Calman's dE numbers and Chromapure's dE numbers (Excel calc. sheet) do not match.

CalMAN is likely using more decimal places for its calculations than it displays onscreen, while you are inputting the numbers CalMAN displayed into the spreadsheet. And I believe it may be using raw XYZ data in its calculations. If so, you have to take this rounding up or down as well the conversion from XYZ to x,y into account.
Edited by Rolls-Royce - 3/9/13 at 11:05am

You didn't take a 100% white reading in CalMAN, without it all our calculations are based off a 100cdm target.

Also by default in calman we normalize the target gamut luminance targets to the 75% white reading (still needs the 100% white to nonormalize). so that may also be a contributing factor.

Sound like chromapures calculator is incomplete then

Think for a second, 5% grey and white look very different and have the same x,y

The autocal built into the Samsung SP-800 and SP900 projectors (Jor Kanes specifications) didn't require a reading for Y for white.

In theory the three Ys for the respective primaries should sum to 1 or 100% if correct, should they not.. Calibrated white Y is a sum of the three Y for the calibrated white and that sum obviously can be calculated rather than measured.

Of course to do calibrate the three Ys to the correct values (or as close as one is able) which are a set percentage a 100% stimulus white baseline would need to be measured initially to establish the 100% sum number. I hope I have this right if not I am sure the multitudes will rejoice in correcting me.

I would add that with a front project you will run out of one primary first and the other two will be set to less than their max in order to allow the run out of primary to meet its correct percentage of the total for calibrated white. That's why a calibrated whitre will be less than the maximum uncalibrated white where the other two primaries are running higher than they should. Torch mode.

The CMS's on projectors are build around the fact that light is additive.

If you have the x,y for white, and the x,y for red, green and blue at the same stimulus you can derive the normalized luminance levels for red green and blue. But that isn't particularly relevant to dE formulas.


If you look at any deltaE formula they are all based on converting your colormetric data into the L*a*b* color space to calculate from. In order to make that conversion you need to have the relative luminance of the value you are converting to L*a*b* and the reference white level in order to properly calculate the samples L*a*b* values. If you aren't including the luminance data, you aren't properly seeding all the information into the algorithms.


That is also why CalMAN needs both the 75% reading and the 100% reading. The 75% establishes the target values, but you still need the 100% value to properly normalize all the values going into the L*a*b* conversions.

I was away all weekend checking this on my phone.

After looking at the chromapure.xls worksheet, the discrepancy with their calculator is that it's only designed to work at 100% luminance.

If you run CalMAN with 100% luminance gamut targets, the numbers should agree.

Whenever you have any question about whether a dE calculation for CIE94 or CIEDE2000 is correct, simply consult.

http://www.brucelindbloom.com/index.html?ColorCalculator.html

and

http://www.brucelindbloom.com/index.html?ColorDifferenceCalc.html

On the first page, set reference white to D65, type the xyY values and then click the xyY button.
This will provide conversions for all of the color formats. Focus on the Lab value. The Lab value for x0.3188, y0.3248, Y1.0 is 100, 5.3877, -0.5167

Next, copy, paste the Lab values from that page into the Lab Sample on the second page. In the Lab reference simply enter the Lab equivalent of whatever reference color you wish. For white (0.3127, 0.329, 1.0), that is 100, -0.0025, -0.0139.

The CIE94 dE for x0.3188, y0.3248, Y1.0 is 5.4. CIEDE2000 is 6.9.

This is exactly what ChromaPure reports. The discrepancy is based, I am guessing, on how CalMan treats luminance in dE calculations with white.

The industry standard is, and has always been, to NOT consider luminance for dE calculations when measuring white. All you look for is errors in chromaticity. The luminance data point is accommodated by gamma. This is why you always use unity as the Y value for white dE calculations. CalMan has an option for including luminance data in dE calculations for white, which yields unexpected results and will conflict with every other white dE calculation in this business with which I am aware. With color there should be no differences.
Edited by TomHuffman - 3/12/13 at 11:49am

What is the 100% white reading?

In order to convert XYZ to L*a*b* you need to normalize the XYZ values against the Y value for white. Chromapure omits this and is wrong for it.

Bruces Calculator is completely accurate.

If white Y is 124.98 and your white reading is 15.06 cd/m then that reading has a Yn value of .1204 Yn

If you are in CalMAN and using 75% white as your target luminance (the default setting), then your target Y for red is the 75% white reading * 0.2126 or in this case 14.125

When you normalize the target to white you get 0.113 as the Yn target.

so now we have

Measured x,y,Yn value of 0.648, 0.3301, 0.1204
Target x,y,Yn value of 0.64, 0.33, 0.113

So using D65 as our white point we get

Measured L*a*b* 41.27, 67.52, 59.74
Target L*a*b* 40.08, 64.87, 54.43

The dE94 between those two L*a*b* values is 1.9


I'm not entirely sure what Tom does that raises his dE numbers, but when you correctly normalize luminance you'll get the correct result.

As I have said previously, I think it is a mistake to use this "normalizing" method for the grayscale, so nothing more needs to be said about that.

However, with color it is a more complicated question. Technically speaking, neither of us do 75% color precisely the correct way (for 100% stimulus color, both are the same). This would require having the user select a preferred gamma and then using that gamma value to establish the correct target based on a 100% white reference (For example, 0.113 would not be the correct luminance target in your example if you had selected a gamma of 2.4). This is the method we use for establishing targets for the Lumagen 125-point color cube calibration, and I am sure they do the same for their LUT calibrations.

Having said that, I honestly think that both methods are acceptable for basic 7-point gamut calibration at 75%. The CalMan method is closer to the theoretically correct approach and our method is the more commonly used approach by video engineers, or at least so I am told (I asked one). The reason I say that both are acceptable is that the difference between a CIE94 value of 1.9 and 2.2 is negligible. However, if you were calibrating using 50% stimulus color (which very few do) then the difference would be bigger, and our dE value would be considerably larger than the CalMan value. However, even here the only consequence for our users would be that they would be prompted to calibrate to tighter tolerances than he or she otherwise would.

So, to be clear, the small differences you have seen are not related to the dE calculation itself, but rather to the method used for establishing the correct target against which the sampled color is compared.