Wednesday, September 19, 2012

Fifteen years of HD displays in one morning!

I had to visit a new client to calibrate four different display devices and it struck me that they represented the gamut(!) of HD displays since the mid-90s. They didn't have an OLED display for me, but a Sony BVM-D24, a VuTrix Pro-24, an AutoCue G-series and a JVC D-ILA 2k projector.

The CIE 1931 chromaticity chart shows the extents of human vision with the RGB primaries mapped onto a pair of X-Y primaries that represent the colour content only and says nothing about the illumination of a colour. In the centre of the triangle is an "equal mix" of red, green and blue which is the very definition of white light. The various standards for the white-point used in different territories and applications is shown as a curved line. In TV pretty much the whole world now regards 6500k (actually, 6504k - had to be updated in the seventies when physicists realised they'd got Planck's constant wrong!). When I test myself I can only just discern the difference between 6500 and 6600k colour temperature.
Anyhow, metamerism aside (and that's another blog post!) you can't use the same probe to calibrate different display technologies. My venerable old CRT probe is no good for LCDs and vice versa (and that's before you consider plasma and OLED displays). That's why the £200 Huey-type probes you can buy on Tottenham Court Road are worse than useless.

  • Sony BVM-D24 - Old faithful, the HD display we've all been using forever. I'm starting to notice that the '24s I see now are in the autumn of their tube's lives. They are still the gold-standard for colour and unless the EHT is wrong or the tube is really knackered it's a nice and easy to job to get a D24 correct in the blacks and the whites and generally they track splendidly. As ever they look best when the white point is set below 100Cd/m2
  • VuTrix Pro-24 - These could be could be considered first generation broadcast LCDs as they are rear-illuminated with fluorescent strips. This particular one had to be really wound out from it's factory defaults to get it even near standard. They also have no sensible way of setting the colour in dark areas of the picture ("bias" on a Sony monitor). These have poor off-axis black performance as well.
  • AutoCue G-series - This was a new monitor, the G-series are their best ones with LED backlit LCD panel and a really nice look. Easy to get it looking correct from a colour point of view and it's factory preset was pretty darn close to illuminant-D. They must have some sort of fancy dichroic glass on the front because the off-axis performance is pretty good.
  • JVC D-ILA projector - because I don't have a spectral-radiometer (a wide band colour analyser), only photometers (colour probes that rely on having the same metameristic failure as the the device being analysed) I have to eye-match projectors to a know calibrated LCD or CRT. This is the bit of the job I like the best. Since I spent a decent amount of the nineties racking studios cameras I'm used to colour matching. I don't have a good colour memory (I'm always surprised how milky D6500 white is compared to what I've remembered).

Once I've got the probe happy with the light coming from the front of each display I do an eye-match as a final tweak. BBC test-card F is perfect for this as it has lots of grey-scale and real pictures. It's always nice to give Carole an outing; she's the person who's been on TV the most in the whole world, ever!

You can see here the monitor on the floor is a bit redder in the whites than the projector; since I'd got the monitor correct I eye-matched the projector as best I could; but it's always a struggle because different display technologies just look different because they make colour in different way.

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