Why Are Contrast Ratios Important?
One reason contrast ratios are important is because separation between
intensity levels in an image help us differentiate things. That separation
can also be a factor in how 3-Dimensional the images look. When lights in a
room are turned on and the image is washed out to the point that it is hard
to tell what is happening on the screen, this is basically because the
simultaneous CRs off the screen have dropped very low with the extra light
added. On/Off CR is important both because it indicates the absolute black
level for a particular white level and so gives a good idea of how
realistically the projector can produce blackout conditions, and because
On/Off CR is usually the limiting factor in maintaining good simultaneous CR
in the darkest non-blackout images. ANSI CR is important because it is often
the limiting factor to simultaneous CR in brighter mixed scenes and it also
has some relevance to visibility of shadow detail in mixed scenes. We can
experience both blackouts and good simultaneous CR in real life, so in a way
these determine how realistically the display can provide those things.
I should mention that while I am discussing that a higher CR is desirable, a person can rightfully prefer less CR for whatever reason,
just like they can prefer that colors be different than standards call for,
prefer much brighter or dimmer images than the standards call for, watch
with lots of lights on in the room that change the images from the standards, etc. I see
the advantages of higher CR, and the standards call for video black to be no
light (or high CR), so that is what I will discuss. The bottom line is that
if the projector is capable of a very high contrast ratio, you will have the
option of seeing a better image, because if the movie scene has black
shadows, you will see them as black, and not dark gray.
Contrast Ratios and the Room Environment
One thing that is nice about On/Off CR is that the colors of the walls have
basically no effect on it, as long as there aren't lights on in the room,
light leaking out of the projector where there shouldn't be light, or light
coming into the room, and the only light is that going from the projector to
the images. The reason is that the amount of light reflecting around the
room and back onto the screen will be the same for the full screen 100%stim
image as for the full screen 0%stim image. Even if 20% of the light came
back, 1.2x divided by 1.2y is still the same as x divided by y. *1
Now, as to
*1, I did simplify things a little bit here, as 100%stim images and 0%stim
images can have different color balances, and so there can be minor
differences in how reflections affect them (like if the image is stronger in red
and the walls are red), but that is a complication beyond the level I want
to discuss here and likely to fall into the margin of error for most people
doing measurements.
ANSI CR is much more room-dependent. It is different than On/Off CR as far
as the effect of reflections around the room, because reflections from white
areas that go around the room do fall on the black areas, whereas with
On/Off
CR, the full white image and full black image are only raised by their own
reflections. If 20% of the light comes back and splits between the white and
the black rectangles, each will be raised by about 10% of the original level
of the white rectangles. For a projector providing 100:1 ANSI CR, raising
both the white rectangles and black rectangles by 10% of the white
rectangle's intensity would result in the brighter rectangles being 110% of
their original values, and the darker rectangles being 11% (from 10% + 1%) of
the original intensity of the white rectangles, for an ANSI CR of 10:1 just
considering the first reflections that return.
Even a projector with the best ANSI CR can have its contrast killed by a bad room, and the
best room for ANSI CR retention (basically black velvet everywhere) can only
do so much for a projector weak in ANSI CR. But ANSI CR can be helped by
things like gray screens when the projector is bright enough to maintain a
good white level with that screen. Gray screens basically help ANSI CR by
reducing the effect of secondary reflections. For instance, ignoring
directionality, a screen with 0.5 gain will kill half the light hitting it
compared to a standard 1.0 gain white screen. That won't help the CR for the
light going from the projector to the screen the first time, but only half
the light will go toward the walls with the 0.5 gain screen and any
reflections that come back will get reduced by 50% again, and so on.
So, ignoring 3rd order and beyond effects, where the darker screen continues
to help, this is similar to 0.5 gain for the initial light from the
projector and 0.25 gain for the reflections coming back from the room, or a
2:1 ratio in favor of the light you generally want to see and against the
light you want to reject. The result is less washout effect than with a
standard 1.0 gain white screen where the initial light from the projector
and the reflections both get 1.0 gain, or a 1:1 ratio.
Screens with directional layers can also help ANSI CR off the
screen for a viewer in less than perfect rooms by giving more gain for the
projected light for a viewer at a particular location compared to the
average gain for the secondary reflections. For instance, a person sitting at
a 2.0 gain position for a high gain screen should only get about 1.0 gain
average at most for reflections around the room that come from random
directions and then fall back on the screen.
I expect to see more advancements in screen design over the next couple of
years to help ANSI CR retention in less than perfect rooms and both ANSI CR
and On/Off CR retention with external lighting. In looking back at the
contrast calculator located at
http://home1.gte.net/res18h39/contrast.htm,
which I mentioned earlier in this article, one thing that these gray and
other screens designed for ANSI CR retention could be thought to do is to
decrease the "room gain (reflectivity)" value that a person should use for
that calculator, even with no changes to the walls or anything else in a
room. The reason is that the screens help reduce the effects of those
reflections, as mentioned above.
Go to Part III.