Foot Lambert Calculations for 2.35 Screen - Home Theater Forum and Systems - HomeTheaterShack.com

Old 08-03-11, 10:21 AM Thread Starter
Shackster

Join Date: Jan 2011
Posts: 13
Foot Lambert Calculations for 2.35 Screen

I had a quick question about calculating foot lamberts for a 2:35:1 aspect ratio screen when an anamorphic lens is not used.

When calculating foot lamberts for a 2.35:1 aspect ratio screen, do I use the screen area or do I have take into account the black bars? I do not plan on using an anamorphic lens.

For example, if I were get an 138" Diagonal 2.35:1 screen, do I use 54" x 126" (2.35:1) or 70.8" x 126" (1.78:1) as the screen area?

Thanks!
jhan1000 is offline

Old 08-03-11, 03:09 PM
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Don

Join Date: Oct 2007
Location: Central PA
Posts: 3,772
Re: Foot Lambert Calculations for 2.35 Screen

Quote:
jhan1000 wrote: View Post
I had a quick question about calculating foot lamberts for a 2:35:1 aspect ratio screen when an anamorphic lens is not used.

When calculating foot lamberts for a 2.35:1 aspect ratio screen, do I use the screen area or do I have take into account the black bars? I do not plan on using an anamorphic lens.

For example, if I were get an 138" Diagonal 2.35:1 screen, do I use 54" x 126" (2.35:1) or 70.8" x 126" (1.78:1) as the screen area?

Thanks!
This is an interesting question and one that could have several answers depending on the projector used and how it attains different aspect ratios. I'm going to assume that the PJ has a native 16:9 AR. Now I'm not up-to-date on this, but the last I knew most such PJ's simply turned off pixels on the top and bottom of the image to get a 2.35 AR image. In theory this would decrease the lumens being output, but I don't believe the difference is worth calculating.

BTW, the best way to find actual image brightness is to use a light meter designed for the job (about \$35).
Harpmaker is offline
Old 08-03-11, 03:59 PM Thread Starter
Shackster

Join Date: Jan 2011
Posts: 13
Re: Foot Lambert Calculations for 2.35 Screen

Buying a light meter makes a lot sense and will take the guess work out of this. Do you have any recommendations for a good light meter for \$35?

I did a quick Amazon search and many of the light meters seemed north of \$100. There is also an Iphone / Ipad light meter app that I may check out as well.

jhan1000 is offline
Old 08-03-11, 07:09 PM
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Don

Join Date: Oct 2007
Location: Central PA
Posts: 3,772
Re: Foot Lambert Calculations for 2.35 Screen

Quote:
jhan1000 wrote: View Post
Buying a light meter makes a lot sense and will take the guess work out of this. Do you have any recommendations for a good light meter for \$35?
This is the one I bought several years ago. I see it's now on sale for only \$17.00, the only problem with that deal is it's below the \$25 total needed to get free shipping.
http://www.amazon.com/Light-Meter-LX...2415193&sr=8-5

If I was going to get a light meter today it would be this one. It allows you to change from measuring in Lux to Foot Candles with the flip of a switch (we use mostly FC when talking about screens and PJ's). It's easy to convert from one to the other, but you don't have to do that with this meter. Cost is only \$35.56 with free shipping.
http://www.amazon.com/Mastech-Digita...2415193&sr=8-1

Quote:
I did a quick Amazon search and many of the light meters seemed north of \$100. There is also an Iphone / Ipad light meter app that I may check out as well.
The really weird thing is that the inexpensive meters above are actually BETTER for reading low levels of light than the much more expensive light meters designed for photography! We don't want to measure in f-stops or EV, we just want basic readings in Lux or FC.

BTW, when a screen has a gain of 1.0 the light being reflected from the screen (in foot lamberts) is the same as the light striking the screen in foot candles. Since we almost always use a screen gain of 1.0 when calculating screen recommendations we always use fL (foot lamberts) as the unit of image brightness for both the light from the PJ and the light reflected from the screen instead of the more correct foot candle unit to describe PJ brightness to keep things simple.

Quote:
No problem, that's why we're here.
Harpmaker is offline
Old 08-10-11, 08:23 AM Thread Starter
Shackster

Join Date: Jan 2011
Posts: 13
Re: Foot Lambert Calculations for 2.35 Screen

I just purchased the Mastech light meter... I had a question. How should I position the probe? I'm assuming I should be measuring a white screen and the probe near the screen and facing the screen. Is this correct? Thanks!
jhan1000 is offline
Old 08-10-11, 12:30 PM
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Don

Join Date: Oct 2007
Location: Central PA
Posts: 3,772
Re: Foot Lambert Calculations for 2.35 Screen

Quote:
jhan1000 wrote: View Post
I just purchased the Mastech light meter... I had a question. How should I position the probe? I'm assuming I should be measuring a white screen and the probe near the screen and facing the screen. Is this correct? Thanks!
The probe should have a round dome on it that will help average out the light hitting it so you shouldn't have to aim the probe at the PJ like a rifle sight.

