Since I have a few grey cards, which have served as a “standard” for processing during the years, I thought it would be of interest to see how “grey” they really are. Here are the results, expressed in sRGB:
The colorimeter I have is an X-rite ColorMunki Photo. The software that comes with the gadget only works under Windows and presents its readings as Lab, HTML and sRGB.
In Linux I have ArgyllCMS installed. Using spotread -s I will get readings as D50 Lab as well as XYZ.
I just made a quick comparison, using Colour Confidence’s grey “mat”:
The comparison colour was derived from the measurement by simply setting the a and b channels to 0.0 while keeping the L. The usual interpretation of deltaE is that everything below 2.0 looks the same to the human eye, so while those might not be good enough for scientific measurements I wouldn’t worry too much for actual use. In case someone wants to play with that you can find the libreoffice spread sheet here, zipped to please the forum software:
You mean my dirt has a colour cast?
Actually, I do not think so, since almost all of the cards
are kept in a nice drawer and they are not handled
more than when in use.
@Claes I guess the material, construction, condition and age would have an impact too. Also, not all targets are made equal. Personally, I am not serious enough to use a target but I am sure this thread would be instructive for many.
@houz I don’t know much about deltaEs. How should I read the two columns?
My old Mennon grey card gives somewhat different values depending from which part of the card the measurement is taken. It has been used a lot and got dirty. Eventually it started to give consistently too high color temperature with tint trending too high on the green side, and therefore I replaced it with Spyder Cube. I was told in a camera store that UV radiation can damage grey cards if they are used a lot outdoors (as I have done).
I am rather happy with my Spyder Cube. Paid 19 euros for it (normal price here in Finland is 59+ euros… ridiculous!). It fits in pocket. You can get two grey card readings with single shot if necessary: one for lit part, one for shadows. White and black sections are more or less useless, but the reflective part is perfectly usable for self-portraits…
delta E is basically the distance between two colors in Lab space. If it is less than 2 the two colors are considered to be indistinguishable by the human eye. Of course it’s not a scientific fixed truth but a rule of thumb, so some people might be able to tell colors apart that have delta E of 1.5, some might not see a difference of 5. The column “deltaE (neutral)” has the difference between the measured Lab color and the grey with the same L value. For example, the Lastolite grey card measured as L = 41.811898, a = -0.362705 and b = -0.717014. Comparing that to L = 41.811898, a = 0.0 and b = 0.0 gives a delta E of 0.8 which is darn close and therefore the color tint is not something anyone would see.
The column “deltaE (50%)” is comparing all grey cards to the same 50% grey Lab color. It’s not very useful to do that as the cards are probably not meant to be neutral grey in the first place, but it might be interesting to see how far they are off.
The Wikipedia page has more about the theory. I used the 1976 model btw.