[mechman]

If you go to PJCentral and use their calculator, they automatically adjust for the initial bulb dropoff and for content. Bill had asked them about this a few years ago and this was their reply:

Our calculator pre-ages the lamp by 25% before it does its calculations. We do this so that people do not setup their projectors at maximum lamp performance, but rather halfway through the lamp life. This ensures a quality viewing experience throughout the life of the lamp. Therefore, you can expect our fL to be 25% lower than you would calculate with a new lamp.

We also assume a projector has an ANSI lumen rating calibrated for presentations (9000+K) unless the manufacturer states that it is calibrated at a different color temperature, such as 6500K. This means that if you take a projector that is calibrated for presentations and select Video/Movies or Games as your Primary Use, we will automatically reduce the lumen output of the projector based on the new color temperature that is required for video.

Dave

So if you use their calculator, the numbers should be pretty close to what you should expect. That's where I got the 60fL number from. :T In a typical Home theater environment, the minimum fL should be around 12. I ran my Mitsubishi HC3000U at around 10fL at times in ambient light. It still performed fine to my eyes. And yes, pre-calibration and post calibration fL readings will be different - they'll be lower.

 

[bobbyt]

Yes, I would but unfortunately PJ Central doesn't allow the selection of the LNS-T21 long lens.

EDIT: I used the stock lens in the calculator and derated it by 40% and came up with the 60fl that you did, Mech. Still, if I back down to a 35fl PJ, I'd only have a 5:1 CR -- pretty washed out.

Am I looking at this the wrong way?

-- Bob

 

[bobbyt]

Since the only difference between the Scorpion8 and an OTS color is gain and since I have lots of PJ light, I think I've decided to go with the TV Winter Mountain in Valspar Signature matte. Much simpler with no mail ordering of the components. If after seeing it I need something darker, I'll go with BW. The N8 seems to be a reasonable compromise for full lights on and mostly dark.

Does anyone know what the gain of an OTS painted screen is in a matte finish?

Thanks,
Bob

 

[Harpmaker]

Since the only difference between the Scorpion8 and an OTS color is gain and since I have lots of PJ light, I think I've decided to go with the TV Winter Mountain in Valspar Signature matte. Much simpler with no mail ordering of the components. If after seeing it I need something darker, I'll go with BW. The N8 seems to be a reasonable compromise for full lights on and mostly dark.

Does anyone know what the gain of an OTS painted screen is in a matte finish?

Thanks,
Bob

That makes perfect sense Bob. :T If you have the lumens to spare you don't need a high gain screen mix. We haven't tested the gain of regular paints, but it IS something that should be done. The gain of the Signature Matte gray will be higher than a true flat paint the same shade, but still not enough to hot spot.

I used the stock lens in the calculator and derated it by 40% and came up with the 60fl that you did, Mech. Still, if I back down to a 35fl PJ, I'd only have a 5:1 CR -- pretty washed out.

Am I looking at this the wrong way?

I see how you are coming up with your CR figures, and I don't believe that is the correct way to do it. It might work that way for a white totally diffusive screen, but when you start using a gray screen, and especially a gray with reflective elements in it, that changes the situation quite a bit. While no passive screen can actually increase the CR beyond the projector specs, it CAN increase perceived image contrast. This is due to an optical illusion where the gray screen absorbs light to darken the black levels (and white levels too) of the image, but the human eye/brain still sees a "white" object (like snow, a white dress, white shirt etc.) as being truly white when it is in fact a light gray.

The On/Off contrast figures that most projector manufactures use are pretty much worthless. A better contrast spec is ANSI Contrast where an image consisting of rows of alternating black and white rectangles is shown and the ANSI contrast is the average luminance difference between the white and black blocks.

 

[bobbyt]

I see how you are coming up with your CR figures, and I don't believe that is the correct way to do it. It might work that way for a white totally diffusive screen, but when you start using a gray screen, and especially a gray with reflective elements in it, that changes the situation quite a bit. While no passive screen can actually increase the CR beyond the projector specs, it CAN increase perceived image contrast. This is due to an optical illusion where the gray screen absorbs light to darken the black levels (and white levels too) of the image, but the human eye/brain still sees a "white" object (like snow, a white dress, white shirt etc.) as being truly white when it is in fact a light gray.

