Re: Hue Angle for Black
Re: Hue Angle for Black
- Subject: Re: Hue Angle for Black
- From: Dan Wilson <email@hidden>
- Date: Thu, 27 Aug 2009 14:34:22 +0100
- Thread-topic: Hue Angle for Black
Hi Bill,
Great to hear you are doing G7 on a Flexo press, I am doing one in Belfast
at the moment. Just waiting for press time for the confirmation run.
For K, all you really need to focus on is the L* value.
For type 1 paper it is 16.
The a*b* values are not really important in black.
So depending on the substrate, and use the target lab* for your c,m,y as a
guide, adjust the K L* to match.
For instance, if the cmy lab/hue angle gets you around type 1 paper an L*for
the k of 16. If it is closer to Type 3 a different L* would be the target.
All that said K is the most forgiving colour in the G7 process and I have
seen the printers use how solid black type prints as a guide to what L*
target is appropriate for them.
I may be corrected on this but I believe G7 refers to Delta E 1976 - which
is what most ISO specs refer to as well.
I am very interested to hear your results, please let me know offline how
you are going with the project?
PS: years ago we exchanged emails about flexo printing.
Many thanks,
Dan Wilson
G7 Expert
Ugra Expert
> ------------------------------
>
> Message: 4
> Date: Wed, 26 Aug 2009 09:16:53 -0700
> From: email@hidden
> Subject: Hue Angle for Black
> To: email@hidden
> Message-ID: <email@hidden>
> Content-Type: text/plain
>
> Greetings,
>
> (FYI, I posted a similar question on the Print Planet list - for those that
> are concerned about cross posting. Thanks, as
> always, to Mike E for your reply)
>
> Can black have a hue angle?
>
> To help me in my G7 qualification runs on our flexo presses, I measure 3
> consecutive press sheets and average 6 readings
> (3 from each side of the press). I have created an Excel file that enters
> these 6 Lab measurements as I measure them with
> my 528 (using a MAC) and then calculates chroma, hue, dE (under CMC 2:1); and
> plots L*a*b* to tolerances, and shows an
> approximated hue angle view from the average of these measurements.
>
> My challenge is creating the dE for the black. I use the formulas below my
> name that I received from a colleague. These
> formulas work for CMY when compared to the results achieved on Bruce
> Lindbloom's web page.
>
> However, I am not able to compute a hue angle for black. Since my math
> education ended with Geometry, I am a little
> challenged with these formulas, but I did notice that with black's a* and b*
> being 0, the formula for dH*ab returns a 0
> divided by 0, and I know dividing by 0 is undefinable. My guess is that this
> is what is "breaking" the formula to compute
> hue for black. But once again can black have a hue angle?
>
> I am able to get a tolerance when comparing two blacks using Bruce's site, so
> I am hoping some one can share with me
> how that is computed.
>
> Thanks to anyone that takes a little time to help me and if seeing my Excel
> file would help, I will gladly send it.
>
> -Bill-
>
>
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