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Some of us have been having a raging "discussion" about measuring on-off contrast of projectors over on the RS20 owners thread. In this "debate", GlenC asked a question that got me to thinking about something I've never considered: how much contrast is enough?


The basic problem is that you can have a projector with a great on-off contrast (say, 35,000:1), but you can be using it in a room that has much less than ideal light control. The effective contrast of the projector in that room could be much less than 35,000:1. Here is an example:


Suppose my projector puts out a 100IRE white image of 25 fL at the screen. Also suppose that with the projector off and all of the lights off in the room (viewing conditions), the room still has 0.001 fL due to imperfect light control.


My projector (with a 35,000:1 contrast ratio) would be putting out 0.000714 fL for a 0IRE image. But, the room is contributing 0.001 fL to the projected image, for a total 0IRE image of 0.001714 fL. In this case, the contrast ratio of the projector in my room would be 14,500:1 (and not the 35,000:1 that I paid for). This is a 58% drop in on-off contrast.



Now, suppose I do the above math, and decide that I should get a projector with only 15,000:1 contrast (and save myself some money to buy blu-ray disks with
). If this projector puts out the same 25 fL 100IRE then it will put out 0.0017 fL at 0IRE. In my (0.001 lumen) room, the black level will be 0.0027 fL for a contrast ratio of 9,375:1 (a 37.5% drop)


OK, so Now I decide on a 9:000:1 contrast ratio projector (to save myself some money for the cool hot-oil popper I read about in the theater equipment forum
). If this projector also puts out 25 fL at 100IRE, it will put out 0.0028 fL at 0IRE, and in my room, I will have 0.0038 fL at 0IRE for a contrast ratio of 6,600:1 (a 26.5% drop).


I could keep playing this game for a while and eventually I should buy a flashlight and a sock puppet for my projector.
Clearly for a given room, the contrast of the projector matters, but so does my ability to control the ambient light in my room.


Suppose I decide to go back to the 35,000:1 projector, but I fix the light control of my room so that the room now has a black level of 0.0005 fL. Now, my projector would have an effective contrast ratio of 20,500:1 in my room. (still a 41% drop). The 15,000:1 projector would have an effective contrast of 11,500:1 (a 23% drop), and the 9000:1 projector would have an effective contrast of 7,600:1 (a 15% drop).


Now, I go and put black tape over all of the LEDs on my equipment to get the ambient light level of my room down to 0.0001 fL. The 35,000:1 projector would have an effective contrast ratio of 30,700:1 (a 12% drop), the 15,000:1 projector would have an effective contrast ratio of 14,150:1 (a 5.7% drop), and the 9000:1 contrast ratio projector would have an effective contrast ratio of 8,700:1 (a 3.5% drop).


It looks like I would certainly be better off controlling the light level in my room, and I could then make a choice of projector based on my budget. However, the question is (sorry it took me so long to get to the point
). How do I measure the ambient light level in my room to determine if I need to do a better job of light control, or look at buying a less expensive projector because my effective contrast ratio won't be all that high anyway?


We had a lot of discussion on the other thread about how you cannot use a light meter that reads down to 0.001 fL with an error of +-0.002 fL (like the Minolta LS-100) to measure the room (there is way too much error in the reading to make the kind of determination I am trying to make above). So, what should I do?


[I wonder if this is the question that Glen was asking, an we all just misunderstood for 4 pages
]


--Mark
 

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There's another thing to consider, intuitively we know that in a room with "significant" ambient light, you will reach a point where more CR from the projector (lower black level) will not yield any improvement. So we could ask the question, where is that point?


From the other thread, Darin posted that the sensitivity to differences in light level is about 22% in the 0.001ftL range. If this is the case, the projector's black would be indistinguishable from the ambient light level if it's less than 22% of the ambient light, or in our example (0.001ftL ambient light)

So for our 25ftL white level, we would want 25ftL/0.00022ftL = 113,000:1 CR to avoid the projected black from being distinguishable from ambient black.


If the room were worse, 0.005ftL, then the projector's contribution could be 0.0011 (assuming the same threshold), so 25ftL/(0.0011ftL) = 23,000:1.


