Re: 16 bits = 15 bits in Photoshop?
Re: 16 bits = 15 bits in Photoshop?
- Subject: Re: 16 bits = 15 bits in Photoshop?
- From: bruce fraser <email@hidden>
- Date: Thu, 14 Apr 2005 14:30:39 -0700
Ray,
All reflective spectrophotometers measure relative to some absolute
standard, but not to the same absolute standard! Until you get into
research-grade instruments, the calibration tile is serialized to the
instrument, and the calibration tile is the absolute white reference
used by that instrument. Some vendors provide a path that traces the
individual unit's performance back to some accuracy relative to an
NIST standard, but plenty of others do not.
The imprtant qualifier to "the minimum and maximum L* values that a
particular hard copy proofing system can produce" is "as illuminated
by the light source in this spectrophotometer!"
On a lot of papers, you'll get very different L* values for paper
white from the same spectro with a D50 filter, a D65 filter, and a UV
cut filter. Calibration tiles are designed to come very close to 100%
reflectance, but they can only reflect what's already present in the
illuminant.
In practice, the variability in the measurements is probably drowned
by the variations in paper manufacturing and the sheet-to-sheet
variability of the output processes-I'm just cautious about using the
word 'absolute" unless it really, really means absolute!
And yes, you did indeed qualify everything you said as being about
reflective hard copy. I was just trying to point out that reflective
hard copy is not the be-all and end-all in digital imaging. I wasn't
really disagreeing with you, just pointing out that there are
scenarios beyond the ones that you were discussing that nevertheless
real.
best,
Bruce
At 2:15 PM -0700 4/14/05, Ray Maxwell wrote:
bruce fraser wrote:
100 L* isn't a dynamic range. It's simply maximum white relative to
whatever your white reference is, and is hence a relative rather
than an absolute value.
Hi Bruce,
I agree with everything you said and thought I had qualified
everything I said. However, with regard to reflective
spectrophotometers I believe that all measurements are done to an
absolute standard rather than relative. I also understand that what
I said only applies to non-fluoresent papers. I have exchanged data
with many color scientists and, with regard to reflective
measurements, we have always talked about the minimum and maximum L*
values that a particular hard copy proofing system can produce.
Can you confirm or correct this practice.
Thanks,
Ray Maxwell
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