How Much Simultaneous CR Can We See
at
Once?
I don't have an absolute answer to this one, but
I know the number isn't as low as I've seen some claim. I have seen
several people say that projectors don't need to be able to do more than
100:1 CR at once since humans can't see more than that. I don't know
where that number came from, and I'm somewhat baffled since I can easily
prove that people can see transitions much higher than that to anybody
with close to normal vision in less than 5 minutes in my theater, but
for some reason it seems to persist. If we could only see transitions
with a 100:1 CR or less range at once it would make projector design
easier, but that isn't the case. When bright and dark objects are very
close in proximity, our ability to discern levels can be low, but with
images on projection screens, levels of very different intensities do
not have to be right next to each other.
I have talked with one
company who works on High Dynamic Range displays that can do way more
than 100:1 at once and can been seen by people viewing their displays,
and they don't know where that number came from either. I was told that
vision specialists have said that the human eye can perceive somewhere
in the range 100,000:1 without any adaptation of the iris. I haven't
devised any test for measuring the most CR I can see this way since I
have trouble measuring things close to that high with any measuring
equipment I have, but I know that transitions that are beyond 2000:1
simultaneous CR from the brightest objects can be easy to see. This
doesn't mean that I can see beyond 2000:1 in every image, but there are
conditions where I can see beyond that easily. One test I've run is
using an image I have of a skeleton on a "black" background. The
skeleton takes up only a small part of the image, so the simultaneous CR
is much higher than the ANSI CR from my projector and room, and lower
than the On/Off CR. By zooming the image from my projector down to
smaller than the screen area, I can get transitions from the projected
"black" to an area that is lit from washout effects and then from that
to the screen border. In this test, it is easy to see some detail in the
whites on the skeleton, then the projected "black", then the area beyond
the projected border, then the screen border. I cannot see the
difference between the screen border and the black velvet behind it
though. While I can do measurements to find that the CR from the white
on the skeleton to the area inside the screen border is more than a
couple thousand to one in my mostly black velvet room, I don't have
anything that can measure the light level off the screen border
accurately.
For a while I thought that the 100:1 simultaneous CR
limit floating around came from confusion with the Contrast Sensitivity
Function, but I'm not sure if that is the main source of this. In a book
by Poynton, he says that humans can discern different luminances across
about a 1000:1 range at a particular state of adaptation, but there are
references on the Internet to him claiming a 100:1 limit. This may have
ultimately come from his discussion of CSF and threshold of 1.01:1
though.
In any case, I will explain why the CSF does not tell us
the upper limit for how much simultaneous CR we can see. The main reason
is that CSF doesn't measure the upper limit, it is a measure of the
lower limit, or how we perceive low contrast cycles, not high contrast
cycles. The Contrast Sensitivity is the inverse or reciprocal of the
Contrast Threshold. That is, as the Contrast Threshold gets smaller
(like off-white on white), the Contrast Sensitivity goes up. A Contrast
Sensitivity score of about 100:1 means that a person started to be able
to differentiate levels when they got to around 1% or CR of 1.01:1
(although those aren't exact), and as the separations got larger than
1.01:1, they would continue to be able to see them. Because some people
miss the inverse or reciprocal part here, they may falsely assume that
higher Contrast Sensitivity is higher CR when the opposite is true. Or
that Contrast Sensitivity going down as our eyes age means we would want
less CR, when in fact lower Contrast Sensitivity scores mean that we
need more differentiation (or more CR) between levels to be able to see
them as our eyes age. However, we also might want brighter whites as we
age and our eye's ability to pick up light decreases.
Readers are
welcome to try their own experiments putting black posterboard over part
of their screens with images of some white levels and a lot of video
black or by using their hands or something else to create shadows in
those kinds of images and see if they have any trouble seeing those dark
transitions and white transitions at the same time. There is a spot in
Sin City in chapter 20 at 1:28:45 on the regular DVD that can be
useful for this, although it will probably require lowering the
brightness setting a few notches since the background isn't
encoded completely as video black. Here is a shot of that scene:
Copyright Dimension Studios
A projector with just 100:1 ANSI CR and 1000:1 On/Off CR
could do much more than 100:1(and less than 1000:1) simultaneous CR in a
scene like that, and most of the projectors discussed here can beat both
of those. A white room could hurt the simultaneous CR off the screen,
but the brightest part of this scene only takes up part of the image,
and part of that could be blocked with dark material (leaving some white
with detail) while testing to reduce the effect of reflections around
the room, if desired. If the simultaneous CR off the screen is too high,
a person might not see the difference between the screen and black
poster-board or a shadow created in the darkest parts of the image, but
I don't know of any projectors which would create that problem at the
moment. Note that shadows with setups which have light coming from more
than source (like dual projectors or projectors with three lenses) are
not the same as shadows created from a single light source.
Why On/Off CR Matters More Than Absolute
Black Level From a Projector
A common question I see
is how projectors compare for absolute black level. I steer people
toward On/Off CR even if what they ultimately care about is absolute
black level, because front projectors cannot give you images on their
own. They require a surface. Without a surface there are no images to
see. And the user gets to determine what surface is used to a large
degree, along with whether they will use any kind of filtering (like a
neutral density filter) on the projector, although these mostly apply to
digital projectors and not to CRTs. Until a surface (usually a screen)
and filtering (or not) are chosen, there is no absolute black level in
ft-lamberts or cd/m2 (values for light coming off the
screen), but rather only in lumens (values for light going toward the
screen). And if filtering and screens are chosen such that two setups
produce the same ft-lamberts for white, then the one with the higher
On/Off CR will have the lower absolute black level. The math just works
out that way with the following equations which are all forms of the
same thing:
(On/Off CR) = (white level) / (black level)
(white
level) = (On/Off CR) * (black level)
(black level) = (white level) /
(On/Off CR)
With any two of the above, we can determine the
third, and the last one shows that for the same white level, a higher
On/Off CR means a lower black level.
As an example, let's
consider two projectors with the following properties:
Projector
A: 1000 lumens and 2000:1 On/Off CR
Projector B: 500 lumens and
1500:1 On/Off CR
If we just look at the absolute black levels we
get:
Projector A: 0.5 lumens
Projector B: 0.33
lumens
We can see that projector B has the better absolute black
level if we just consider what is coming out of the projector. However,
if we look at a complete solution which includes a screen (and possibly
a filter), the results can change. Projector A is fairly bright, so
let's put it on a screen with a gain of 0.5. And we'll pick a size which
results in 15 ft-lamberts for white. Projector B isn't bright enough to
use with that screen and have the whites be bright enough if somebody
wants in the 15 ft-lamberts range, so for that one we will use a 1.0
gain screen. The results here are the following:
Projector A: 15
ft-lamberts for white and 0.0075 ft-lamberts for absolute black
Projector
B: 15 ft-lamberts for white and 0.01 ft-lamberts for absolute
black
As can be seen, the projector with the higher On/Off CR
ended up with the lower absolute black level once things were set up to
the same white levels, even though it had the brighter black level
coming out of the projector lens. This is also not even counting that
the darker screen that could be used with projector A will help kill
reflections off the walls and help the ANSI CR. Even if a person was
happy with 7.5 ft-lamberts for white and would have used the 0.5 gain
screen for projector B, they could have put a 2x neutral density filter
on projector A (if it was a single lens projector like a digital) for
the same white level and once again that lower absolute black level off
the screen.
People do not have infinite choices for screen gain
among commercial screens out there, but even so, I would encourage
giving the On/Off CR more weight than the absolute black level out of
the projector lens (or lenses).
Go
to Part V.