AVS Forum banner

1 - 5 of 5 Posts

·
Registered
Joined
·
28 Posts
Discussion Starter #1
Hi, I am new to the forum, and thinking about getting a first projector.


I am a bit confused about gray vs. white screens. Knowing very little about the physics of light, this would seem intuitive to me:


Lets say the amount of light reflecting off a white 1.0 gain screen to your eye is x (for blacks) and y (for whites). Then the ratio of black to white is x/y.


Now lets say we switch to a gray screen, which reflects 80% as much light as the white screen. Then the blacks would be .8x and the whites would be .8y


The ratio would then be .8x/(.8y)= x/y , the same as before.


Now lets say we add ambient light (z on the white screen). On the gray screen this would be .8z


Then the ratios would be:


white screen: (x+z)/(y+z)


gray screen: (.8x+.8z)/(.8y+.8z)= .8(x+z)/(.8(y+z)) = (x+z)/(y+z)



EDIT:


But the gray screen would produce less ambient light in the room to begin with. So the above variables aren't accurate. With less ambient light, the equations might be more like:


Let "z-c" be the level of ambient light that would reflect off the white screen if the amount of ambient light in the room were equivalent to the amount of ambient light while using the gray screen


white screen: (x+z)/(y+z)


gray screen: (.8x+.8(z-c))/(.8y+.8(z-c)) = (x+z-c)/(y+z-c)


Then, because x>y, the gray screen would have more contrast.

For example, let x=10, y=50,000, z=10 and c=2. Then:


white screen: 20/50,010 = ~4/(10^4)


gray screen: (8+6.4)/(40,000+6.4) = ~3.6/(10^4)


So the gray screen would have a better contrast ratio.


In this case, it would seem like the gray screen would increase contrast over the white screen.....

-more in rooms where ambient light from walls/ceiling/furniture was high (where c would be greater)

-not as much in rooms where light from windows/lamps were higher, and light from walls etc. was lower. (where c would be lower)



Here's another example, this time keeping screen brightness and ambient light constant on both screens:


Lets say we have the same gray and white screens as above.


But when using the gray screen, you increase the brightness of the projector by 25%.


Then the White and gray screens should have the same picture brightness and produce the same amount of ambient light.


But when this ambient light hits the screen again, it will show up 20% less on the gray screen:


Both screens will be x for black and y for white (because the projector is notched up in brightness when using the gray screen). Then the ambient light will show up as z on the white screen and .8z on the gray screen.

Then we have:


white screen: (x+z)/(y+z)


gray screen: (x+.8z)/y+.8z)


Then because x


For example: x=10, y=50000, z=10


white screen: 20/50010=~4/(10^4)

gray screen: 18/50008==~3.6/(10^4)


The gray screen would have the better contrast ratio.









But the linear equations don't seem to make sense when going to an extreme, at least with respect to what the human eye perceives. After all, if we had a screen which only allowed 1% as much light as the white screen, then my earlier equation would be:


.1x/(.1y) =x/y same ratio as the white screen.


But I doubt anyone would say the contrast ratio on the 1% screen was anywhere close to the white screen.


(lets say on the white screen we have x = 1 lumen and y = 1000 lumens, then on the 1% screen we'd have x = .01 lumens and y = 10 lumens. I would suspect the human eye would perceive much more difference between 1 and 1000 than .01 and 10


Maybe there's a sweet spot? For example, maybe the eye would perceive more difference between .5 and 500 than 1 and 1000?


I am probably really botching this up with all the guessing, so I hope you guys can shed some light (sry for that) on the subject.


I'd like to try to keep fancy light focusing/ambient light killing specialty screens out of the discussion for now, it's complex enough as it is.


Thanks



Ok, part 2:


The Mitsubishi HC4000 looks like a good projector for what I'm looking for:


-lots of movies, 1080p but also a lot of 480p.

-a fair amount of sports (fast motion clarity)

-like a less "digital" look to screens (when LCDs Flat panels first starting booming, I liked the old 480p sets better (blurrier but less "fake" looking) and plasma way better. I love my current 21.5 inch LCD monitor, but the pixels are really, really small)

-950 possible lumens in "best mode" (brill color on) sounds nice if I'd like the flexibility to use it on a larger screen or a less controlled room.


But in several reviews I read its blacks were lacking compared to the next step up in projectors.


How much would a DLP be that had the same clarity, pq, and ability to play 480p as the HC4000, but with better blacks.


Also, the rooms I am thinking of putting it in have dull blue/green walls, and off-white ceilings. Maybe for blacks it would be better to invest in black velvet or felt wall and ceiling coverings than in a better projector?


Thanks for your help : )
 

·
Registered
Joined
·
9,755 Posts



Above are photos of the apparent contrast similar to the experiment you posted. There was a desire in making these samples to keep the diffusion of the two samples the same.

I see no issues in your math above.


Keep in mind you are talking about absolute CR measured at the screen closer to a ANSI type measurement and similar to what is seen in the photos. Our eyes have iris and are in constant adjustment over a range of about 22f stops. This plays a huge role in the perception of contrast as do gray screens , brighter projectors that help put back the attenuated lumens and the effects of ambient light. Room treatments especially closest to the screen are the next factor in intercepting rebound light. Then placement and direction of desired sources of ambient light. They all play a part in what our state of vision is while viewing also. A measured black on the screen may be fairly poor but given enough brightness in the overall field of view of our eyes will look inky black thru perception.
 

·
Registered
Joined
·
4,854 Posts
So... Use gray if there is ambient light and white if room has no ambient light and dark walls and ceiling?
 

·
Registered
Joined
·
4,854 Posts
Thanks.
 
1 - 5 of 5 Posts
Top