If the setup were dimmer, say 16ftL per DCI, it would be 16ftL/0.00022ftL = 73,000:1.


Or if the setup were better, say your 0.0005ftL: 25ftL/0.00011ftL = 227,000:1


But then as you say, how does the average person, heck, how does anyone measure/know the ambient light level of their theater when even some of the best meters (eg the Konica-Minolta CS-2000) are only specified down to 0.003 cd/m^2.


What is a reasonable goal for ambient light level?
 

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I thought some would start thinking about what I was doing and where I was going with this.


I think the discussion will reveal that the highest Projector CR possible is best in nearly every situation.


As for the CS-2000, yes, it is accurate to .4% at .003cd/m2, but they indicate it will go to .00002cd/m2, we just don’t know how accurate. On the other hand, the LS-100 (my interpretation) is +/- 2% @ .001cd/m2, however the display could be off by 2 digits. It wouldn't surprise me if they left something out of the specs. I'll contact Minolta to get clarification.


This basically started for me with my RS1, the black level was poor at best. The RS2 was an improvement, and now with over 2K-hours on the bulb, the blacks have never looked better.


I wanted to reference what was happening, therefore the LS-100. It’s not perfect, but the only affordable option to read something at .001 as indicated.


Some Measurements I have taken:

My room - RS2: 110” 1.0 screen

• Bulb 2000+hours

• Room=0.001

• “hide” black = 0.002

• White = 12.15

• Performance: projector/room CR=6075:1


• Bulb NEW

• Room=0.001

• “hide” black = 0.006

• White = 31.7

• Performance: projector/room CR=5283:1
Customer A - RS1 + ISCO III 130" 2.35 curve 2.5 gain

• Bulb 1105 hours

• Room=0.001

• “hide” black = 0.007

• White = 32.17

• Performance: projector/room CR=4595:1
Customer B - RS1 92” 1.3 screen

• Bulb 705 hours

• Room=0.004

• “hide” black = 0.025

• White = 45.7

• Performance: projector/room CR=1828:1


Customer B (Same room) – RS2 92” 1.3 screen


• Bulb 0 hours

• Room=0.004

• “hide” black = 0.016

• White = 66.09

• Performance: projector/room CR=4130:1
Customer C – Sony VW60 110” 1.3 screen

• Bulb 84 hours

• Room=0.002

• black = 0.009 (auto1) .026 (iris off)

• White = 34.45

• Performance: projector/room CR=3827:1 / no iris 1325:1
Now on their own, the performance results may be the obvious choice, and I think, to my eye or opinion, the higher CR rooms performed the best. Whether the measurements are accurate or not, the lower measured black levels do look better to me. This is the reason I took the new bulb out of my RS2.


I look forward to a productive discussion of this, because everyone is not able to have a black velvet room.


DrMark, "How much on-off contrast is enough?"


..... I think it is as much as necessary to get a desired CR in the specific application. IIRC, the human eye, while having an enormous dynamic range, capable of extremely low light levels, this only comes in steps and there can be significant time for the eye to adjust. i.e. walk into a dark commercial theater from out in daylight...... you can't see anything for a while, or in the house at night with the lights on and look outside, can't see dark detail, then there is the "headlights in the eye", you can no longer see the road....... Too much on-screen contrast can create problems for the human eye, depending on the brightness level. Too bright of an image on a very high contrast application with a high gamma can cause the eye to crush black detail. an Iris doesnt help here because there can be too high of an APL for the Iris to efectively reduce the white level to be able to see all the dark details.
 

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Quote:
Originally Posted by GlenC /forum/post/16848017


I thought some would start thinking about what I was doing and where I was going with this.

Yeah I was hoping the discussion would get moved, it was/is an interesting discussion, just didn't belong in the RS20 thread.

Quote:
I think the discussion will reveal that the highest Projector CR possible is best in nearly every situation.

I think with today's projectors at least, that is true.

Quote:
As for the CS-2000, yes, it is accurate to .4% at .003cd/m2, but they indicate it will go to .00002cd/m2, we just don't know how accurate. On the other hand, the LS-100 (my interpretation) is +/- 2% @ .001cd/m2, however the display could be off by 2 digits. It wouldn't surprise me if they left something out of the specs. I'll contact Minolta to get clarification.


