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Thanks. Those posts I made took days to trickle
through to the list - I attempted to quickly let you all know that
I didn't need this any more, but the list delays
were just too long. :) But no harm done - I've learnt something
in the process.
Cheers,
Greg.
----- Original Message -----
Sent: Friday, February 06, 2004 8:38
AM
Subject: Re: [Lcms-user] Re: Convert a
lot of Lab measurements to a different observer temperature?
>>I need to convert many D50 Lab measurements over to D65
illuminant. =
>>I.e, I need the new measurements to be
equivalent to
>>the measurements I *would* have measured, had I set
my colorimeter to =
>>D65.=20
>
>
> This is
called calculating "corresponding colors" and you use Bradford
>
transform to do those.
As I understand it, Bradford will give you a
perceptual change. If you want an approximation of the numbers your meter
might have given had it been set on D65, I find that a 3x3 matrix in XYZ gives
a closer result:
D50 LAB -> XYZ
XYZ -> 3x3
matrix -> XYZ
XYZ -> D65 LAB
You can find a 3x3 matrix
by (for example) measuring a macbeth with your spectrophotometer set on D65
and on D50, and doing a least-mean square.
I see an average error of
just under 1 dE doing this, with a peak error possibly much higher. For best
results, redo your
measurements.
John
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