D'nardo Colucci: Light output and resolution
- Posted on behalf of D'nardo Colucci:
Wow, this is a great discussion of resolution and brightness demands
for dome theaters. I'd like to piggy back on and add to the comments
by Ka Chun, Ed and Phil.
First, about the Elumenati specs: We use manufacturer lumens ratings
as a way to compare our products against each other. If you have
seen one of our systems in action you have a reference point and
suddenly these numbers ARE useful. With so many different system
configurations, we have find it easier to help clients calculate
expected screen brightness on a per case basis. We also provide help
on our website for calculating what screen brightness you can expect
using our products in a given configuration.
On to the discussion:
Ka Chun's write-up is fantastic. I'd like to add a couple of
comments. The first is a concept call the Kerr Factor. This is the
decrease in effective resolution due to the fact that the pixels are
in a rectangular array. There is higher resolution for line pairs
drawn directly along the vertical or horizontal axes. Lines drawn
diagonally yield a significantly lower resolution. The Kerr factor
for a rectangular grid of digital pixels is about 0.7. I don't know
if this changes given the fact that the fisheye changes the array
geometry on the dome.
My second comment is about the "hard lower limit" of 3
cycles/degree. Ka Chun says this corresponds to 600 pixels across a
180 deg. dome. I think the actual number is 1080 pixels/180
degs. In order for there to be a cycle, you need two pixels. So 3
cycles/deg = 6 pixels/deg. 6 x 180 = 1080 pixels. This agrees with
what we have determined empirically - that less than 1024 pixels
across the dome is unacceptable for extended viewing of anything
other than ambient or cartoon imagery.
Ed's write up is also great. This time I have several comments. The
first is about how one defines "cinematic quality". Ed states that
18 pixels/deg is equal to 35 mm film. This is true i the 35 mm film
is projected onto a dome but not for the true cinematic experience of
a theater where your field of view is </= 90 degs. I used to say
that our saving grace was that film transport mechanisms cause huge
amounts of jitter in the projected image - often 5"-6" on the
screen! But now that I have experienced digital movie theaters, I'm
thinking that the illusionary effect of higher resolution for a
moving image actually helps 35 mm film. Maybe I was drinking at the time....
To add to Ed's discussion of screen brightness, it's interesting that
he empirically determines that 0.5 ft-L is "pretty good" and >1.0
ft-L is "great". Studies have shown that the transition from rods to
cones (Scotopic to Photopic vision) occurs around 1.0 ft.-L. I too
feel that 1 ft-L should be the minimum design goal for screen
brightness. Remember, no matter how dark your room is, if you have
not achieved 1.0 ft-L screen brightness colors will never look great
as you have not "activated" the audience's cones.
Moving to IPS specs, Ed I think Phil's idea of a normailized dome
illuminance measure is great. The spec should be independent of the
final system but allow users to calculate the final screen brightness
given their system specifics. I would drop Peak Light Output. It's
as useless as peak Contrast Ratio - I will forever hate Texas
Instruments for changing the contrast specs for projectors!
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