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post #1 of 25 Old 06-16-2014, 07:51 AM - Thread Starter
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Brightness Calculation Question

I have a pretty large disparity in measuring how bright my projector is and I am hoping for some input.

using a 100% pattern:

If I measure using an i1pro I get 11.5 FtL calibrated.
Using a LX1330B Lux meter, I get close to 200 Lux. I am using the LuxtoLumens excel sheet someone here posted, and I have also done the math manually. I get 551 Lumens/20.4 FtL.

That is a big difference and I am not sure which to trust (if either)

The screen is supposed to be 1.1 gain and I have a 144" throw.
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post #2 of 25 Old 06-16-2014, 11:31 AM
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This is probably mostly due to a difference in measuring angle between the two. IIRC, the I1Pro's measuring angle is something on the order of 2 degrees. Don't know about the other.

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post #3 of 25 Old 06-16-2014, 11:37 AM - Thread Starter
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Originally Posted by Rolls-Royce View Post
This is probably mostly due to a difference in measuring angle between the two. IIRC, the I1Pro's measuring angle is something on the order of 2 degrees. Don't know about the other.
That may be. I have to have the i1pro positioned exactly right to get a peak reading or it drops off very fast. The other is a lux meter, so I can angle it many different ways and the reading is pretty much the same within reason.

Question is, how do you know which reading is closer to actual?
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post #4 of 25 Old 06-16-2014, 12:01 PM
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Quote:
Originally Posted by curlyjive View Post
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Originally Posted by Rolls-Royce View Post
This is probably mostly due to a difference in measuring angle between the two. IIRC, the I1Pro's measuring angle is something on the order of 2 degrees. Don't know about the other.
That may be. I have to have the i1pro positioned exactly right to get a peak reading or it drops off very fast. The other is a lux meter, so I can angle it many different ways and the reading is pretty much the same within reason.

Question is, how do you know which reading is closer to actual?
They both may be, then again, they both may not be. Without a reference instrument, you won't really know. Since the I1Pro is measuring a small spot, it can only see a percentage of the total light reflected from the screen. If it's aimed in the center and there is a hot spot there, the percentage of total reflected light the Pro reads could be higher. You pay your money and you take your choice...

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post #5 of 25 Old 06-16-2014, 12:56 PM
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The spreadsheet you're talking about is based on a certain throw distance (192 inches). I don't think you can use the same conversion from illuminance to luminance with a different distance.
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post #6 of 25 Old 06-17-2014, 07:56 AM - Thread Starter
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Originally Posted by spacediver View Post
The spreadsheet you're talking about is based on a certain throw distance (192 inches). I don't think you can use the same conversion from illuminance to luminance with a different distance.
No, you can input your own throw distance and screen size/gain. I also did the math without the sheet.
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post #7 of 25 Old 06-18-2014, 08:02 AM
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A Lux meter and the i1Pro read light differently. The i1Pro has a narrower reading angle where the Lux meter measures luminance based on a larger area. One lux equals one lumen of light spread across a surface one square meter.

Since the i1Pro should be approximately 18 inches from the screen. Here is Xrite's spec for the optics of the i1Pro; Measurement geometry: 45°/0° ring illumination optics, ISO 13655:2009. Remember that emmisive measurement reading differs from that of patch readings with the i1Pro. Try switching the i1Pro to ambient reading to see if the readings differ.

From minor research based on the information you provided it seems that the Lux meter is used to measure ambient light. The meter you should be using is a spotmeter with a 2 degree reading area. The spotmeter is a well known device used for evaluating contrast and luminance.
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post #8 of 25 Old 06-18-2014, 11:44 AM - Thread Starter
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Quote:
Originally Posted by spacediver View Post
The spreadsheet you're talking about is based on a certain throw distance (192 inches). I don't think you can use the same conversion from illuminance to luminance with a different distance.
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Originally Posted by randal_r View Post
A Lux meter and the i1Pro read light differently. The i1Pro has a narrower reading angle where the Lux meter measures luminance based on a larger area. One lux equals one lumen of light spread across a surface one square meter.

Since the i1Pro should be approximately 18 inches from the screen. Here is Xrite's spec for the optics of the i1Pro; Measurement geometry: 45°/0° ring illumination optics, ISO 13655:2009. Remember that emmisive measurement reading differs from that of patch readings with the i1Pro. Try switching the i1Pro to ambient reading to see if the readings differ.