This basically started for me with my RS1, the black level was poor at best. The RS2 was an improvement, and now with over 2K-hours on the bulb, the blacks have never looked better.

Frankly that doesn't really surprise me. This is just the feeling I get, but it seems as though even the vaunted RS20 fails to produce what some/many would consider "good" black level. What it does produce is exceptional compared to much of the competition but not near the 100k+ necessary to really do black, or at least to not be the largest contributor.

Quote:
I wanted to reference what was happening, therefore the LS-100. It's not perfect, but the only affordable option to read something at .001 as indicated.


Some Measurements I have taken:

My room - RS2: 110 1.0 screen

Bulb 2000+hours

Room=0.001

hide black = 0.002

White = 12.15

Performance: projector/room CR=6075:1


Bulb NEW

Room=0.001

hide black = 0.006

White = 31.7

Performance: projector/room CR=5283:1
Customer A - RS1 + ISCO III 130" 2.35 curve 2.5 gain

Bulb 1105 hours

Room=0.001

hide black = 0.007

White = 32.17

Performance: projector/room CR=4595:1
Customer B - RS1 92 1.3 screen

Bulb 705 hours

Room=0.004

hide black = 0.025

White = 45.7

Performance: projector/room CR=1828:1


Customer B (Same room) - RS2 92 1.3 screen


Bulb 0 hours

Room=0.004

hide black = 0.016

White = 66.09

Performance: projector/room CR=4130:1
Customer C - Sony VW60 110 1.3 screen

Bulb 84 hours

Room=0.002

black = 0.009 (auto1) .026 (iris off)

White = 34.45

Performance: projector/room CR=3827:1 / no iris 1325:1
Now on their own, the performance results may be the obvious choice, and I think, to my eye or opinion, the higher CR rooms performed the best. Whether the measurements are accurate or not, the lower measured black levels do look better to me. This is the reason I took the new bulb out of my RS2.

Glen, what would be really interesting, would be for you to pick a room and run several CR tests at varying distances from the projector. The CR should be constant close to the projector, up until the room/ambient light begins to play a significant role.
 

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Quote:
Originally Posted by stanger89 /forum/post/16848419


Glen, what would be really interesting, would be for you to pick a room and run several CR tests at varying distances from the projector. The CR should be constant close to the projector, up until the room/ambient light begins to play a significant role.

I plan on doing that, I've been preoccupied with trying to get a hip replacement. I plan on measuring where the screen is 1sqft then up from there. I will also be able to use the lux/fc meter to calculate light hitting the screen vs. light coming off the screen. Since the theater is in the living room, when recovering from surgery, I may totally blackout the windows to use the projector in the day. I'll have lots of time on my hands for a few months.
 

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Quote:
Originally Posted by DrMark /forum/post/16847378


Suppose I decide to go back to the 35,000:1 projector, but I fix the light control of my room so that the room now has a black level of 0.0005 fL.

Or leave the room the same and go with a projector that is brighter with lower on/off CR, except combine it with a lower gain screen. For instance, lower the black level to .0005 fL with a screen that is half as bright, but a projector that is twice as bright for white. Now the images will retain more of the on/off CR from the projector and may end up with more on/off CR in the images. The screen thing can get fairly complicated as far as getting screens with more directionality (basically higher gain for the projected light than off-angle ambient light if the viewer is in a good viewing position) and screens with gray layers or not.


In the presence of other lighting the lumens for white make a lot of difference. With enough other light the lumens for white can be a bigger determining factor to on/off CR in the images than the on/off CR of the projectors, if they are all high enough.


And in the above case a person didn't need to get a darker screen to retain more on/off CR, just a brighter projector. But the darker screen doesn't force them to have the images be really bright in order to retain on/off CR. And the darker screen tends to help ANSI CR.


