Experience with color calibration between lamps and lasers, and their matching.

[Marco Giustini]

James,

I know this will sound harsh but I think you're misunderstanding how metamerism works. It has nothing to do with colour correction. It's not about questioning the validity of colour-science, which is very valid.

If it was a well-known phenomenon that exists between many display technologies and has been managed in colour workflows for decades, current RGB cinema projectors would have a way to compensate for it.

Hong Sung
I'm not sure you can "convince" a colourist! If they don't like what they see, there's not much you can do about that! That's why most productions are graded on Xenon. And that's why those projectors you mentioned should not be used in a colour-critical post production environment.

[James Gardiner]

Hong Sung,
Colour science does work on laser — the tools are valid. The problem isn't the science, it's that narrow-band laser primaries expose personal variance in cone sensitivity that broadband sources naturally masked. That's a new variable to manage, but it is manageable. A personalised metameric correction built around a specific observer and a broadband reference is a legitimate and workable solution for a grading environment.

The wider exhibitor variance — different people in the same theatre seeing slightly different colour — is a real limitation, but it's the same category of compromise we already accept with contrast performance varying from cinema to cinema. Neither is ideal, neither makes the technology wrong.

And practically speaking, Xenon isn't a long term option. The industry is moving to laser whether we're ready or not. The more useful conversation is how we manage the known limitations — and we do have the tools to do that. Dismissing laser from colour critical work isn't a position that's going to hold.

[Leo Enticknap]

The reality is that xenon lamps will soon become very difficult, or at least extremely expensive, to obtain.

I remember that prediction being made by the instructor at the first "Barco school" I attended, in the summer of 2015, and regularly since. The reality is actually that only a small number of specialty lamp models have been discontinued (e.g. the tiny lamp for the NEC NC800 and iS8-2K), and the cost of the most commonly used ones has not increased much beyond inflation.

Barco and SharpNEC have an incentive to push the "xenon is dead" line, because they make higher margins on the sale of laser-illuminated projectors than they did on lamp projectors, and money spent on lamps is money that they don't see any of. This is likely why only Christie still makes lamp projectors (the Cinelife -Xe line) - the bulbs that go in them are made by its parent company, Ushio.

In the nine years and nine months I've been working as a field tech, I would estimate that the proportion of xenon-lit screens I service (which is a pretty even split between first run houses, arthouse/college campus type theaters, VIP residence theaters, and post houses) has declined by about a third. If that is representative of the industry as a whole, xenon arc lamps will still be with us well into the 2030s.

[Harold Hallikainen]

I previously suggested that lasers at the peak wavelengths of the XYZ filters could reduce metamerism because the slope of the filters is zero at that point so slight variations in the peak frequency and differences in skirt slope would have less effect than putting the lasers on the filter skirts. HOWEVER, thinking about this more, putting a laser at the peak of the X filter would also result in about 70% output from the Y filter. This would not give enough independent control over the XYZ levels.

It looks like the best way to currently reduce metamerism with laser projectors is to use more lasers (6P instead of 3P). There are other techniques, but they are apparently expensive. A Google AI discussion of this is at https://share.google/aimode/pGMkawpNjgczyzBtH .

Harold

[Leo Enticknap]

I suspect that the cost of 6P was why the DP4K-xxL line was so expensive as to be essentially stillborn. It was only viable for PLF screens, and not even most of those. One cool thing about 6P, though, is that it has built-in 3-D capability, requiring only passive glasses for the viewer: the left eye is reproduced using additive color (RGB), and the right subtractive (YCM). But even there, Barco shot themselves in the foot by charging more for the glasses than some of the active ones on the market at the time. I once installed a DP4K-45L with an XpanD emitter, because that worked out a lot cheaper than 700 pairs of the Barco glasses at not far short of $100 a pair (this was a very large auditorium).

[Harold Hallikainen]

Would the Barco glasses be similar in design to Dolby 3D glasses, which I believe were also not cheap.

[Steve Guttag]

As it is now, if you look at the spectral response of both Christie and Barco, they are not using monochromatic lasers. There are several peaks on red and green. I suspect that this is, in part due to trying to combat speckle as well as metamerism. I will say that the look of the Christie CP4415-RGB with an Ultra High Contrast Premium lens looked VERY good...quite "filmic" to me. I didn't have, what I perceived, to be a red shift that I normally get with laser based projectors. I was judging it on a Stewart Snomatte 100 screen. The image was really good. I didn't notice any speckle either. I'm not saying there wasn't any..just from where I was sitting, I didn't notice it. When I shot colors, I definitely noted the multi-spiked areas in Red and Green.

[Harold Hallikainen]

I definitely noted the multi-spiked areas in Red and Green.

Are there multiple lasers for red and green? Or is some other technique being used?

