grayscale gradient anomaly

[Lucas Iaccarino]

Hi everyone,

While reviewing some older equipment, I came across an issue that seems a bit strange to me.

During the screening of a test chart, I noticed that the grayscale gradient appears differently on two projectors—even though both are the same model (CP2220). While the overall image doesn't look drastically different, I did notice some convergence issues on one unit, especially in the cyan gradient. However, it’s not as pronounced in the image itself as it is in the gradient ramp, which really caught my attention.

Do you have any thoughts on how to approach this or what might be causing the difference?

I've attached photos of both images for reference.

Thanks in advance!

Best regards,

[Ioannis Syrogiannis]

Since cyan is not a DMD but the combination of blue and green, I would guess that the red DMD is misaligned, but -since no DMD is permanently fixed- that's not a given.
I won't get into convergence of the DMDs, since I only have done that once on a Christie and I far from mastered it. (Mostly, I managed to hop from touching hot metal surfaces.)
Yet, Christie also offers a digital convergence setting. I would first try that and see if what seems cyan turns into red (or other). Best case scenario, colors align.
The projector has test patterns that are better than this one to check convergence.

And one last thing. The red DMD shows a little bit larger than blue and green. So, I would go for perfect alignment at the center and maybe towards the subtitles (high black and white contrast), if subtitles are something common in my country. Choose your battles.

[Pete Naples]

Also consider on a Christie, PCT calibration.

[Marco Giustini]

it's hard to judge from a photo - are both "whites" calibrated with a colour meter?

DLP machines don't need "gamma" or "white balance" calibration, with the exclusion of 100% of course. Once the 100% is calibrated, the gamma should be perfect down to 0%. It was SXRD, Sony, which needed constant calibration as the SXRD technology would drift differently at different levels, that is, you'd have perfect DCI coordinates at 100% but not at 80% or 60% etc.

I wouldn't expect that on a professional 3-DLP machine.

So first please confirm that the white is adjusted at the same coordinates on both machines.

Then you could try to find some 80%, 50% patches, project them on both and confirm that they still read the same coordinates. What I mean is that if you read 0.3140 x 0.3510 at 100%, you should read the same at 80%, 60% etc. Only the brightness would decrease.

I would try removing all calibrations: MCGD, PCT, screen files, everything and then check again. Also remove electronic convergence, I remember it was not recommended because of some possible artefacts.

[Marcel Birgelen]

Even if the convergence would be massively off, it shouldn't make your gradients more blue or green-ish.

Like Marco indicated: DLP doesn't really need a gamma or white balance calibration, unless the environment around it changes drastically, like a new screen, etc. That's the "beauty" of using PWM to create colors instead of using some kind of light valve technology like SXRD.

I'd also rather expect that someone messed up the calibration of the machine a while ago. Redoing the calibration and screen files is a nice point to start.
IMHO, you should only do electronic convergence as a last resort method, as it wil most likely impact picture quality: It needs to shift pixels around...