Q: CRT phosphors/chromaticities

[Mahatma Kane-Jeeves]

I am seeking information about the design and manufacture of CRTs used
in common computer RGB monitors.

More specifically, I would like to know the exact colors and behaviors
of the phosphors in common use, and whether, without making actual
measurements, any useful a-priori assumptions can be made about the
color rendition and chromaticity profiles of popular monitors.

As a bit of background, this question is coming from a community of
photographers and visual artists, studying how to best present our
works on the web. We do have some engineering background.

Can anyone point me to some useful resources on this question? Thank
you in advance for your help.

--m321

[Mahatma Kane-Jeeves]

Mahatma Kane-Jeeves (m3...@world.std.com) wrote:
: I am seeking information about the design and manufacture of CRTs used

: --m321

Sorry, I think I should have been clearer in my original query.

I think what I actually need are the "coordinates", in CIE
color-space, of the phosphor formulations in common use. Also
information about how much those color-points may vary under normal
conditions, and why.

Finally, I need information on just how "commonly used" each phosphor
formulation actually is. I'm hoping I can make at least one or two
reasonably reliable assumptions about unknown, remote systems.

I do understand about variations in gamma, white point, user
adjustments, and so forth. But if I could find even just ONE
chromatic point to rely on, that would be very helpful!

Any and all replies appreciated. Thanks again.

--m321

[Tom Lianza]

Well, you are asking for some information that is not particularly hard to
find, but there is a lot of folk lore about phosphors that might get in the
way of your task. Also, having the phosphor information alone, will only
lead to wrong assumptions about the imaging characteristics that should be
of interest to you.

First I would recommend that you review the sRGB specification
http://www.color.org/sRGB.html . I am no great fan of this particular
thinking, but the document does give some background on the chromaticities
of phosphors and the rationale for selection of some chromaticities to use
as a basis for this color space. You should also take a look at B.R.M.E.A,
Radio and Electronics Engineer, 38, 201, (1969), this will give you a handle
on the acceptable variance of phosphors as defined by the European
Broadcasting Union. You will note that these are NOT uniform in CIE Yxy
space.

For reference purposes, there are three "families" that are defined by
chromaticity (as opposed to chemical content), EBU, SMPTE C, and CCIR 709

Red
EBU x = 0.64 y =0.33
SMPTE C x=0.63 y = .34
CCIR 709 x=.64 y=.33

Green
EBU x = 0.29 y =0.60
SMPTE C x=0.31 y = .595
CCIR 709 x=.30 y=.60

Blue
EBU x = 0.15 y =0..06
SMPTE C x=0.155 y = .07
CCIR 709 x=.15 y=.06

You should note that many of these values are specified only to the second
decimal point. The third decimal point is significant visually, but
measurement uncertainties realistically limit estimates of x and y to +/-
.004. When you combine the measurement uncertainties with the manufacturing
uncertainties, specifications beyond the second decimal point become
meaningless. You should note that these values are somewhat different than
the original NTSC phosphor set. That particular set offered a very wide
gamut at the expense of maximum luminance and limited life.

For the photographer, there are three very important issues that overwhelm
any differences in phosphor type: Setting Black Point, Knowing Gamma and
Specification of Viewing conditions. If you read the sRGB specification,
you will notice no mention of Black point, the surround illuminance is
assumed to be very low (64 lux?) and the white point is D65. None of these
conditions represent the normal photographic viewing environment and these
conditions are far more important than any variances due to phosphor type.

When I talk to artists and photographers about the hierarchy of adjustment
and relative "strength" of an error I use the following:

Match White point ( white balance)
Match Tone reproduction (i.e. gamma is the same)
Match Contrast Ratio (Ratio of White to Black matches)
Similar Saturation among all hues
non-preferential saturation within a hue (i.e. No Day-Glo colors unless they
existed in the original scene. )

The phosphor issue doesn't enter the hierarchy until #4 . The hierarchy
should also reflect the order of operations.

Hope this helps.....

TL

Tom Lianza (tli...@sequelimaging.com)
Technical Director
Sequel Imaging Inc.
25 Nashua Rd.
Londonderry, NH 03053