Consider the RS20. Most reliable stuff I've seen and my measurements indicated that the on/off CR goes up as the iris is closed. That doesn't mean that the on/off CR off the screen goes up as the iris is closed because people can have other lights on. This isn't JVC's fault at all and doesn't mean the projector was speced incorrectly, it just means that the on/off CR off the screen isn't the on/off CR of the projector if people have other lighting. Reflections around the room from the light coming from the projector to the screen, then around the room, then back to the screen basically don't matter as far as what I am talking about here. Just other lighting, unless the projector is spewing lighting out the sides or something, in which case a light colored room could be a problem for on/off CR even without other lighting (but the projector is basically at fault for not controlling its light spill and a person could try to put something to block that stray light from the projector).
Quote:
Originally Posted by DrMark /forum/post/16847378


How do I measure the ambient light level in my room to determine if I need to do a better job of light control, or look at buying a less expensive projector because my effective contrast ratio won't be all that high anyway?

That is a tough one, especially if a person doesn't have a projector. Most people aren't going to be able to afford a high end meter and even many high end meters aren't going to be that accurate for measuring that directly. One method I would suggest if a person has a projector is to estimate it by figuring out the approximate black level of the projector off the screen, which can be done by calculating from measurements with smaller images (closer to the projector or putting the projector closer to the screen for that, but don't change zoom as it changes the light coming out of the projector). Then ND filters can be added to the projector to reduce the absolute black level an amount that can also be calculated fairly well. A person can then go about comparing the black level of the room with the projector off to the black level of the projector and room to see how dim they have to make the projector's black level to be in the same league as the room light with the projector off.


In my off white walled setup with a little bit of light leaking around some blinds on basically the same wall as the screen (but a foot or two out) even at night, and with a little bit of LED light at the back of the area (about 30 feet from the screen in the 2nd of 2 rooms that are basically combined) the absolute black level with the projector off is low enough that even when I simulated about 100,000:1 on/off CR from a projector the 16:9 screen area was easy to see with the projector like that, but not with the projector off.


ND filter material can be purchased from a place like Adorama and used for some testing like this by stacking layers, along with using some pieces to put over LED lights so that they can still be seen, but don't put much light on the screen at all.


--Darin
 

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Another angle on 'enough' after I'd asked a similar question a few years ago - I think Chris Wiggles then showed me the calculation for the contrast capability of the camera lens that captures the scene and puts it onto a movie film negative - it came out as around 2000:1. I compared the shadow detail of some films such as Spider-Man and Vaders intro in Star Wars on a 2700:1 DLP and an 8" Barco CRT (not sure if this was before or after I'd seen the calculation though). The Barco was capable of much higher on/off and black levels, but there was no extra visible shadow detail to be found.


What I found to be most import was that the Barco had been set for good black levels and was masking a lot of shadow detail (couldn't see Vaders legs for example), whereas the DLP which had calibrated levels using a test disk had more visible shadow detail. Both pjs produced excellent images but with pros and cons for each.


Film stock can have higher on/off than was captured at the lens but that doesn't introduce more detail, just better blacks.


So it could be said that greater than 2000:1 is enough to see all the detail that is present, but I think everyone would agree that a better black level is preferable to a 'greyer' one, all else being equal.


One thing I found with LEDs, and you don't need the pj to be on for this, is to move your arm across your LED displays and see if you can see a shadow created on your screen. I did this in a dark grey room with black felt to 1.5m of the screen on floor walls and ceiling and no lights, and once my eyes were accustomed to the room it was easy to see a shadow. Those LEDs (from a Denon receiver) weren't putting out a great deal of light but it was enough to raise the black level on the screen. At that time though it was lower than the black level my pj was capable of so it didn't have a great impact on the image, but I masked the display all the same.


Gary
 

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Quote:
Originally Posted by Gary Lightfoot /forum/post/16850001


Another angle on 'enough' after I'd asked a similar question a few years ago - I think Chris Wiggles then showed me the calculation for the contrast capability of the camera lens that captures the scene and puts it onto a movie film negative - it came out as around 2000:1. I compared the shadow detail of some films such as Spider-Man and Vaders intro in Star Wars on a 2700:1 DLP and an 8" Barco CRT (not sure if this was before or after I'd seen the calculation though). The Barco was capable of much higher on/off and black levels, but there was no extra visible shadow detail to be found.

