. We see sharp detail and good shadow tones. The obvious next step is to calibrate the monitors and see how similar they look. We match the monitor whitepoints in XYZ using a spectrometer, but the whites look different. Different people see different shifts. Why?

The only explanation is that the CIE standard observer is not accurate enough to predict what we see.

Can we improve on the 1931 standard observer? This is surprisingly hard to do. The CIE model is based on the assumption that we have three detectors in the eye sensitive to red, green, and blue, a bit like a digital camera mosaic. If two colours give the same signal at these three detectors then they should look the same.

The human eye is not really like a digital camera. We have a central region on the retina that we use for our precision vision, and we move this around to capture detail as we need it. The rest of our peripheral vision is very fuzzy and has a lot of chromatic aberration which our brain filters out. The central region has a yellow spot of dye over the top and no blue detectors underneath. The colour image we ‘see in our head’ is a weird synthesis of the sharp, central, yellow filtered RG image and the surrounding unfiltered RGB image. Different people have different densities and different sizes of yellow spot, and this affects how their brains merge their central and perhipheral vision.

The original researchers back in 1931 knew about this. They did colour matching experiments where you mixed red, green, and blue lights to match a reference colour. The two colours patches were halves of a circle with a dark surround. They saw they got different results with different sizes of circle, so they settled on a 2-degree circle – about a third bigger than your thumbnail at arm’s length – because this gave the most consistent results.

When you look at a white patch the size of a monitor, this is a lot larger than the CIE 2-degree patch, so the colour we see is influenced much more by the peripheral vision. This means we can get the paradoxical situation when we match all the colours throughout an image at the 2-degree level, but are left with an overall sense that the white does not match at a larger scale. The Arri image is particularly good as showing this.

This is not something that can be easily corrected. We can try colour matching experiments with larger circles, but our eyes move with subjects larger than 2 degrees so it is hard to be sure which part of our eyes we are using. There is a CIE 1964 standard 10-degree observer, but it is not a trusted standard. Also, different people see different matches, so one standard will not fit all.

* Can I grade on an OLED display?

The OLED is not the same as the CRT, but that does not make the OLED ‘wrong’ and the CRT ‘right’. Ten years ago we might have said that anyone looking seriously at an image used a CRT, and so a CRT represented the best working practice. Now CRTs are rare in home and grading suites, and the choice is much less obvious.

My advice is to use an OLED monitor or a CRT when grading. Either will work, but do not use both, and certainly do not put them next to each other. The OLED primaries are at least as good as the old CRT ones. The fault that creates the visual difference is not in the CRT or the OLED but because out eyes do not fit the CIE standard observer.

D65 white has a spectral definition: it was based on the spectrum of noonday sun at Rochester. However, most of us only judge whether our display matches to D65 by measuring XYZ and invoking the CIE standard observer. If we compare any display to true D65 white, it would probably not look like a match: true D65 has a lot more deep blues and violets than almost all display and projector whites.

* Why do we not see colour errors on other displays?

This is quite a recent thing. Ten years ago, people would not have expected to get a visual match on different media.

I have not seen a digital projector look as different to a CRT when displaying matched in-gamut colours, though some digital projectors have red spectra very similar to the OLED displays. However, we have always had to tweak the white point when doing butterfly tests, which is a similar error when comparing a digital projector to film. We also see similar differences: we can get almost all the image colours to a good match, but we still have an impression that the overall white point is different, with the film version looking more yellow-green and less bright.

I believe when we have two similar media, such as two flat panel displays or two projections on a screen, we are conscious of small differences in the white point; but when we compare different media such as a CRT and projected film, or a CRT and a digital projector, then our brains register the different sizes and natures of the displays and ignores the small colour difference.

* Should we use something newer than the CIE 1931 observer?

We know our eyes do not fit the assumptions behind the CIE standard observer. At FilmLight, we got fifty people to match the white of an OLED display to a fixed white on a CRT, and measured the whites with a spectrometer. There was a significant scatter to the results: what looked correct for one person looked significantly out for another. This means we cannot fix the CIE for one observer without making it worse for another.

The problem is not that the 1931 standard is 80 years old. The 1931 experiments have been repeated, and you can get more modern versions of the XYZ weights. These new weights are slightly different in the deep violets and reds, but these changes make little difference to the XYZ values of sensible displays with little signal at these wavelengths. The 1931 values are sufficiently good that most people prefer to use the standard values which everyone knows, rather than to use the more modern variants.

You mean “Old Ben”? Ben Kenobi?

[Robbie Carman]

great post as always JP. I believe this difference can also be general referred to as metamerism failure. I experience this all the time. In my case a FSI ref monitor and panny plasmas. When measured with a 35k PhotoResearch Spectroradiometer they show the same values however the panny always looks warmer, FSI always greener.

Quick dorky read on the subject of metamerism failure https://en.wikipedia.org/wiki/Metamerism_(color)

Robbie Carman
—————-
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[Knut Jansohn]

Hi,

Have you tried the D65-preset, is the green there still present?

[Florian Hirschmann]

On my PVM 1741 Sony’s D65 preset was also way too green. I calibrated the whitepoint to the rec709 primaries – now the green tint is gone.

[Joseph Owens]

[Knut Jansohn] “Have you tried the D65-preset,”

We couldn’t even use the D65 preset on the old BVM-1916 CRTs.

jPo

You mean “Old Ben”? Ben Kenobi?

[Lee Niederkofler]

How did you calibrated it?

[Florian Hirschmann]

I used my quato silver haze pro together with quato’s software iColordisplay to measure a pure white displayed on the PVM and adjusted the RGB gain (starting with the D65 preset) until I got the right white point (0.3127, 0.3290). Then I sent different levels of grey to the monitor to be sure that the color temperature doesn’t shift on different luminance levels. The temperature always stays very, very close to D65.

The resulting gain Levels on my PVM are: R 491, G 470, B 512.

Florian

[Knut Jansohn]

Hi,

In a company I worke for, I have seen a PVM-2541 (I assume no difference than size), which matched perfectly (for our eyes :)) rec709.
We were impressed with its picture compared to a BVM-L231 which stood beside. The BVM was calibrated to 709 with a Minolta cs-200.

They told me that the service that calibrated the BVM hadn’t had success with calibrating the OLED. The visible result was useless!
They simply copied the D65-preset to the user-preset (the result was far better, but not close to the BVM) and after that they had matched (adjusting gain and bias, same way as adjusting a crt) the pvm by eye, comparing to the BVM.

I am not an expert to colormeter. But I guess that the 40 sensors of the cs-200 can not resolve correctly the small OLED emission bandwith.

Regards

[Knut Jansohn]

Hi Florian,

as I understand it (color) it is not enough to calibrate with an achromatic pattern, because this lets you not now anything abaout saturation. Or in other words: changing saturation would not affect the results of your calibration but chromatic images would look different!

regards

[Florian Hirschmann]

Hi Knut

You are right, but the default saturation in different color modes seems to be accurate – at least to my eye – I haven’t measured it right now, but i will.

regards

Florian