Re: OBA's yet again
Re: OBA's yet again
- Subject: Re: OBA's yet again
- From: Hanno Hoffstadt <email@hidden>
- Date: Thu, 18 Jun 2015 15:33:39 +0000
- Thread-topic: OBA's yet again
Chroma is maybe too general (it could put a yellowish, say b*=+9, and a standard uncoated paper accidentally in the same group :-)
As a rule of thumb, assume +1 a* per -4..-5 b* for a neutral paper tint. (This can be approximated from the relationship between CIELAB and CIE whiteness and tint via CIEXYZ. It depends slightly on your chosen lightness level and uses different observers, but is good enough. L* does not change much.)
So I'd rather use grouping in b* with a corresponding a* tolerance band (e.g. +/- 1.5 a* units). Typical ab* values would be along:
0.0 -2 (FOGRA39, FOGRA47),
1.5 -6 (FOGRA51 candidate),
2.5 -10 (FOGRA52 candidate); see FRED15 on www.eci.org
A deviation of +/- 2 b* units is maybe just acceptable. So you could set one group center at a* 2.5 b* -10, the next one at a* 3.5 b* -14 (which is already quite bluish, high fluorescence. You probably won't need another one beyond that), a previous one at a* 1.5 b* -6 etc.
Unless you have distinctly reddish (or greenish) papers, there is no need for groupings away from this tint-neutral axis (in plus or minus a* direction).
For the L* grouping I would agree which Terry that less than 3 units (+/- 1.5) is not necessary/reasonable.
Hanno
>
> ------------------------------
>
> Message: 4
> Date: Wed, 17 Jun 2015 14:28:58 -0400
> From: Terence Wyse <email@hidden>
> To: Mike Stewart <email@hidden>
> Cc: "'colorsync-users?lists.apple.com' List"
> <email@hidden>
> Subject: Re: OBA's yet again
> Message-ID: <email@hidden>
> Content-Type: text/plain; charset=utf-8
>
> I would probably use delta L* and delta Chroma as a metric for grouping. Since we know when dealing with OBAs, your delta chroma is going to increase from near-neutral to blue in hue (yellow papers would throw a wrench in this).In my experience, you’re going to see increasingly positive a* and increasingly negative b* values as OBAs increase.
>
> To keep it manageable, I would say deltas of 3 on L* and chroma for groupings would be reasonable.
>
> Terry
>
>
>> On Jun 17, 2015, at 1:06 PM, Mike Stewart <email@hidden> wrote:
>>
>> As I've mentioned before. we are a pre-press provider for publishers and
>> printers worldwide. We use an Indigo Press as a proofer and 95% of the time
>> print/proof is on the actual press stock. This paper is mostly supplied to
>> us by the printer themselves. As I look into our press room I would say
>> that I probably have about 150 different papers; three quarters of which
>> are uncoated stocks heavily laden with OBA's. Using the i1Pro2 in M1 mode I
>> have measured all the papers.
>>
>> I cannot possibly have 150 different colour managed Hot Folders with
>> individual colour tables based on each stock we proof on. (It would be a
>> nightmare to build and control). I want to group papers into groups based
>> on LAB M1 measurements. Example: 94, 1.0, -9 might be one group. My colour
>> managed Hot Folder would have a colour set up created from 94, 1.0, -9 and
>> I'm wondering if there are any "rule of thumb" tolerances I could use for
>> my groups. Say Group 1 has been set up with 94, 1.0, -9 and my tolerance is
>> L+/- 2, *a +/- 1, *b+/- 2. I could now group all papers that fall within
>> those specs. I realize colour would not be an exact match for each sheet;
>> but should be close enough to allow a printer to "match" the proof.
>>
>> Has anyone done this and does anyone know if my tolerances are out of
>> whack? My +/- 2 could become quite a large tolerance if ..
>>
>> 94, 1.0, -9
>> 92, -1.0 -7 Is this to large of a gap?
>>
>> Any help would be appreciated.
>> Mike Stewart
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