From minor research based on the information you provided it seems that the Lux meter is used to measure ambient light. The meter you should be using is a spotmeter with a 2 degree reading area. The spotmeter is a well known device used for evaluating contrast and luminance.

Hmm...I've never tried it at 18" away. Is that the distance for calibration as well? I've always run it close to the screen...especially since it is not great below 30 IRE.

When you say switch it to ambient light readings, do you mean use the diffuser cap? If so I don't have one.

I've heard of others using a lux meter for this purpose. The readings I got are in line with what others have measured as well....actually both the high and low numbers. I attributed the differences to have more to do with individual rooms/screen gain/high lamp mode. So either 11.5 FtL or 20FtL seemed possible.
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post #9 of 25 Old 06-18-2014, 01:54 PM
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Lets face it, you really have no way to increase the output of your projector other than replacing the projector lamp, projector itself or screen. SO, pick a meter, pick a spot in the room and measure. Write it down where you won't loose it.. or in my case, give it to the wife! In 500 hours repeat.. ... You will get an idea of how much the lamp has faded.. let your eyes be the judge if it time to change.. On the other hand, if it is too bright, your discomfort will also let you know.

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post #10 of 25 Old 06-18-2014, 02:16 PM - Thread Starter
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Originally Posted by airscapes View Post
Lets face it, you really have no way to increase the output of your projector other than replacing the projector lamp, projector itself or screen. SO, pick a meter, pick a spot in the room and measure. Write it down where you won't loose it.. or in my case, give it to the wife! In 500 hours repeat.. ... You will get an idea of how much the lamp has faded.. let your eyes be the judge if it time to change.. On the other hand, if it is too bright, your discomfort will also let you know.


True enough. It's plenty bright for me, but it would be nice to know more precisely. If I was getting over the recommended 12-14ftl I would like to employ the manual iris to get a bit more contrast. If the high reading is correct, I can do so. But if the low reading is correct I really don't have the headroom.

The main thing I want to accomplish is to track relative changes in brightness over time. I'll probably use the lux meter simply because I don't need to get out the laptop and tripod/meter for that.

Btw Doug, I just saw your post on the custom c6/i1pro mount you made. I'm going to get a c6 and newer version of calman soon, so I ordered the i1pro mount so I can make one too. How close to the screen are you using your c6?

Last edited by curlyjive; 06-18-2014 at 02:22 PM.
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post #11 of 25 Old 06-18-2014, 02:35 PM
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Quote:
Originally Posted by curlyjive View Post
True enough. It's plenty bright for me, but it would be nice to know more precisely. If I was getting over the recommended 12-14ftl I would like to employ the manual iris to get a bit more contrast. If the high reading is correct, I can do so. But if the low reading is correct I really don't have the headroom.

The main thing I want to accomplish is to track relative changes in brightness over time. I'll probably use the lux meter simply because I don't need to get out the laptop and tripod/meter for that.

Btw Doug, I just saw your post on the custom c6/i1pro mount you made. I'm going to get a c6 and newer version of calman soon, so I ordered the i1pro mount so I can make one too. How close to the screen are you using your c6?
I have a High Gain HP 2.8 and put he meter right in front of my seat at the same angle as my vision.. i.e. sitting in the seat the meter is in my line of sight.. the HP is a little tricky as you sweetest spot is directly under or over the lens do to the reflective properties of that screen fabric .. With a standard utility gain screen you would want it a foot of 3 from the screen. You want to move the head around very slowly till you get the maximum amount of light.
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post #12 of 25 Old 06-18-2014, 03:13 PM - Thread Starter
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Originally Posted by airscapes View Post
I have a High Gain HP 2.8 and put he meter right in front of my seat at the same angle as my vision.. i.e. sitting in the seat the meter is in my line of sight.. the HP is a little tricky as you sweetest spot is directly under or over the lens do to the reflective properties of that screen fabric .. With a standard utility gain screen you would want it a foot of 3 from the screen. You want to move the head around very slowly till you get the maximum amount of light.


I've been using my i1pro as close to the screen as I can get to help with lower readings. Will give it a try further back. The C6 will be nice to have when I do get one this summer.
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post #13 of 25 Old 06-18-2014, 08:42 PM
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In THX classes the standard is about 18" and angled to avoid shadowing while achieving the maximum input of light.

Yes, to the cap.