The way to measure the light hitting your screen from the PJ is to project a 100% (or 100 IRE) white image and then hold the probe flat against the middle of the screen with it facing the PJ.

If the probe was facing the screen you would be measuring screen reflectance, but this reading can't really be used to calculate actual screen gain because of the geometrics involved (calculating gain requires the use of a 1° spot meter and being able to position the meter to get the highest reading).

BTW, you can also use this meter to find your true contrast ratio by using an ANSI checkerboard pattern (if your calibration DVD has one). This is an image that has 16 black and white rectangles on it. Measure the brightness of the center of each rectangle and average them for total black and total white readings then find the ratio. Don't be surprised if the result is way less than the PJ specs.

Also don't be surprised to find that the center of your screen is getting hit with more light than the corners.
Harpmaker is offline
Old 08-10-11, 02:49 PM Thread Starter
Shackster

Join Date: Jan 2011
Posts: 13
Re: Foot Lambert Calculations for 2.35 Screen

Quote:
Harpmaker wrote: View Post
The probe should have a round dome on it that will help average out the light hitting it so you shouldn't have to aim the probe at the PJ like a rifle sight.

The way to measure the light hitting your screen from the PJ is to project a 100% (or 100 IRE) white image and then hold the probe flat against the middle of the screen with it facing the PJ.

If the probe was facing the screen you would be measuring screen reflectance, but this reading can't really be used to calculate actual screen gain because of the geometrics involved (calculating gain requires the use of a 1° spot meter and being able to position the meter to get the highest reading).

BTW, you can also use this meter to find your true contrast ratio by using an ANSI checkerboard pattern (if your calibration DVD has one). This is an image that has 16 black and white rectangles on it. Measure the brightness of the center of each rectangle and average them for total black and total white readings then find the ratio. Don't be surprised if the result is way less than the PJ specs.

Also don't be surprised to find that the center of your screen is getting hit with more light than the corners.
Haha... I'm wondering if I should return the thing now. Sometimes, ignorance is bliss.

Thanks again!
jhan1000 is offline
Old 08-10-11, 05:06 PM
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Don

Join Date: Oct 2007
Location: Central PA
Posts: 3,772
Re: Foot Lambert Calculations for 2.35 Screen

Quote:
jhan1000 wrote: View Post
Haha... I'm wondering if I should return the thing now. Sometimes, ignorance is bliss.
Actually, I've known people that were satisfied with they PJ's until they measured them and then couldn't get over the fact that every square foot of screen didn't have the same brightness. So you really do have a point.

I was very surprised when I found that my PJ is a full 1/3 dimmer in the corners and edges than in the center, but the change is fairly linear so it really is not visible to the human eye except on a pure bright white image and even then only when you look hard to see it.

Quote:
Thanks again!
Harpmaker is offline
Old 08-10-11, 05:19 PM
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Join Date: Aug 2010
Location: Torrance, CA
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Re: Foot Lambert Calculations for 2.35 Screen

To calculate foot lamberts for a 2.35:1 screen when using the zoom method (no anamorphic lens) - just base your calculations on a 16:9 screen that's the same width as your 2.35:1 will be.

For example, a 120" wide 2.35:1 screen would measure 120"x51"; the 16:9 of the same width would measure 120"x67.5". 120x67.5 = 8100 square inches, divide by 144 = 56.25 square feet.

For a hypothetical 700 lumen output projector, and 1.0 gain screen, you'd divide the 700 lumens by the 56.25 square foot area of the screen, and get 12.44 ft/Lamberts. If you had a higher gain screen, you'd first multiply the lumens by that gain before dividing by the screen area. Another factor to consider is that most projectors will produce more light output when mounted closer to the screen, and less further away - the amount varies by projector, but sometimes reviews will tell you the variation between the extremes of the zoom range.

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Old 08-10-11, 08:41 PM
Moderator Emeritus
Don

Join Date: Oct 2007
Location: Central PA
Posts: 3,772
Re: Foot Lambert Calculations for 2.35 Screen

The really cool thing about having a light meter is that you can know exactly how many foot candles the screen is being hit with, and no math involved. Also, you won't have to guess at how much brightness you lose over time.
Harpmaker is offline

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