RGB values of the grey swatches?

[jamie C]

Hi all

I'm new to WhiBal and CO, and have searched but can't find what the RGB vales of the WhiBal might be expected to be in an image correctly exposed and with the black/white points correctly set? just thought the figures might give me a hand when assessing exposure etc before I add my 10% to the picture and hit the print button. Upto now I've always referred to the grey values when using my MBth colour checker,

regards, Jamie

[MichaelT]

I am not an advocate of this technique. The Black and white points have to do with dynamic range, and not exposure, although they can be used as an aid to see that given dynaimc range is with the exposure of the camera. Lighting and reflectance and many factors go into an exposure, so I might us the values as a check in the beginning of the process, but my eyes in the end ( on a properly calibrated and profiled monitor).

The RGB values obviously should all be equal but will vary from image to image as to their absoultue values, and will also vary according to the color space used.

As an attempt to answer your question, the Absoulte values as measured in Lab* color space by a Spectrophtometer (and we test each and every card) for the gray cards are:

L* = ~65 (dark card)
L* = ~75 (light card)
These values do not affect the White balance reference so we do not test for these values.

a* and b* = between -1 and +1 (each card measured to meet spec)
Many of the card are typically more neutral in the range of -.5 to +.5, but -1 to +1 is our overall spec and what i consider to be required for a neutral reference.

I hope that this helps..

[charles beasley]

Jamie-I'll go further out on the limb than Michael. I had the same curiosity as you, and created a Lab document in Photoshop using the values MT published for the WhiBal, disregarding the a & b channel differences, just the L values.
Then converted to 3 popular working spaces using Adobe Engine, Rel Col & BP Compensation. For Lab and those 3 working spaces they are in this order:

Shiny Black, Black, Dark Gray, Light Gray, White:

Lab: 5, 19, 64,75,94

Colormatch: 14,34,137,170,234

SRGB: 17, 46,155,184,238

Adobe RGB: 24,49,153,183,237


Keep in mind that as MT says, the end points are prey to the camera's dynamic range and the converter's tone curve. The two grays should play out pretty close though.

[jamie C]

Thankyou Michael and thank you very much Charles, very much appreciated. Charles is that a typo for the light grey value in the AdobeRGB space - I'm sure it is? really appreciate you going to the trouble to reply
_______________
regards, Jamie

[charles beasley]

Yes Jamie thanks for catching it.

I did these calculations a while back and started to post them, but thought I'd wait to see if anyone asked for them.

The astute amongst us may ask why the differenece between sRGB and Adobe since both are 2.2 gamma spaces.* The differences from 153 to 238 I can only attribute to rounding errors, but the differences between 17 to 49 are due to some sort of "hook" that sRGb introduces in that area that I have just recently seen referred to, and am in the process of investigating. If anyone who knows about this happens upon this thread, please enlighten.

*I also did ProPhoto and they are exactly the same as Colormatch, so the disparity between sRGB and Adobe can't be due to the spaces' "sizes" as difference between ProPhoto and Colormatch is much greater than the difference between sRGB and Adobe.