Re: [Lcms-user] Profile Illuminant

[Carles L. <car...@in...>]

Hello Marti,

then there is something that I don't understand and the best way that I have to explain it is with an example:

Let's assume that we have made an RGB profile, using any profiling tool. Normaly we use EyeOne in D50 2º setting,
Now we want to convert an Lab spot measurement to this profile, but this measurement is done by a more precise
spectrophotometer like X-Rite SP-62. This device is configured to work in D65 10º (recommended for textiles),

To do this conversion we must adapt the spot color Lab to D50 prior to convert it to the profile, isn't it ?

In our workflow we should know the illumninant used to make any profile in order to perform the chormatic adaptation or not.
(At least, is how we worked since now!)

Regards
Carles


El 01/02/2012, a las 22:34, mar...@li... escribió:

> 
> Hi Carles,
> 
>> We are currently porting our applications to LCMS 2.3 (from 1.16) and we've found that the original profile illuminant is lost when opening and saving.
> 
> Wow, 1.16 is really old. Hope you would find the new code easier to deal
> width.
> 
>> It seems that this new version assumes that all profiles will be in D50 illumninant, what it's not necessary true in our cases.
>> The cmsTakeIlluminant function disappeared and we cannot perform the right crhomatic adaptations at our side.
> 
> I'm afraid you are confusing the PCS illuminant with the source of
> light used to create the profile. The PCS illuminant (than's what is
> in the header) is just a reference. It is there for future ICC extensions
> and this is what the ICC spec saya about this field:
> 
> "7.2.16
> PCS illuminant field (Bytes 68 to 79)
> The PCS illuminant field shall contain the nCIEXYZ values X = 0,964 2, Y = 1,0 and Z = 0,824 9 encoded as an XYZNumber. See Annex A for further details."
> 
> At that point, the old function to retrieve PCS illuminant is useless as
> should return same values for all valid profiles, so it was removed in 1.17
> if I recall correctly.
> 
> Otherwise, if what you really want is to create profiles on media illumined
> by light sources other that D50, then you should use the chromatic adaptation
> tag. The spec is very specific on that. See 9.2.15 chromaticAdaptationTag
> and also the annex D. I think there are some white papers in the ICC site
> talking about that.
> 
> 
> Regards
> Marti
>