This is something a lot of people get confused about. Even if a camera can only capture 2,000:1 (I'm not going to argue whether or not that's correct, lets just say that it is) that's 2,000:1 simultaneous contrast.


So what it means is that the projector/display needs to be capable of 2000:1 simultaneous contrastwhat ANSI CR tries to measure. (though I think the EBU pattern for measuring simultaneous contrast is more realistic)


A camera is exposed differently depending on the lighting conditions, so even if it is limited to capturing 2,000:1 at once, the overall dynamic range of a film is much greater, and requires a much higher contrast ratio from the display.


I threw this together quickly to illustrate it:



Here there are two scenes being captured. One might be a very dark scene at night, and the other, a scene shot on a bright day. In both cases the camera is capturing 2,000:1 but the on/off contrast ratio required by the display to accurately represent both scenes is 12,500:1.


So you would need a projector capable of both 12,500:1 On/Off CR, and 2,000:1 simultaneous contrast to accurately display both scenes.


A projector with 2,000:1 would, in theory at least, only be able to accurately display one of the two scenes, not both. (in reality though, it won't be able to display either correctly)




As for the question of how much contrast is enough?


Well, digital displays are so far off what they need to be, the answer is basically It's never enough.


With projectors being limited by the viewing conditions a lot more than direct-view displays you're going to hit a limit sooner, but even in a room that isn't properly light controlled (blacked out, velvet on the walls etc.) I'd be surprised if any current projector is going to hit that limit.


Of course, you're best to work on light control if you can. Even if your projector's contrast ratio isn't being affected by the ambient light level, the light from the projector itself will be bouncing around, lowering simultaneous contrast. (possibly significantly)



If you want a completely flat 2.2 gamma, you would need approximately 160,000:1 on/off CR. I don't think anything is even close to approaching that yet. (a CRT projector might be able to do that, but I suspect non-linearity towards black would swallow up some shadow detail, even if the on-off was 160,000:1)


Some people suggest that you should use 2.5 gamma in a completely dark room (personally, I feel that this is wrong) which would require a display capable of around 720,000:1!
 

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35mm film is a lazy S curve, in the middle linearly 100:1 (6.5 stops) and at each end non-linearly another 5:1 (2.2sotps) for a total of around 2,000:1. At the extremes the contrast is crushed making details difficult or impossible to extract. During mastering white balance point is fixed at the onset of white crushing, leaving about 1,000:1 (10 stops)

Traditional film cameras can over or under expose then adjust it when transfered to film print. Film print has a higher gamma/contrast than the real world to compensate for the lower peak white level of the display in comparison to the real world, to compensate for the dark surround effect lowering perceived contrast, to make the image more appealing and for artists intent. This contrast expansion is about 50% so 1,000:1 is expanded to 1,500:1

The DCI standards requiring mastering review room on screen 1,500+:1 contrast makes sense.

DCI comercial theaters are still within spec as long as they are 1,200+:1 but DCI uses relative luminance encoding below 5% so all levels are displayed, but with luminance levels dictated by (relative to) the projector/screen black level. When the master is encoded a high contrast projector is used and the lowest code value of luminance is assigned to the lowest luminace / black level it can display. When displayed with a higher black level - lower contrast than the master the bottom curve is made gentler - compressed so all the shadow detail is displayed.

Digital Cinema System Specification
Version 1.2
Nominal reference White 48 cdm2(14fL)
Sequential Contrast 2000:1 minimum

To display 2000:1 contrast in a room with no ambient lighting the reference projector must be capable of 2000:1 contrast

Review room
White 48 cdm² +/-2.4 cdm² (14fL +/-0.37fL)
Sequential Contrast 1500:1 minimum

48 cdm² reference white level / 2,000 minimum reference constrast spec (what a projector capable of meeting the DCI reference spec must be capable of) = 0.024

48 cdm² / 1500 minimum review room contrast spec = 0.032

0.032 review room black with 1500:1 contrast - 0.024 projector black with projector capable of 2000:1 contrast = 0.008 cdm² the amount of ambient light that they could have hitting the screen while still maintaining 1500:1 contrast.