Harold

[Christos Gartaganis]

I agree with Harold that a 6P laser would be the best option for dealing with the color metamerism problem. We ran a few demo shows with a Barco 6P laser projector back in 2018 in Athens and the picture quality was amazing. No speckle and no color shifting issues observed. The 3D was also very good. Too bad that Barco abandoned this technology. On the other hand we have installed a few 4K BLP projectors since 2018 with high contrast lenses, and we are very happy with the picture quality. Again no speckle and color convergence issues. Try watching a Black and White movie on any RGB laser projector, and you will be very disappointed with the picture quality displayed on the screen.

Also worth noting that the Barco SP2K series projectors are still based on a laser phosphor source. Only the SP4K series are available with RGB laser technology. Maybe there is a good reason behind this decision.

[Marco Giustini]

Can someone clarify the 6P story for me please? I am very curious. I thought 3P was the beginning and 6P - either via multi-laser LEDs or dual projector - was the future. I read Steve saying that both Christie and Barco show multiple peaks while Christos says Barco abandoned the 6P technology.

What is the current state of the Laser implementation?

[Steve Guttag]

The original intent behind the 6P laser was for (Dolby) 3D since using a color wheel wasn't going to work (if you only have one wavelength, what are you going to filter?). Christie's approach was to use 2 projectors each having different wavelengths for the primaries. Christie's approach also means that both eyes are always projecting versus Barco's approach of still alternating eyes.

What I see on my spectroradiometer when I shoot S4 laser projectors (RGB) are typically a single spike for blue and multiple spikes for green and red. I'll see if I can find it but on a Christie CP4415-RGB, the green looked like quite a few spikes.

[Leo Enticknap]

Would the Barco glasses be similar in design to Dolby 3D glasses, which I believe were also not cheap.

They were so similar to the Dolby glasses that I suspect that they were made in the same factory. The only difference was to the tint in the lenses. IIRC, the Dolby (reusable) ones were $20-30 a pair to the theater, but Barco sold theirs for well over double that.

Can someone clarify the 6P story for me please? I am very curious. I thought 3P was the beginning and 6P - either via multi-laser LEDs or dual projector - was the future.

I can't tell you much about the other manufacturers, because I wasn't trained on their products at the time. Barco's first line of laser-illuminated projectors was the DP4K-30L/45L/60L, which it branded as "flagship laser," and which was 6P. Shortly afterwards, it launched variants of the DPxK-B and C line with the lamphouse and lamp power supply modules replaced with a laser phosphor light source unit (so from 6P down to 2!) and its power supplies. The DP4K-Ls produced a beautiful picture, but were horrifically pricey (between $250K and $350K roughly), were so physically large as to cause problems with some booths, and had exacting ambient temperature and humidity requirements. I've only installed one and serviced two during my entire time as a service tech. The one I installed was bought in 2015 or '16 and in storage for four years while a long delayed building project was completed. The launch of the 3P Series 4 line in 2019 essentially killed 6P stone dead, at least in the Barco product line.

I believe that NEC only ever made one 3P projector (the 1040L), which was also very expensive, and went entirely to laser phosphor immediately after that.

[Marco Giustini]

The launch of the 3P Series 4 line in 2019 essentially killed 6P stone dead, at least in the Barco product line.

Thanks Leo

What confuses me is the "3P" mention but Steve says he can see multiple spikes with the Barco S4.

Maybe the "pure 6P" was abandoned - that is, having 2 Red, 2 Green and 2 Blue which could also be independently driven for wavelength-filtering 3D - and a new generation of 3P was introduced: 3P as in "Red/Green/Blue" (no phosphor) but with multiple frequencies on some of the colours. That would make it even better than 6P.

Is my understanding correct?

[Christos Gartaganis]

Marco, a 6P (six-primary) laser projector produces a distinctive spectrogram consisting of six narrow, high-intensity spectral spikes. In other words 6P technology uses two slightly different wavelengths for each of the primary colors, Red, Green, and Blue resulting in significant reduction of laser speckle and also addressing color metamerism issues. In 3D mode the laser modules would flash alternatively to create 3P Left and Right images that could be easily separated with specially designed passive 3D glasses available from Dolby and Barco. For a short period Dolby was selling those 3D glasses in 2 different editions (Christe and Barco) to exactly match the 6P color primaries of the laser light sources used by Christie and Barco.

Unfortunately the 6P technology was abandoned by both manufacturers after the covid pandemic due to their high production and maintenance costs.

[Marco Giustini]

Thanks Christos,

Yes, I understand how 6P works.
But Steve says that modern Barco and Christie machines show multiple peaks of Green and Red.
Does that mean that "pure 3P" doesn't exist anymore (that is, ONE Red wavelength, ONE Green wavelength, ONE Blue wavelength) and modern machines all feature multiple wavelengths to mitigate speckle and metamerism? Maybe the 6P which was abandoned is one that can be independently controlled for 3D reasons?