Contrast discussion - Home Theater Forum and Systems - HomeTheaterShack.com

Old 08-25-10, 01:23 AM Thread Starter
Shackster
Robert

Join Date: Feb 2009
Location: Sweden
Posts: 93
Contrast discussion

As I came across a fellow Swedish screen DIY guy trying to find the solution to front projection in medium to high ambient light I started to reflect (no pun intended) on the measuring you guys do at HTS.

Is it possible to define an ambient level of light and then measure the ratio between the only-ambient-light-lit screen and a screen lit with a normative light source? Am I totally of target here or am I stating something you guys allready are doing but in another way?

1. Measure ambient light level
2. Measure light level from screen (this would be the blackest black of the screen)
3. Turn on normative light source
4. Measure light level from screen (this would be a selected norm white)
5. Calculate ratio.

Alternative

1. Measure ambient light level
2. Measure light level from screen (this would be the blackest black of the screen)
3. Turn on 100% level white on a projector
4. Measure the level of light but sensor aimed at the projector
5. Measure the level of light aimed at the screen
6. 4/2 would equal theoretical max contrast and 5/2 equals screen contrast.

What I am after is some sort of objective way to measure a screen parameter often discussed. Why not do a measurement of reflected contrast (as described above) for a set of ambient light levels ranging from pitch black to normal living room levels?

I am just brainstorming here...Thoughts?

Last edited by robert_1967; 08-25-10 at 01:36 AM.
robert_1967 is offline

Old 08-25-10, 02:40 AM
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Don

Join Date: Oct 2007
Location: Central PA
Posts: 3,772
Re: Contrast discussion

Brainstorm away Robert! And BTW, go with the pun, it was funny.

Quite a while ago I wanted to do something like you are suggesting (and still think it's a good idea), but it was pointed out that while such testing would give interesting results, and meaningful within it's own paradigm, they really wouldn't mean much in the "real world" since there are so many variables in ambient light conditions; one of the largest being the color of the ambient light. What light is hitting the screen? Is it daylight or from an incandescent or fluorescent source; these are all very different colors. Is it a combination? And then the colors of the room surfaces can even come into play.

Total lumens of ambient light will affect image contrast, but the color of that light will affect the color of the image as well. I personally find that the further the color of the ambient light is from the color of the projected light the more it affects the quality of the image - but this might just be me.
Harpmaker is offline
Old 08-25-10, 05:37 AM Thread Starter
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Robert

Join Date: Feb 2009
Location: Sweden
Posts: 93
Re: Contrast discussion

Sure you are correct. The tests of screen materials such as Black Diamond speaks for them selves. Still, if we do not measure anything we never will get any closer of quantifying a parameter most HT ppl talk about daily.

Is it so simple that a so called (very incorrect yes, but still) "High Contrast" Screen is only a gray screen with optical carateristics for a narrow viewing cone? Dispite all efforts from DIY and the industry, tyhat is all it boils down to?
robert_1967 is offline
Old 08-25-10, 07:03 AM
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Location: Gainesville, FL, USA
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Re: Contrast discussion

I believe that his point is that the variablity between settings is too great for ambient light measures to be meaningful.

Looking for me, just google my username. I have used the same one for most sites for many years.
lcaillo is offline
Old 08-25-10, 12:24 PM Thread Starter
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Robert

Join Date: Feb 2009
Location: Sweden
Posts: 93
Re: Contrast discussion

Yes, harps point came across clear, I am just stubborn and challenging to my nature

Put it this way....when a manufacturer claims that it can "free up" more contrast given by the projector it is a pure subjective statement as there is no way to objectively proove it?
robert_1967 is offline
Old 08-25-10, 07:18 PM
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Don

Join Date: Oct 2007
Location: Central PA
Posts: 3,772
Re: Contrast discussion

Quote:
robert_1967 wrote: View Post
Sure you are correct. The tests of screen materials such as Black Diamond speaks for them selves. Still, if we do not measure anything we never will get any closer of quantifying a parameter most HT ppl talk about daily.
I agree Robert that readings such as you suggest should be done. The proper instrument for doing them is a 1° spot meter like Mech has. I believe he has a contrast study planned, but free time to do it is lacking. Please me believe me that we are as frustrated with our lack of free time to do experiments as you folks are.

Quote:
Is it so simple that a so called (very incorrect yes, but still) "High Contrast" Screen is only a gray screen with optical carateristics for a narrow viewing cone? Dispite all efforts from DIY and the industry, tyhat is all it boils down to?
When it comes to true image contrast vs apparent image contrast nothing is simple. One of the many problems in defining screen/image contrast is that the human eye is SO ADAPTIVE. Not only does the iris of the eye let in more or less light by opening and closing just like a camera lens diaphragm, there are also chemical changes taking place when things go dark so that the eye become more sensitive to light. This adaptability is why the eye is a horrible light meter and colorimeter for judging absolute light levels and colors. It's also why it's impossible to look at a screen and tell what gain it is. Some people think they can do this (and I'm not talking about Robert here), but they are wrong. In side-by-side testing with the same image on two different screens it is possible to say that one screen appears brighter or more contrasty than the other, but we CAN'T tell HOW MUCH brighter or contrasty using our eyes alone.

Projector manufacturers are using contrast numbers to sell projectors without explaining what those numbers really mean (and they don't mean much). I found a page that has, imo, some really good information on the whole situation here.

Here is one of the most important things I read there:

Note: Some definitions for contrast ratio imply that an increased contrast ratio yields better subtle shadow detail and increased color palette. This is NOT true. These definitions fail to differentiate between an imaging device ability to display finer image detail for which a sufficient level of contrast is necessary for the eye to perceive the finer detail, and subtle shadow and color palette.
A device ability to display subtle shadow arises out of its ability to display various levels or intensities of gray - referred to as shades of gradation - from complete black to complete white. This depends on its grayscale capabilities rather than on a higher contrast ratio.
There is also the issue of grayscale uniformity - the ability to maintain all shades of gray close to the D65 (or 6,500K) standard across the entire brightness range to preserve color fidelity at all brightness or luminance levels. Many TVs fail here - shifting towards red in the darker areas and blue in the lighter parts of the image.
Harpmaker is offline
Old 09-11-10, 01:00 PM Thread Starter
Shackster
Robert

Join Date: Feb 2009
Location: Sweden
Posts: 93
Re: Contrast discussion

Sorry for beeing afk..
robert_1967 is offline
Old 09-11-10, 04:57 PM
Plain ole user

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Join Date: May 2006
Location: Gainesville, FL, USA
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Re: Contrast discussion

What is "afk?"

Looking for me, just google my username. I have used the same one for most sites for many years.
lcaillo is offline
Old 09-11-10, 07:04 PM
Moderator Emeritus
Don

Join Date: Oct 2007
Location: Central PA
Posts: 3,772
Re: Contrast discussion

Quote:
lcaillo wrote: View Post
What is "afk?"
Urban Dictionary says it means "Away From Keyboard"; used a lot by on-line gamers.
Harpmaker is offline
Old 09-12-10, 04:21 AM Thread Starter
Shackster
Robert

Join Date: Feb 2009
Location: Sweden
Posts: 93
Re: Contrast discussion

My bad . I do have a history as gamer. In this context I shall endevour to use "non urban" words
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