50.4 cdm² highest permitted white still in spec for review room / 2,000 minimum reference constrast spec = 0.0252

50.4 cdm² / 1500 minimum review room contrast spec = 0.0336

0.0336 review room black with 1500:1 contrast - 0.0252 projector black with projector capable of 2000:1 contrast = 0.0084 cdm² the amount of ambient light that they could have hitting the screen while still maintaining 1500:1 contrast

DCI states the ambient light level of a mastering environment reflected by the screen is required to be less than 0.01 cdm²
 
Theatrical Presentation
White 48 cdm² +/-10.32 cdm² (14fL +/-3fL)
Sequential Contrast 1200:1 minimum

48 cdm² reference white level / 2,000 minimum reference constrast spec (what a projector capable of meeting the DCI reference spec must be capable of) = 0.024

48 cdm² / 1200 minimum theatrical presentation constrast spec = 0.04

0.04 theatrical presentation black with 1200:1 contrast - 0.024 projector black with projector capable of 2000:1 contrast = 0.016 cdm² the amount of ambient light that they could have hitting the screen while still maintaining 1200:1 contrast.

58.32 cdm² highest permitted white still in spec for theatrical presentation / 2,000 minimum reference constrast = 0.02916

58.32 cdm² / 1200 minimum theatrical presentation constrast spec = 0.0486

0.0486-0.02916 = 0.01944 cdm² the amount of ambient light that they could have hitting the screen while still maintaining 1200:1 contrast

DCI states for theatrical environments, the ambient light level reflected by the screen is encouraged to be less than 0.03 cdm² . Safety regulations and the placement of exit lights or access lights can result in a higher ambient light level. But, it is noted that this will reduce the contrast of the projected image.


Color film print is also contrast limited pre 1997 to about 400:1 intra-scene, 1600:1 sequencial, and from 1997 just under 4000:1 sequencial. Kodak Vision Color Print Film print is in theory capable of a density range of over 4.0 10,000:1, Kodak Vision Premier Color Print Film density range over 5.0 100,000:1 these immense range are limited by the dynamic range of the negative and further reduced by projection flare.

SMPTE 196M-1995, 35mm film print reference white 16ftL open gate center of the screen.
Which due to film transparency and the different lenses used is about 13.92 foot Lamberts with 2.39:1 aspect ratio film, 11.658 foot Lamberts with 1.85:1 aspect ratio film. Which is where I assume the often quoted 14 foot lambert and 12 foot lamberts comes from.

The 0.0029fL black level with projector off, for a cinema review room makes sense, with 14fL white that gives up to 4,827:1, with 12fL up to 4,138:1

Screen black level with the projector off for theatrical presentation is encouraged to be
 

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Quote:
Originally Posted by andrewfee /forum/post/16851051


This is something a lot of people get confused about. Even if a camera can only capture 2,000:1 (I'm not going to argue whether or not that's correct, lets just say that it is) that's 2,000:1 simultaneous contrast.

There's no confusion - the contrast in question was on/off/sequential, not ANSI/simultaneous.


Here's the relevant thread from the archive:

http://archive2.avsforum.com/avs-vb/...3&page=1&pp=20


It was Mrwiggles, not Chris who posted the calculation (my apologies):

Quote:
Originally Posted by MrWiggles /forum/post/0



I will try and answer the original question somewhat scientifically.


Given that our projectors typically have more dynamic range than the cameras used in original shooting, it is a very valid question to ask "why do we need XXXXX:1 contrast?"