Harpmaker, I follow what you say about perceived contrast increase with a gray screen. Makes perfect sense.

My understanding of the actual (observed at the screen) CR though is that it is simply the incident white light from the PJ (peak white) divided by the incident ambient light (darkest black). This assumes that the CR of the projector is infinite, which for my level of ambient light (7fl) is good to a first approx.

For my case: 80fl / 7fl ~ 11:1 whether white screen or gray. As you point out though, the gray will appear to have more contrast.

For the Sanyo PLC-WM5500 the CR is rated at 800:1 so black from the PJ is 80/800 = 0.1fl and is in the noise when compared to the ambient.

Is this what you are saying is not correct?

Thanks for your patience,
Bob

 

[Harpmaker]

Harpmaker, I follow what you say about perceived contrast increase with a gray screen. Makes perfect sense.

My understanding of the actual (observed at the screen) CR though is that it is simply the incident white light from the PJ (peak white) divided by the incident ambient light (darkest black). This assumes that the CR of the projector is infinite, which for my level of ambient light (7fl) is good to a first approx.

For my case: 80fl / 7fl ~ 11:1 whether white screen or gray. As you point out though, the gray will appear to have more contrast.

For the Sanyo PLC-WM5500 the CR is rated at 800:1 so black from the PJ is 80/800 = 0.1fl and is in the noise when compared to the ambient.

Is this what you are saying is not correct?

Thanks for your patience,
Bob

It's getting late for me Bob, let me look into this further tomorrow and get back to you.

 

[Harpmaker]

Sorry for the delay in answering this CR question. My main PC recently died and recovering the data from it (including bookmarks) it proving to be a bit problematic so I had to hit the search engines to refresh my memory on the subject.

ANSI Contrast: Contrast is the ratio between white and black. The larger the contrast ratio the greater the ability of a projector to show subtle color details and tolerate extraneous room light. There are two methods used by the projection industry: 1) Full On/Off contrast measures the ratio of the light output of an all white image (full on) and the light output of an all black (full off) image. 2) ANSI contrast is measured with a pattern of 16 alternating black and white rectangles. The average light output from the white rectangles is divided by the average light output of the black rectangles to determine the ANSI contrast ratio. When comparing the contrast ratio of projectors make sure you are comparing the same type of contrast. Full On/Off contrast will always be a larger number than ANSI contrast for the same projector.

ANSI Contrast image:

It finally struck we why I was feeling a little uneasy with your method for determining CR. You are using INCIDENT light levels which only works when using a pure white Unity gain screen. You are also not adding the ambient light level to your projector output.

A better way of determining CR is to measure the amount of light being reflected, not hitting, the white and black areas of the target image. This method even takes screen gain into account, which is why projector manufactures don't use it.

Using a gray screen will absorb some of the ambient light thus decreasing it's perceived impact on contrast. This brings us back to that optical illusion I mentioned. A N8 perfectly diffusive surface (a really, really flat paint can come close) reflects 56.7% of the light striking it. Most flat paints will reflect more than that, but since we haven't actually measured their gain we don't know how much.


The site below has a bunch of good stuff on contrast, and links to further good stuff.
http://www.presentationtek.com/2006/07/31/contrast-ratio-sectrets-uncovered/

 

[mechman]

Another good reference is at the Chromapure site. Hit the Color Science button on the top. :T Here's the link for understanding contrast.

 

[bobbyt]

Thanks for your reply, Don. I hope your PC recovery is relatively painless. (I know, fat chance!)

I don't understand why you say that I'm not adding the ambient light. I'm only including the ambient and ignoring the PJ black since it's so much lower than the ambient level.

I fully agree with you in using the ANSI chart for evaluating contrast performance. :T But this is for evaluating an existing system, not for specifying a new design. For a new design we have no direct performance measurements -- we only have:
1. PJ white lumens
2. PJ black lumens
3. ambient light lumens
4. screen gain

A terrific article upon which I based my calculations is on the Da-Lite web site:
http://www.da-lite.com/education/angles_of_view.php?action=details&issueid=8

In it there is an equation for effective contrast which takes into account these four variables:

where LA is the PJ white lumens * gain
LB is PJ black lumens * gain
rho = 1.0 for front projection
Lamb = incident ambient in fc

At the end of the article is a handy formula for minimum PJ brightness required for an observed 10:1 CR:

which assumes a modest 50:1 PJ CR.