The ISF classes state that a 2 degree spot meters are required for measuring contrast and luminance. Upon doing minor research, no where can I find that it was designed for the purpose you are using it for; references only refer to it's usage as an ambient light measuring device.
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post #14 of 25 Old 06-18-2014, 11:44 PM - Thread Starter
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Originally Posted by randal_r View Post
In THX classes the standard is about 18" and angled to avoid shadowing while achieving the maximum input of light.

Yes, to the cap.

The ISF classes state that a 2 degree spot meters are required for measuring contrast and luminance. Upon doing minor research, no where can I find that it was designed for the purpose you are using it for; references only refer to it's usage as an ambient light measuring device.
See this thread. Different meter, but same type I am using. Though I'm not trying to measure on/off.

Using the CA813 light meter to measure your front projector
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post #15 of 25 Old 06-19-2014, 07:33 AM
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For clarity, you are trying to get a proper foot Lambert reading. I am not really familiar with the meter you are using but I was wondering about the settings. Here is something that I came across that might help;


I myself use a spotmeter (also referred to a luminance meter). This is what was recommended in class and has worked flawlessly for me.
(right tools for the right job).

I have read that you are looking at the C6 colorimeter. Excellent budget meter and very accurate. How ever I would never trust any colorimeter as a stand alone measuring device. It should always be profiled against a spectrometer for the maximum accuracy.

Well, good luck and I hope you solve your situation.
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post #16 of 25 Old 06-19-2014, 08:23 AM - Thread Starter
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Quote:
Originally Posted by randal_r View Post
For clarity, you are trying to get a proper foot Lambert reading. I am not really familiar with the meter you are using but I was wondering about the settings. Here is something that I came across that might help;

https://www.youtube.com/watch?v=2zXA0M_HHM0

I myself use a spotmeter (also referred to a luminance meter). This is what was recommended in class and has worked flawlessly for me.
(right tools for the right job).

I have read that you are looking at the C6 colorimeter. Excellent budget meter and very accurate. How ever I would never trust any colorimeter as a stand alone measuring device. It should always be profiled against a spectrometer for the maximum accuracy.

Well, good luck and I hope you solve your situation.
This is the meter I have:
http://www.amazon.com/Dr-Meter-Digit...ords=lux+meter

I'm not sure it is much different than yours in that it still measures Lux and Foot Candles. Obviously, yours is going to be more accurate and the price is way higher. But in terms of how they measure and what they measure, I think they are the same type of meter.

I will be using the i1Pro I have to profile the C6 with, so it should be a nice setup.
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post #17 of 25 Old 06-19-2014, 12:03 PM
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The i1pro is a good budget spectro. I have two of the i1pro 1 series but my favourite is the i1Pro2. Many on the forum will try and talk it down but many professionals will have it in their arsenal of equipment. Of those who talk it down the most are those who have no formal training. Even the pros who own $10000.00 - $20000.00 spectro's have a i1Pro series of some sort.

I have in my own personal opinion found that the C6 and the i1 Display Pro are identical with the exception of the internal look up tables. When you profile he colorimeter you bypass those tables and utilize the created color correction matrix which will give you the best accuracy than any Look up table can provide.

I have included the manual for your meter and I could not see how to set it for ftL. Try contacting the company for assistance with the setup.
http://www.histest.com/blog/files/20...ew-version.pdf

I have included a link of my meter. With this new forum design I hope I did it right.
http://www.spectracine.com/product_3.html

Last edited by randal_r; 06-19-2014 at 12:13 PM.
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post #18 of 25 Old 06-19-2014, 01:09 PM - Thread Starter
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Originally Posted by randal_r View Post
The i1pro is a good budget spectro. I have two of the i1pro 1 series but my favourite is the i1Pro2. Many on the forum will try and talk it down but many professionals will have it in their arsenal of equipment. Of those who talk it down the most are those who have no formal training. Even the pros who own $10000.00 - $20000.00 spectro's have a i1Pro series of some sort.

I have in my own personal opinion found that the C6 and the i1 Display Pro are identical with the exception of the internal look up tables. When you profile he colorimeter you bypass those tables and utilize the created color correction matrix which will give you the best accuracy than any Look up table can provide.

