Re: linearization - luminance, chroma or density?
Re: linearization - luminance, chroma or density?
- Subject: Re: linearization - luminance, chroma or density?
- From: "Clark Omholt" <email@hidden>
- Date: Tue, 23 Jan 2007 07:31:29 -0800
- Thread-topic: Re: linearization - luminance, chroma or density?
-----Original Message-----
From: Clark Omholt
Sent: Tuesday, January 23, 2007 7:04 AM
To: email@hidden
Subject: Re: linearization - luminance, chroma or density?
>Date: Sun, 21 Jan 2007 11:20:04 -0600
>From: Scott King <email@hidden>
>Subject: linearization - luminance, chroma or density?
>To: "email@hidden List"
><email@hidden>
>Message-ID: <email@hidden>
>Content-Type: text/plain; charset=US-ASCII; delsp=yes; format=flowed
>Can anyone point me to a web reference, text book or white paper that
>discuss the pros and cons of the various inkjet printer linearization
>methodologies? In particular the use of luminance, chroma or density?
>I've searched the colorsync-users archives and cannot seem to find
>anything conclusive... or even close to conclusive. A few references
>here and there about each approach, but nothing that compares the
>three.
>Any references or comments are much appreciated.
>Scott King
>Austin, Texas
This is an area where we've done a fair amount of investigation. The
RIPs we've worked with most are EFI Colorproof XF/Best (Lab), Onyx
(density), Colorburst (Lab), ImagePrint (no linearization currently).
The ones that use Lab I believe use chroma for linearizing colorants and
L* for linearizing K.
Our experience has shown linearization to be pretty mechanistic,
resulting in good results in combination with an ICC profile using
either density or Lab. The area to pay the most attention is in
per-channel ink limits. There's a number of reasons for per-channel ink
limiting.
1) Hue Hooks: Most inkjet inks will experience a hue hook at higher ink
lay down levels. It's not uncommon to see 20 degrees of hue shift from
70% to 100%. ICC profiles perform much better in environments where
there's little hue shifting. Quite often, the last 20-40 percent of ink
will increase density and look darker to your eye but will actually have
minimal or an adverse effect on chroma/saturation, the parameter you're
really trying to maximize. As others have commented, the key is to get
the maximum saturation with the least amount of ink and then have the
profile add K to achieve darker colors.
2) Overprints: Typical per-channel ink limiting processes do not
explicitly address overprints (RGB), which are an absolutely critical
element of your overall gamut. When I use aggressive (high) per-channel
ink limits and examine my overprints in 3D Lab, typically I'll notice a
"folding over" in the G&B channels, meaning 100% G has a lower chroma
and L* than say 80%.
3) Total Ink Limit: Most RIPs perform best if you're not overly
aggressive with the Total Ink Limit (TIL). I get nervous when I need to
drop TIL below 200. Appropriate per-channel limiting helps keep TIL in
the desired range.
In general, we find it's best to be on the aggressive side with
per-channel ink limiting. Once proper per-channel ink limits have been
set, the rest of the calibration usually goes smoothly.
Incidentally, the tools we've used to perform this kind of analysis are
X-Rite's Colorport (for generating the CMYKRGB patches, free download)
and Colorshop X (I'm sure Colorthink could do the graphing, too).
Regards,
--Clark
Clark Omholt
email@hidden <mailto:email@hidden>
Spectraflow, LLC
Color Workflow Solutions
Phone: 415-472-0130
Mobile: 415-225-6644
122 Paul Dr., Suite B1
San Rafael, CA 94903
www.spectraflow.com <http://www.spectraflow.com/>
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