The simple reason is that people generally like higher display gammas in dark viewing enviroments (if someone mentioned this already I appologize for repeating). For an ideal projector it has been argued that the "best" gamma is upwards of 2.5. So if the video is shot at a gamma of 2.2 you run into the following:


Camera Gamma = 2.2

Camera Dynamic Range = 1000:1 (This is about the limits of both digital and film cameras)

Display Gamma = 2.5

Display Dynamic Range = XXXXX:1


So we have on the encoding side for a dark image,


Camera Output* = (1/1000)^(1/2.2) = .043 (or 4.3% of full white signal)


thus


Display Output = .043^2.5 = 0.00039 or 2600:1


So I would say the dimenishing returns start kicking in at about 3000:1 for normal cinematography


Now some images are digitally created to begin with (i.e. infinity:1 dynamic range). Or, often digitally underexposed in post production to make them darker meaning the 1000:1 mentioned above essentially will have a multiplier effect attached to it making it effectively 3K-4K:1. Or, in many cases to get rid of low level noise in the image, they just shift values downward thus "clipping" the blacks and a total loss of shadow detail at the source level (this last one is a real no-no but happens ALL THE TIME and is a serious limiting factor to the Home Theater experience)


So, a few images/scenes out there will require higher than 3000:1 at the projector level to resolve to their best possible level. Well, there goes my scientific analysis and I'm left with giving you a subjective answer. Taking everything into effect, the only thing you can do is just assign a letter grade to the performance level and leave it at that.


I give,


2000:1 = B+

3000:1 = A-

5000:1 = A

>10000:1 = A+


The point I'm really trying to make is that with darkened viewing enviroments you need far more contrast at the display than you did at the camera level.


-Mr. Wigggles


*This equation is not entirely accurate because I have ignored the linear tail of REC 709 for two reasons. First, to make the math simpler. Second, it is hard to say how many cameras, film scanners, etc actually follow REC 709 to begin with.

Even so, it does seem to be a the case that when you get above 2000:1 on/off there's very little difference in any of the displays with regards to detail, all else being equal.



Gary
 

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Discussion Starter · #11 ·

Quote:
Originally Posted by Gary Lightfoot /forum/post/16851493


There's no confusion - the contrast in question was on/off/sequential, not ANSI/simultaneous.


Here's the relevant thread from the archive:

http://archive2.avsforum.com/avs-vb/...3&page=1&pp=20


Gary

Well, I'm no expert here, and I didn't have time to read the referenced thread, but I wonder if the 1000:1 is on/off contrast for a single setting of the iris, or if that takes into account the ability to change the appature of the camera when shooting day vs. night.


--Mark
 

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Discussion Starter · #12 ·
Glen,


If you are looking for an experiment to do, you could take measurements of a 0-IRE image using your LS-100 at 1-foot intervals from the projector and plot the light readings vs. distance. You should see the light level drop off with the inverse-square law (it won't be exactly distance^2 because of screen gain -- pointing the meter at the screen. It should be exactly distance^2 if you point the meter at the projector). There will be a point when you start to hit the limit of the meter that you will see the measurements start to flatten out. This will give you the acceptable range of the meter. I would be interested to see how the meter works in practice.


--Mark
 

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To approach this from a different angle ideally an infinite contrast ratio is required to prevent crushing near black when utilizing a constant gamma to maintain the darkest to lightest portions of the signal. Anything less than less than infiinte CR will require a trade-off or balance between near black crushing and gamma deviations near black to maintain decent looking (near) blacks. Using your 35K CR example and let's assume for a moment that the viewer wishes to see/measure differentiation between digital value 16 (ref black) and digital 17 which would be about a 1% stimulus. At a constant gamma 2.4 your .000714 black level is used up at about digital value 18, therefore values 16 and 17 will be crushed. If the goal is to have these values visible without deviating from a 2.4 gamma then clearly this scenerio is not suitable. With a 35K PJ in perfect black theater requires a gamma of less than 2.27 to prevent crushing into the black floor of digital value 17, and that's just measureable, to see digtial value 17 a gamma of much less than that would be required.


So at a CR of 35,000:1 or any other ratio there's a few trade-offs that can be employed, lose black detail at the expense of a constant numerically higher gamma, sacrifice what is considered reference constant gamma to maintain black detail, or utilize a variable gamma curve to maintian both black detail and ref gamma above 1-2% black. At anything less than infinite CR you can't have it all. To put this into perspective, for a 1% signal at 2.4 gamma not to be lost into ref black requires a CR greater than 63,000, .5% signal not to be lost greater than 633,000 CR, .1% signal over a CR of over 15 million:1 is required.