I have included the manual for your meter and I could not see how to set it for ftL. Try contacting the company for assistance with the setup.
http://www.histest.com/blog/files/20...ew-version.pdf

I have included a link of my meter. With this new forum design I hope I did it right.
http://www.spectracine.com/product_3.html

You cannot set the meter I have for FtL. Only Lux and Fc. I did the math using the LUX reading.


For my purposes, an i1Pro and a C6 will be very good. I think you'd have to spend much much more to do better! Probably more than my theater is worth, and I think I have a pretty good setup!
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post #19 of 25 Old 06-19-2014, 07:23 PM
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By all the information that I looked into regarding your meter it appears its purpose is for ambient light measuring.

Do not underestimate the combination of the i1Pro and the C6. I don't know which software package you use but I prefer CalMAN 5. I also use the AccuPel 5000 video generator. You don't have to spend a lot of money just spend it wisely.

I personally feel that the i1Pro will give you a proper ftL reading in order to track your lamp. Most find that around the 1200 hr mark (on a 2000 hr bulb), that the bulb's luminance will start to diminish to the point it will become noticeable.

Enjoy your home theatre.

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post #20 of 25 Old 06-20-2014, 04:51 AM
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Quote:
Originally Posted by curlyjive View Post
I have a pretty large disparity in measuring how bright my projector is and I am hoping for some input.

using a 100% pattern:

If I measure using an i1pro I get 11.5 FtL calibrated.
Using a LX1330B Lux meter, I get close to 200 Lux. I am using the LuxtoLumens excel sheet someone here posted, and I have also done the math manually. I get 551 Lumens/20.4 FtL.

That is a big difference and I am not sure which to trust (if either)

The screen is supposed to be 1.1 gain and I have a 144" throw.
You have a 100" 16:9 diagonal screen, yes? Are you measuring lux with the sensor placed right against the screen surface?

For there to be a discrepancy that large, either there is some problem with your measuring technique (placement of the meters), the i1Pro is defective, or the manufacturer's 1.1 gain specification is not correct (pretty common).

Tom Huffman
ChromaPure Software/AccuPel Video Signal Generators
ISF/THX Calibrations
Springfield, MO

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post #21 of 25 Old 06-20-2014, 10:08 AM - Thread Starter
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You have a 100" 16:9 diagonal screen, yes? Are you measuring lux with the sensor placed right against the screen surface?

For there to be a discrepancy that large, either there is some problem with your measuring technique (placement of the meters), the i1Pro is defective, or the manufacturer's 1.1 gain specification is not correct (pretty common).
Yes and Yes.

I ran the diagnostics on the i1pro...not sure if that means anything but it passed.
My guess is meter placement is a factor. I tend to place it very close to the screen to help with low IRE patterns (5-20). Maybe this is part of the problem. When I get a C6, that will help.

I also suspect that the 1.1 gain is probably less. So it might be a combination of the two.
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post #22 of 25 Old 06-20-2014, 03:25 PM
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Tom has a lot of experience and it is wise to follow his advice.

One of the major problem with spectro's is that they have difficulty reading low (dark) levels. This is why you will find many who will pair up a spectro and a colorimeter. Once the colorimeter has bee profiled to the spectro, the colorimeter should almost be a accurate as the spectro but read low levels with ease as colorimeter's do.
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post #23 of 25 Old 06-21-2014, 12:07 AM - Thread Starter
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Quote:
Originally Posted by randal_r View Post
Tom has a lot of experience and it is wise to follow his advice.

One of the major problem with spectro's is that they have difficulty reading low (dark) levels. This is why you will find many who will pair up a spectro and a colorimeter. Once the colorimeter has bee profiled to the spectro, the colorimeter should almost be a accurate as the spectro but read low levels with ease as colorimeter's do.
I know who Tom is and my comments are in agreement with his.

I also noted that I intend to get a C6 to be profiled by my i1pro for the reasons you stated.
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post #24 of 25 Old 06-21-2014, 08:47 AM
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I know who Tom is and my comments are in agreement with his.

I also noted that I intend to get a C6 to be profiled by my i1pro for the reasons you stated.

My apologies, by your posting it appeared that you were a newbie. As of this point I will no longer give you any further assistance.
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post #25 of 25 Old 06-21-2014, 09:19 AM - Thread Starter
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My apologies, by your posting it appeared that you were a newbie. As of this point I will no longer give you any further assistance.
Not a newbie, not an expert either.

I wasn't offended by your post, just pointing out what I had already stated in earlier posts. Why so "all or nothing?"
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