(comp.sys.ibm.pc.hardware.video added back in; please leave it,
as this is the group in which I normally see these discussions, and
this is quite a bit more relevant to a discussion of electronic display
technology - nominally the field covered in that group - than in
digital photography per se.)
No problem. I just didn't wish to xpost to a bunch of groups that I
weren't sure were interested in the topic.
Unfortunately, no one seems to be able to describe precisely
what these "limitations of LCD backlighting" ARE.
Well, that lack is easily remedied.
The limitation is due to the fact that LCD pixels work by phase-shifting
the light from the backlight through a colour filter & a sheet of
polarising film. Because the backlight must illuminate the entire screen
at the brightest possible luminance that may be required at any point on
the screen, & because it's not possible to make a pixel, (or the border
around it) 100% opaque, there will inevitably be some degree of light
leakage at any usable brightness setting.
(Note for non-experts: The brightness control on an LCD only sets the
brighness of the backlight - nothing else - so the brighter you set the
control, the more leakage you get.)
OTOH, CRTs illuminate each pixel by modulating an electron beam, & it's
trivial to drive the modulation to 0%. While it's true that you'll get a
certain amount of internal reflection in the faceplate of the CRT, it'll
be quite minimal on any correctly adjusted monitor, of even average
quality. The big weakness of CRTs is external reflection from that big
slab of glass. In a professional enviroment, you deal with that by
working under subdued, indirect lighting, & ideally have a light hood &
a CRT with a high quality anti-reflection coating.
Nor do these
factors ("black level" and "shadow detail") have anything to do
with color gamut, which was the original point of disagreement.
I'm not arguing that particular point, as I don't think it's a question
that has an answer in the general case. Some CRTs will have a 'better'
gamut than some LCDs, & some LCDs will have a 'better' gamut than some
CRTs.
However, since you've brought these items up, let's dispose of
them as well.
It is absolutely correct that the LCD panels as typically used in
current monitor products provide a higher luminance at the
"black" level than a properly-adjusted CRT. However, this
is not necessarily indicative of an inherent inability of the LCD
to match the CRT's performance in this area.
Well, yes & no. In engineering terms, it's inherently harder to design &
manufacture an LCD panel that is capable of similar black-levels to
those of a CRT, at similar brightness levels. That said, I agree that
it's not an insoluble problem *if* you throw enough effort & money at
the problem.
As they have
been driven by today's marketing expectations, LCD monitor
panels also offer a very significantly higher white luminance
(as much as 3-4X) over typical CRT monitor products.
Yes, but with the drawback that using your LCD at maximum brightness
dramatically brightens your blacks. Each LCD pixel only has a specific
contrast range, & its darkest value is not black. My experience has been
that the only way to get a usable (for photographic purposes) black is
to wind the brightness down to the minimum level that'll give me a
minimally-usable white level.
Since the LCD does operate as a "light valve" - and it is
impossible to completely block the light from the backlight
- it is also correct to say that the LCD will, in practical terms,
never provide an absolutely zero-luminance black. But then,
neither does the CRT when properly adjusted. A CRT
monitor's black level must be set very slightly above zero,
or else the CRT cutoff point would have to be constantly
adjusted (the black would tend to slip below blanking,
making the lower end of the luminance range very non-linear).
Yes, all true. But that black level is still a hell of a lot lower than
anything I've seen on an LCD.
Typical real-world values for the black-level luminance will
be in the range of 0.05-0.5 cd/m^2 (most often toward the
upper end of this range),
Yep. My CRT's currently reading a black level of 0.12 cd/m^2.
vs. somewhere around 1 cd/m^2 or
so for a typical monitor LCD.
Yes, that's the kind of number I've seen when calibrating my own LCD
monitors. (It's worth mentioning that in terms of human perception,
that's around a 9dB difference, or in photographic terms, 3 F-stops in
luminance. That's a *very* dramatic difference!)
But since the LCD's white
these days is up around 350-400 cd/m^2 (vs. something
more like 100 for the CRT), simply cutting down the backlight
output (or, in a more brute-force approach, filtering the LCD)
would bring its black and white values down to something
very comparable to the CRT.
Great minds think alike.
That is exactly how I optimise the
black-levels on my LCDs. Unfortunately, I've found that they then fall
short on whites.
That few products do this
says much more about the demand in the market than it does
anything at all about inherent differences in the technology.
In terms of the "shadow detail" (i.e., the ability to
discriminate "shades of gray" at the low end of the luminance
range, we get back to differences in the response
characteristics of the two technologies - which, if you'll
look back over this thread, was my point in the first
place.
Sure, I agree completely on that. (And we haven't even gotten to the
problem of poor monotonicity/tonal-resolution of LCD drive technology in
comparison to CRTs.) But unless I'm misunderstanding you, you're
agreeing that LCDs *are* weak on accuracy in shadow detail, & the fact
is that for my photographic work, tonal accuracy in the darker areas is
critically important. That's obviously not a major issue for probably
99% of the monitor market, but it is for many photographers.
To summarise my position:
(1) I'm not arguing gamut one way or the other, just contrast,
black-levels & tonal accuracy.
(2) In a typical environment, LCDs will usually give you a much better
contrast range, but in carefully controlled environment that's been
highly optimised for these purposes, a CRT is usually greatly superior
in those areas.
In short, calculations are