IT8 Scanning Standards
IT8 Scanning Standards
- Subject: IT8 Scanning Standards
- From: Robin Myers <email@hidden>
- Date: Wed, 31 Jan 2001 09:10:29 -0800
- Organization: Robin Myers Imaging
Some people recently have described the IT8 standards as weak. I disagree with
that assessment and propose that it is based on a lack of understanding of
what the IT8 standards are and their purpose.
The IT8 standards, and I am referring to the IT8.7/1 and IT8.7/2 transmissive
and reflective targets, are for calibrating and profiling for scanning PHOTO
FILM TRANSPARENCIES and PHOTO PRINTS. When used for this, they are very good.
The grayscale at the bottom of the target represents the entire tonal range
possible, FOR THAT MEDIA. The "white" patch (GS0) and the "black" patch (GS23)
represent the minimum and maximum, respectively, densities possible in that
medium. For that medium, there can be nothing lighter or darker.
The columns 13-15 are comprised of a 12 step scale of the pure cyan, magenta
and yellow dyes, from the minimal perceivable density to the maximum density.
Column 16 is a black scale made from the equal addition of the cyan, magenta
and yellow dyes from columns 13-15. Columns 17-19 are the secondary colors,
red, green and blue made from combining two of the dyes, in equal amounts,
from columns 13-15. Thus, in columns 13-19, you have the pure scales made from
the pure dyes, their secondaries, and their tertiary combinations.
The columns 1-12 are comprised of colors sampled from the L*C*h0 space at 12
hue angles, 3 lightness levels and 4 chroma steps. Columns 4, 8 and 12 are the
maximum chromas possible at the selected hue and lightness levels. So columns
1-12 sample the ENTIRE GAMUT for the medium.
If you have followed this so far, and have some knowledge of color science,
you can see that this is a very good representation of the color reproduction
space for the medium of the target. It is possible to set your scanner for a
variety of tonal reproductions and renderings, including a very exact
perceptual rendering.
However, since each target is keyed to its medium, then individual targets are
necessary for each medium (e.g. Kodak transparency, Kodak print, Fuji
transparency, Fuji print, et cetera). For exacting reproduction, it would be
necessary for Kodak, Fuji, Agfa and the other manufacturers to make targets
for each material. Most have standardized on one transparency and one print medium.
Please remember to place the IT8 standards in their proper historical context.
When the IT8 committee first drafted this standard, the ICC did not exist. The
only color management available on a non-proprietary platform was ColorSync 1.
That the IT8 standards have done so well for scanning calibration and
profiling and lasted so long is a testament to a well done job by an unsung
group of color experts. Instead of dumping on them and their work, we should
thank them for their help.
The difficulty people are having with using the IT8 targets is not in scanning
what they were designed for, but in scanning what they were not designed for.
The gamuts possible with printed materials, cloth, paints, inks, etc. are
different. Unfortunately, there are not any targets available for these media,
with the exception of the new GretagMacbeth ColorChecker DC. This target is
used for digital camera calibration and profiling. Due to its physical
construction, it is not suitable for scanners since the colors do not lay flat
to the scan surface and the target does not bend for attachment to a drum.
So, use the IT8 for what it was designed for, and understand that when its use
is extended beyond its original intent, it may require some tweaking of the
scanner profile to give satisfactory results.
If new standards are necessary for scanning targets, I urge all those
interested to get involved with the standards committees and work with them to
create the new targets. The best way to effect change and get what you want is
to get involved.
Robin Myers