Re: Photoshop Lab values
Re: Photoshop Lab values
- Subject: Re: Photoshop Lab values
- From: "Fleisher, Ken" <email@hidden>
- Date: Mon, 09 Jul 2007 10:17:09 -0400
- Thread-topic: Photoshop Lab values
Okay, thank you Thomas and Graeme for the explanation. I was aware of the
non-real colors possible in Lab in ProPhoto, but I didn't realize that the
blue colorant has such a low L* value. I would have expected that the full
range of RGB would have been mapped into real Lab colors. Interesting...
Thanks again.
Ken
On 7/9/07 9:39 AM, "Graeme Gill" <email@hidden> wrote:
> Fleisher, Ken wrote:
>> Hello. I have encountered a strange result that I don't understand.
>>
>> 1) Create an RGB document in Photoshop in ProPhoto RGB color space.
>> 2) Fill some blocks with solid colors where R=G=0
>> and B = [0, 17, 34, 51, ..., 255]
>> 3) Read Lab values with the color picker (single pixel)
>
>> What confuses me is that L* has a value of zero for all color patches.
>> Shouldn't L* increase as B increases? It doesn't seem logical that an RGB of
>> 0, 0, 255 would have an L* of zero.
>
>> What am I missing? Any thoughts?
>
> ProPhoto has an extreme Blue colorant value. The blue colorant in fact
> has an almost zero Y value, hence pure blues have an almost zero
> L* value. In this respect, since ProPhoto has a non-real world
> blue colorant value, the colorspace itself shouldn't be treated
> as a real world space anymore than XYZ or L*a*b* are.
>
> [The blue colorant has a D50 L*a*b* value of about 0.08 90.25 -172.3].
>
> Graeme Gill.
--
Ken Fleisher
Photographer
Imaging & Visual Services
National Gallery of Art
Washington, D.C.
Phone: (202) 712-7471
email@hidden
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