For my application with a Lamb = 7fc, screen area = 40sq ft. and screen gain of 0.9, the min PJ output is 3600 lumens (less for a PJ with a CR better than 50:1). At 3300 lumens for the PLC-WM5500 with the long lens, I'm just about there.

-- Bob

 

[Harpmaker]

Thanks for your reply, Don. I hope your PC recovery is relatively painless. (I know, fat chance!)

Thanks. The problem I am having is one I didn't foresee and is being caused by the use of an add-on IDE hard drive controller card (Maxtor) that I had assumed used a standard format - it turns out the drives used with this controller can't be read by a standard on-board IDE controller so I must install the old controller into the new PC to read the old drives. Everything is SATA now and I have run out of molex power connectors so I have to order some adapters to get around that problem, then comes the challenge of installing the old controller card... you get the picture. :doh:

I don't understand why you say that I'm not adding the ambient light. I'm only including the ambient and ignoring the PJ black since it's so much lower than the ambient level.

My bad, I see that the formula does take ambient light into consideration. :blush:

I fully agree with you in using the ANSI chart for evaluating contrast performance. :T But this is for evaluating an existing system, not for specifying a new design. For a new design we have no direct performance measurements -- we only have:
1. PJ white lumens
2. PJ black lumens
3. ambient light lumens
4. screen gain

A terrific article upon which I based my calculations is on the Da-Lite web site:
http://www.da-lite.com/education/angles_of_view.php?action=details&issueid=8

In it there is an equation for effective contrast which takes into account these four variables:

where LA is the PJ white lumens * gain
LB is PJ black lumens * gain
rho = 1.0 for front projection
Lamb = incident ambient in fc

Thanks for the link to the Da-Lite article. They have a number of good articles there even though they are a bit dated.

LA is the Lumen output of the projector divided by the surface area of the screen multiplied by the screen gain (if any).

p is the term which accounts for whether the display is front or rear projected. If front, let p = 1.0. If rear, let p = 0.2.

LB is the display black level and Lamb is the ambient light incident to the screen's surface, measured in foot candles.

At the end of the article is a handy formula for minimum PJ brightness required for an observed 10:1 CR:

which assumes a modest 50:1 PJ CR.

For my application with a Lamb = 7fc, screen area = 40sq ft. and screen gain of 0.9, the min PJ output is 3600 lumens (less for a PJ with a CR better than 50:1). At 3300 lumens for the PLC-WM5500 with the long lens, I'm just about there.

-- Bob

While I don't mean anything against Dick Blaha (the author of the formulae) who has probably forgotten more math than I will ever know, I don't understand some of the things this last formula implies. Using your criteria of a 40 sq ft. screen area and a screen gain of .9 he says that a PJ shooting 3600 lumens is required for a CR of 10:1, and increasing screen gain to Unity (1.0) drops the lumens required down to 3150. In the presence of ambient light a gray screen (with no added reflective particles) will provide more perceived contrast than a white 1.0 screen with the same PJ lumens. The key here may be perceived contrast, which I don't think the Da-Lite article addressed. :dontknow:

Something that is very illuminating (pun intended) is the chart included in that article which shows just how little light it takes to RAPIDLY decrease the contrast ratio of a projected image.

 

[wbassett]

Interesting conversation, and I can pretty much say that they are talking about actual CR and not perceived CR. The reason, which is probably pretty clear because of all the math and formulas- 'Perceived' is a variable and differs from one person to another. It's something you can't define or nail down so you can't use it in a math equation.

I too have always been amazed at how little light is needed to destroy CR and bring a 5000:1 projector down to 100:1. It only takes one lit candle in a room to do that. A projector can even create enough light from the images being reflected off the ceiling or adhacent walls to cause this to happen. During professional calibrations if a person walks in the room wearing a white T-shirt it is enough to mess with the calibration. It really is amazing how little it takes to mess up the image.

You guys mention 10:1, if I recall, that is the lowest percieveable CR that the human eye can actually see an image.

 

[bobbyt]

Don and Bill, I think you are both correct as to the perceived vs. actual CR.

