RE: Grayscale perception
RE: Grayscale perception
- Subject: RE: Grayscale perception
- From: "Mark Rice" <email@hidden>
- Date: Mon, 5 Dec 2005 23:20:41 -0500
- Organization: Zero One
Ray, I have to disagree. I have been using an LVT film recorder for 20
years. It has internal 12 bit LUTs and 8 bit output. Since the beginning, we
have been plagued by quantization in the highlight regions. And the LVT is
the best film recorder - I have used the Lightjet as well, and it does not
have as smooth a gradation or as consistent calibration. I have measured the
gradations we tested to assure that there were no "jumps" in bit values. The
eye can detect this on a high quality film recorder. The only way to get rid
of it was to introduce noise, which is very similar to dithering.
One doesn't see this effect very often on printers for two reasons:
1. The dynamic range is about a factor of 10 lower than a transparency film
recorder.
2. Dithering is almost always introduced by the RIP, the driver, or the
device itself.
One of the things I have always wanted to try was to print a very
large sheet of paper on a Lambda or Lightjet with a perfect gradation in 8
bit grayscale, with no dithering, and see if any banding can be observed.
But with most RIPs and print drivers, and even inside the black box that is
the printer, it is usually impossible to tell if dithering is occurring.
Mark
-----Original Message-----
From: Ray Maxwell [mailto:email@hidden]
Sent: Monday, December 05, 2005 10:40 PM
To: Mark Rice
Cc: email@hidden
Subject: Re: Grayscale perception
Mark Rice wrote:
>
>
>
>One measure of this is the Munsell color test. I took it when I got my
>first job in the photo lab business to see what my color discrimination
>was. It consisted of 256 color chips, mounted on wooden circles. The
>examiner mixed them up, and then asked the examinee to put them in
>order. They used subtly different hues - light desaturated green to
>light desaturated cyan, for instance. The differences between colors were
VERY subtle.
>
>
>
Hi Mark,
I believe that you are referring to the Farnsworth-Munsell 100-Hue test.
See this URL for more information:
http://www.gretagmacbeth.com/index/products/products_color-standards/product
s_color-vision-tests/products_fm-100-hue-test.htm
One of the ironies of this test is that it contain something like 88 hues.
I don't remember the exact number, but I know it was less than 100 hues. If
you had 256 hues in this test no one would be able to perform this test
perfectly.
With 88 hues most people make 3 or 4 transpositions if they have good color
perception. The scoring is too complicated to go into in a short note,
however, any score below 70 is considered "normal" color vision.
The ability to distunguish small differences in color has to do with the
magnitude of the color change (hue, lightness, saturation) and the physical
size of each color patch that is adjacent to the next.
Everyone who has made smooth appearing gradients in Illustrator or InDesign
knows that the color change can get larger if the spacing is smaller.
The magnitude of each unit in CIE Lab space, is supposed to be at the
threshold of a typical standard observer being able to distinguish a
difference.
When it comes to gray scales on reflective media, there are people that will
tell you that anything greater than 64 steps will appear smooth.
The bottom line is that it takes much less than 256 steps to make a smooth
gradient. I would suggest that it is between 64 and 100.
Ray
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