Reading and interpreting calibration charts and data for dummies

In the last year or so, home calibration of front projectors has become feasible for the average enthusiast, as the prices of test gear have come down and the quality of the instruments has gone up. I feel that more and more we will see people investing in colorimeters, and much like they have taken on the task of performing some of the more simple calibration procedures, like setting brightness, contrast, sharpness, color decoding adjustments, and convergence, they will begin performing more extensive procedures like measuring and adjusting greyscale, gamma, contrast ratio, black level, and light output.

So I figured this would be a good time to discuss what this data means and how it should be applied to get the most performance possible from your projector. I am not writing this to provide a guide, as I am just beginning on this journey myself, but rather to pose questions that I have concerning charts and data in public forum rather than asking them for myself privately.

And by the way, all pictures of charts and data that I will be posting are actual measurements of my InFocus 7210/Greyhawk (edit - Firehawk, not Greyhawk), using the DVI connection from my Panasonic S-97, measured with a Gretag Macbeth EyeOne Pro spectroradiometer running UMR's i1 Pro DCS calibration software in emission mode.

Ok, let's start with the CIE 1931 Tristimulus chart, a chart which provides the user with an overall picture of the color balance:


The squares are for the primary and secondary colors, but of what? I mean, are the 6 squares representing ideal values for the 6 colors, and not actually the measurements from my projector? Well, that would be my assumption, and I would also assume that regardless of how many times I remeasured and what changes I made, those squares would appear at exactly the same coordinates as they are presently. Is this correct?

Now, I can see that there are 6 triangles, and my assumption is that those 6 triangles are the actual measured values from my projector. The red and the blue seem to be pretty much right where they are supposed to be, but as we can see, green is southeast of its ideal values, and is pulling toward yellow. So what do I need to do with green to make it greener and less yellow? And what do those x and y coordinates represent? It looks like I need to go up .04 and to the left .04 units, but what are those units?

Ok, now let's take a look at the secondary colors. They are all off a bit, though nowhere near as much as green is. But there are no secondary color adjustments in my software - only primary gains and biases. So again my assumption would be, if I can move green so that it is on target as well as red and blue, then the secondaries would be pulled into place as well. Is this sound logic? So how do I get green "on target"?

While I am thinking of it, I have the option to measure using two different color gamuts - HDTV or SMPTE-C. I have read "HDTV is recommended for 720p and
1080i tests and SMPTE-C is for 480p and 480i.". My Panny S-97 is an upconverting DVD player, so the source is 480i (DVD) and it is being upconverted to 720p, so which color gamut do I use?

I know that by posting all of these questions I am showing just how stupid I am, but at my age I really don't care any more... I could probably get most , if not all, of these answers offline, but I figured that there would be a lot more people who would read this and learn from it, and they might be more concerned about showing their thorough lack of knowledge than I am. If things go well and other "dummies" post more questions or at least post that they are interested in knowing more and continuing with this, AND some of the more knowledgeable members decide to help out, then I will continue on with other charts and graphs until virtually all of the dumb questions have been asked.

Hi Kevin,

Thanks for your help!


That's kinda what I thought. So no matter what adjustments I make to the RGB gains/offsets, the primaries will not change. Is that correct?


Yeah, I'm beginning to figure that out...


I don't think that I have the option, though I will have to check with Jeff on that.


The colors look right...If I needed to change to Rec601 wouldn't it be blatantly obvious that something was wrong?


What is wrong? I assume that you mean that the red triangle should be elsewhere on the chart, but where? And what exactly is a "deeper" red? Oversaturated?

My measuring technique is far from optimum yet, as I using the emission mode and reading directly off of the screen, so my readings below 30% are rpobably quite inaccurate. Once my new, taller tripod arrives, I plan on remeasuring using the ambient mode by putting a diffuser on the sensor and measuring directly from the projector, which should allow me to get more accurate readings below 30%, and hopefully a more accurate picture of the color balance.

Oops, did I say Greyhawk again? It is a Firehawk, but I don't know if it is the RS or not. I bought just the material itself, and all it said on the packing slip was "bulk material" and never specified anything further.

Sure you can, Alan! I represent the "dummy" contingent, so you can contribute by adding to the knowledgeable posts...


Actually I think there are 2 versions, but from what I have heard the shift toward blue on the original Firehawk is so small as to be neglible for these purposes.


Regarding the color space issue of the 7210, I found a menu with 4 settings:

1. Auto (default)
2. RGB
3. Rec709
4. Rec601

My setting is "1. Auto". When I tried unchecking the auto box, the projector was now set to RGB (that was the setting that "auto" had chosen for my S-79 [edit: S-97]). I then tried both Rec709 and Rec601, and either of those settings resulted in a very very very green picture, totally unwatchable in any way, so there was no mistake about it - the correct setting is "RGB". How does "RGB" relate to using either SMPTE-C or HDTV as measurement choices? It would seem to me that RGB probably equals SMPTE-C, right? If I am correct, then I should have measured using the SMPTE-C color gamut and not the HDTV.


Thanks, Jeff. I only posted one graph so far, and when I post the other graphs, I am sure that we will see problems in my mearsuring technique become more obvious. At this point my intent is just to discuss the graphs to better understand what we are seeing and what it means, good or bad, and you and KRAS have shown how your experience and trained eyes have spotted that something was wrong just from that one graph alone...


Yes, and that is my intention. I will remeasure in ambient mode as soon as my new 8.5' tripod arrives...

Thanks for clearing that up, Greg! One more question - when I measure it (Panny S-97 720p DVI to IF 7210) with my EyeOne, should I choose the HDTV or SMPTE-C gamut?

Yes, many thanks to Greg and Kevin for explaining these things to us!

I'm sorry that I haven't posted recently, as I've been plenty busy, but I have been reading and keeping up with the posts. I wonder if Jeff has been lurking as well and might consider adding CIE 1976 charts in a future release.

The next chart is from the same measurement session, and it will hopefully help to explain some of the readings in the CIE 1931 graph:

Here we can clearly see that the readings I took below 30% stimulus are inaccurate. I believe this was due to the fact that my projector was mounted normally in its ceiling mount and the readings were taaken in emission mode off of the screen. AAs I can see it, there would have been two better ways to take these measurements that would have resulted in more accuracy in the low percentage stimuli:

1. Move the projector closer to the screen so that the projected image was MUCH smaller (like around 32"X18"), resulting in much higher ftL coming off of the screen and thus moving the measurements into an easier range for the EyeOne to measure.
2. Leave the projector where it was, use a tripod, put a diffuser on the EyeOne sensor (included in the Beamer package), and then take the measurements in the ambient mode, reading directly from the projector lens from a distance of about 2 feet. This would have a very similar effect of moving the measurements to a higher range like in solution #1, except that any color shift happening in the screen would not be considered. Since I am using a Firehawk, my belief is that this color shift would be neglible, but it is still less accurate (IMHO) than taking the measurements off of the screen.

Please also note that the inaccurate measurements in the low percent stimuli also throws any meaningful CR calculations right out the window, though it appears as if no harm was done to the gamma curve measurement, as no numbers were included in its calculations. I also assume that the color coordinates are being affected, as well as the obvious percentage stimulus errors. My intention is to remeasure in ambient mode from about 2 feet once my new 8.5' tripod arrives here (my current tripod is too short for this type of measuring).

Comments? Further clarifications of what we are seeing here?