It seems to me that going from a white to a gray painted screen (without adding any of the directional components found in a high gain screen) simply decreases the gain of both ambient (black) and white from the PJ, leaving the actual CR unchanged. The difference is that the blacks look darker and if the whites are still acceptably bright (PJ has enough lumens) the perceived contrast is improved.

On the other hand, if the screen is made highly directional, ambient light hitting the screen at an angle is attenuated to the viewer since it is off-axis. However, since all of the PJ light is "on-axis" with respect to the viewer, the real CR will improve at the expense of viewing angle.

Bill, according to Da-Lite a 10:1 is the minimum target CR for a projector system design to produce "acceptable" images. From the article:

One of the very best seminars offered at Infocomm ‘99 was given by Dick Blaha ([email protected]) and was entitled Effective Application of Display Technology. During that part of his presentation which dealt with Contrast, Mr. Blaha maintained that the design goal for a display system's Contrast Ratio should be "at least 10 to 1, white/black in the operating ambient light conditions."

Update on Lowes color matching
BTW, I went to 2 different Lowes stores today to buy some Valspar in TV Winter Mountain. They had no listing for it either by name nor by color code (1982). I settled for Benjamin Moore #1612 which is almost as good (RGB = 200 202 202). I'm painting up a test piece of DW now.

 

[Harpmaker]

Don and Bill, I think you are both correct as to the perceived vs. actual CR.

It seems to me that going from a white to a gray painted screen (without adding any of the directional components found in a high gain screen) simply decreases the gain of both ambient (black) and white from the PJ, leaving the actual CR unchanged. The difference is that the blacks look darker and if the whites are still acceptably bright (PJ has enough lumens) the perceived contrast is improved.

On the other hand, if the screen is made highly directional, ambient light hitting the screen at an angle is attenuated to the viewer since it is off-axis. However, since all of the PJ light is "on-axis" with respect to the viewer, the real CR will improve at the expense of viewing angle.

In my limited experience, on-axis gain caused by adding reflective elements to the paint do not appear to increase actual CR. Black levels are increased along with white levels and I believe physics demands that they are proportional - if whites are 20% brighter than blacks must be 20% brighter as well. What this effectually does is give you a screen mix that has the ambient light properties of the actual shade of gray of the mix, but the directional projected image appears to be hitting a lighter shade of gray screen. Our reflective mixes have a very wide viewing angle.

I believe Mech is scheduled to do some actual testing of this.

Update on Lowes color matching
BTW, I went to 2 different Lowes stores today to buy some Valspar in TV Winter Mountain. They had no listing for it either by name nor by color code (1982). I settled for Benjamin Moore #1612 which is almost as good (RGB = 200 202 202). I'm painting up a test piece of DW now.

After thinking about this I believe I know why TV 'Winter Mist' and 'Winter Mountain' may no longer be in Lowe's system. I'm fairly sure that 'Brown Oxide' was one of the tints used to make these colors, and for whatever reason (it makes no sense to me) it's use has been discontinued by Lowe's. I believe it was their tint #104.

 

[bobbyt]

In my limited experience, on-axis gain caused by adding reflective elements to the paint do not appear to increase actual CR. Black levels are increased along with white levels and I believe physics demands that they are proportional - if whites are 20% brighter than blacks must be 20% brighter as well. What this effectually does is give you a screen mix that has the ambient light properties of the actual shade of gray of the mix, but the directional projected image appears to be hitting a lighter shade of gray screen. Our reflective mixes have a very wide viewing angle.

Yes, for your paint mixes that have a very wide viewing angle, I would agree with you. Actual CR would not change. I was referring to a commercial screen material with a very narrow viewing angle.

My reasoning is as follows. Consider the extreme case of a screen with a 1 degree viewing angle. (I know, no such screen exists but just humor me for the sake of the discussion.) If you sit on-axis, essentially all of the PJ light (white) is delivered to you. The ambient light (black) that is on-axis is also reflected to your eye. However, the ambient that strikes the screen at an angle is attenuated for the same reason that a the PJ image is attenuated to an off-axis viewer. Since the viewer sees all of the PJ light but only a fraction of the ambient, the blacks are reduced while the whites are not, resulting in increased CR.

I would be very interested to see some experimental results from Mech's testing on narrow angle screen material if he decides to include some HC commercial screens.

-- Bob