Introducing any ambient light into the equation obviously raises the black level and being additive numerically lowers gamma in the fist few percent of visible black and in the case of ambient light of .0005 fL numerically will lower gamma to about 1 in the first few visible percent stimulus which quickly ramps up to relatively normal gamma levels above that. With your proposed scenario and regardless of whether you use gamma of 2..2, 2.4., or something in between, the signals primarily affected by introducing ambient light of either .001 or .0005 fL's fall wothin the 1% and 2% stimulus range or approx 17 and 19 digital values.


How much CR is required is simple, if you wish to maintain signal integrity with a constant gamma with no ambient light the answer would be infinite. How much CR is required for a room with ambient will vary depending on light levels, how much black detail is important to you and whether you're willing to numerically lower a reference gamma in the first several percent (of not already black floor crushed stimulus) to see it. At your preferred gamma is a 2.4, you have no ambient light, and you don't mind digital value 17 at the same black level as 16 then you need a CR of 63,000:1 to achieve those goals.


Not sure if this will make any sense, I've been in the sun all day and have consumed non responsible amounts so ya never know, I have the right to delete this post tomorrow morning.
 

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Quote:
Originally Posted by Gary Lightfoot /forum/post/16851493


There's no confusion - the contrast in question was on/off/sequential, not ANSI/simultaneous.


Here's the relevant thread from the archive:

http://archive2.avsforum.com/avs-vb/...3&page=1&pp=20


It was Mrwiggles, not Chris who posted the calculation (my apologies):




Even so, it does seem to be a the case that when you get above 2000:1 on/off there's very little difference in any of the displays with regards to detail, all else being equal.



Gary

But "seeing detail" is not IMO, the determining factor in "enough" CR. For example you can usually see shadow detail more easily on a 400:1 LCD computer monitor than you can on many HT displays.


I really don't think there's a "minimum" CR required to see shadow detail (well, within reason of course).


The issue IMO is one of what the minimum video level stored on the transport medium is supposed to represent and what sort of CR is required to achieve that.


My take is that stored video is intended to hold a perceptually infinite range of luminance. To put it another way, it's intended to store a range from perceptually black (complete lack of light) to perceptually white (bright as is required).


We have simple specifications for white, 12-16ftL, yet there is not such a well known standard for black. The reason for this is IMO, largely because of the variability from room to room, and the fact that minimum video level is supposed to be perceptually black.


We have the DCI standards dovercat likes to quote. They spec 2000:1 for On/Off CR, but what does that really mean? Let's go through all the specs and see what they say.


DCI v1.2 specifies:
  • Peak white level = 48cd/m^2 (14 ftL)
  • Sequential contrast >= 2000:1
  • Ambient light level (cinema)
 

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Quote:
Originally Posted by stanger89 /forum/post/16852052


If we assume an HT ambient light level of 0.001cd/m^2 and a detection threshold of 22% at that level, the projector's contribution can be no more than 0.00022cd/m^2 to the screen brightness. So 48cd/m^2/ 0.00022cd/m^2 ~= 220,000:1

Doesn't this calculate the CR needed where white at 48, black would have no perceptible change (increase) over the room black? This would yield a visible 48K:1. In the cinema example, black at .03, a projector with a 2100:1 CR should perform in a room near .001. Problem is the blacks at .03 are really gray. If you consider cinema mastering, black = .01, 4800:1, this can be achieved with many of the newer projectors. White at 48cd/m2 is the issue. I believe this is necessary with room levels in the .01cd/m2 range, however when rooms are in the sub .005cd/m2, I don't think the white level needs to be that bright as the eye is adjusting to a different light range. There is less of a need to create a perceptual black level, thereby making it easier to see dark detail. With my white level at the 12cd/m2 range, after watching for a while, it really appears bright, the rest of the room appears black. In low APL scenes I can't even see the white mini blinds on the windows next to the screen. When I am done for the night and turn off the projector, it takes a few minutes for the eye to adjust to the darker level (sleeping mode, everything off). After 15 minutes or more, the screen is clearly visible.......


The darker the room, the less perceptual black effects, therefore the better blacks need to be to produce the richest image. Now the selection of screen size, material and room darkening all comes into play.
 
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