Digital BW, The Print

Hello, 

Can anyone explain why the L* jumps considerably in brightness when 
separated from the a*& b* channels. The jump is global, (independent of 
colour information) and usually more than L*10. 

thanks

Eugene

How are you separating L* ?

On 22/11/2007, eappert <appert@...> wrote:
>
>   Hello,
>
> Can anyone explain why the L* jumps considerably in brightness when
> separated from the a*& b* channels. The jump is global, (independent of
> colour information) and usually more than L*10.
>
> thanks
>
> Eugene
>
> 
>


[Non-text portions of this message have been removed]

--- In DigitalBlackandWhiteThePrint@yahoogroups.com, "Michael King" 
<drmrking@...> wrote:
>
> How are you separating L* ?

I am using the Gretag color chart as reference. If I convert the Lab 
file to grayscale via Photoshop menu mode conversion, I will always 
preserve the L* values, no matter what grayscale space is configured 
by default in my color settings.  Equally the info pallet will 
display the exact L* values of the Gretag chart regardless of the 
grayscale space configured in my color settings.  If I deactivate the 
a* and b* channels the file brightens visibly on the screen, even 
though the info palette continues to display the same L* values.  But 
if the lab file is now converted to grayscale ( with a* and b* 
channels deactivated)  or if the L* channel is dragged into another 
file as an alpha channel, (even to another Lab file),  the info 
palette will now display the L* values as they appear on the screen, 
that is at least L*10 brighter.  

Eugene

--- In DigitalBlackandWhiteThePrint@yahoogroups.com, "eappert" <appert@...> wrote:
>
> --- In DigitalBlackandWhiteThePrint@yahoogroups.com, "Michael King" 
> <drmrking@> wrote:
> >
> > How are you separating L* ?
> 
> I am using the Gretag color chart as reference. If I convert the Lab 
> file to grayscale via Photoshop menu mode conversion, I will always 
> preserve the L* values, no matter what grayscale space is configured 
> by default in my color settings.  Equally the info pallet will 
> display the exact L* values of the Gretag chart regardless of the 
> grayscale space configured in my color settings.  If I deactivate the 
> a* and b* channels the file brightens visibly on the screen, even 
> though the info palette continues to display the same L* values.  But 
> if the lab file is now converted to grayscale ( with a* and b* 
> channels deactivated)  or if the L* channel is dragged into another 
> file as an alpha channel, (even to another Lab file),  the info 
> palette will now display the L* values as they appear on the screen, 
> that is at least L*10 brighter.  
> 
> Eugene
>

Eugene,

It is a curious behavior. 

What's happening is that whenever you look at any multi-channel file (LAB,RGB,CMYK)
and turn on just one of the channels it treats it as a "grayscale" file.  Notice if you
take an RGB file and look at just the R it displays as gray.   To do this it has to 
treat it as some "profile" and not too surprisingly it uses your grayscale working space.
It affectively "assigns" your grayspace to the single channel for display purposes.
This can completely change the visible luminosity as seen on the screen.
In fact you can go into Color Settings and try different grayspaces with Preview on.

When you convert to gray you'll be getting a "convert-to-profile" whereas with the
single channel view you are seeing an "assign profile".

Roy

Roy Harrington wrote:

When you convert to gray you'll be getting a "convert-to-profile" 
whereas with the single channel view you are seeing an "assign profile


Roy, thank you this I can understand. 

Eugene