Hi Ninanoki
This will be my last post to your thread. I will provide my answer to a higher accuracy ∆E number and then some mathematical information on why your approach is highly unlikely to get a proper tooth color match. The best I think you will be able to reiably count on is 1 or 2 shades off (not great news yet it may save you some time on your project)
1) To have a higher accuracy ∆E number.mathematically:
The basic difference is to extract out of PS 16 bit values instead of 8 bit values and use them properly in Excel.
Workflow > Image capture in Raw (not JPEG) - Raw process in TIFF 16 bit mode in your desired color space to send to PS. Do not got into 8 bit mode. This reduces accuracy and as well when changing color modes in 8bit, PS by default adds some noise into the image which will also add error (this dither can be turned off in Edit > Color Settings yet only applies to 8 bit mode.
Convert to LAB with Edit > Convert to Profile and use the Absolute Colorimetric rendering intent (not sure this is needed for LAB yet the idea is not to change the white point in the conversion). Again, leave the image in 16 bit mode.
Change the info panel to read Lab values in 16 bit mode (its actually 15 bit mode yet that is a detail). Copy those numbers into Excel.
The scale down the 15 bit Lab numbers by
L=L(15 bit)/327.68 (16 bit numbers for L are scaled different than the "a" and "b" numbers
a=a(15 bit)/128
b=b(15 bit)/128
This scale the numbers in PS to floating point numbers that will be very close to the 8 bit numbers yet in floating point with higher accuracy. This will improve the accuracy of up to ∆E of .8.
NOTE - You would want to have 16 bit values for the tooth references as well.
2) By moving away from actual visual comparisons with the tooth, and moving to a photograph, the level of accuracy to exact correct color I believe is not possible. Here is the math of why.
Almost all tooth samples have a ∆E to another tooth sample of right around 3 or 4. That means if you have an error in your image Lab values and/or the tooth references of only 1.5 ∆E, then you would not be able to get a best match to a reference tooth. It would give you a wrong answer.
That means after going through all the specialized lighting, white balance, tone correction etc, you would need to have an maximum error in each of L,a,b of only 1%
I dare say with the cumulative errors that you could have down your proposed workflow that it would be virtually impossible to achieve.
What you could get is a starting point or accepting that the identified tooth match could be off by one color/shade or maybe two.
That is probably why when getting a color match outside of tooth samples place right next to the teeth, most solutions that I have read about are using photospectrometers and even then.
So this is all in my opinion yet felt obligated along with the "academic" answers on measuring ∆E and accuracy of calculations, that I included a dose of practical advice to consider about why I consider an insurmountable challenge (assuming your are trying to get the exact match in selecting a tooth without reference teeth being used by the dentist.
Hope you find this useful.
John Wheeler (signing out of the thread)
