Correct way to read ftl?

Hi Murray, Nigel here.

Off the screen is the best method. IThe reference method is ideally with a measurement device like a Photo Research PR-521, or SMPTE grade <2deg measurement device.
The best or arguably the most accurate for lumanance is the Klien K10.
All very expensive.

The new xrite i1 display probe is cost effective and really quite accurate aswell.

The traditional units was fl, at aprox 15fl off the screen
The correct si unit is cd/m2 which is aprox 51cd/m2
This is the recommendation for true theatres, many don't achieve this.

For home theatres the reality is most people tend to achieve about 10fl or 34cd/m2 with their setups, where when the bulb ages can run as low as 7fl or 24cd/m2.
This may sound alarmingly low, it isn't as bad as our eyes do adapt within reason. The trick is having total light control, your theatre achieves this. Your problem will always be because of the scope and large screens, the newer projectors are starting to become right for you now.
However because of your setup you will always struggle for high light levels when bulbs age.

The next time im up in Auckland I could point my new jeti 1211 at your gear. Oh we could do a full 3D LUT with your lumagen aswell.

regards
nigel
Edited by <^..^>Smokey Joe - 9/30/12 at 2:33pm

FtL is read off the screen, and in 'film' theaters is 16 FtL open gate, and 14Ftl D-Min (with film base in the projector).
In digital theaters it is 14FtL peak white.

The white point is different, with film being 5400K, and Digital being closer to 5800K

But, all DVDs and Blu-rays are mastered to the TV standard of Rec709, so you need to setup home projectors to that standard...

Cheers,

Steve

You don't use a "formula" for fL because fL is far from the only thing that needs to be correct when you calibrate a projector-screen combination.

If you measure the light beam from the projector only, you can make the projector more accurate. But the screen usually will CHANGE the light in measurable ways. So if you do NOT measure from the screen, you won't really know what you are seeing... either for color or fL (luminanace).

When you calibrate a projector & screen, the home theater illumination standard is 16 fL using d65 as the target for each grayscale step from dark to light., Color temperature is a nearly worthless specification because any given color temperature can be perfectly accurate, but the images could be very green or very magenta because the balance of green and magenta is not (much) taken into account by color temperature calculation.

When you calibrate a projector & screen combination (or a panel display), you want to get the grayscale as accurate as it can be, you want gamma (the luminance of each step) to be as accurate as possible (a fL measurement) for each grayscale step. And you want the color to be as accurate as possible.

You don't do this using formulas, you do it using calibration software running on a computer with the meter used to make the measurements also connected to the computer and feeding the measurement data to the software (so the software has to "understand" the meter you are using). You also need to understand how to use the controls and how to interpret the data displayed by the calibration software.

The best starting point for most beginners is AVS Forums' "Calibration for Dummies" thread where aspects of calibration are outlined and you can learn everything needed to understand calibration. This will take a while. Once you have a meter and software, plan on spending 100 hours or so in study about calibration and practice taking measurements and understanding what the calibration software is telling you about the measurements you are making... that first 100 hours is the learning curve. You won't be a "great" calibrator after that but you should begin getting some reasonable calibrations that will continue to get better with more practice and learning about how to get the most out of your projector/screen or flat panel display.

The motion imaging industry is based upon unifying technical standards, engineering guidelines, and recommended practices. Home systems that don't perform according to these standards and best practices are simply sub-standard systems which cannot faithfully deliver the look and impact of the imaging medium being viewed. Image fidelity and artistic intent are diminished. In my experience in the home theater industry, far too many front projection systems are designed with screens that are too large for the light output of the selected projector. Many consumers settle for dim images in order to have larger ones. I wouldn't want to live like that. All one needs to do to achieve a larger picture is simply to sit closer to the screen. Then the image can be suitably bright, on a smaller screen, without having to spend more on a higher output projector. When a system deviates from industry standards, consequences to performance are inevitable.

Best regards and beautiful pictures,
G. Alan Brown, President
CinemaQuest, Inc.
A Lion AV Consultants affiliate

"Advancing the art and science of electronic imaging"

The SMPTE standard for movie screen illumination is 16 fL with a tolerance range of 12-20 fL. There are also standards for how much darker the corners of the screen can be compared to the center (the center is usually brighter than the rest of the image. This applies to any stage of life of the projection lamp.

Of course there may be reasons you cannot achieve the luminance range of the SMPTE standard. In that case, you just do the best you can.

In home theater, reflections from walls, ceilings, or floors can wash out parts of the screen or entire screen and using less screen luminance may help. But the best "fix" is dark, non-reflective walls.

To set luminance, start with the projector in "Low" lamp mode. Make the projected image fill the screen. Display 100% white and measure the light from the projection screen. If you can get 16 fL with the lamp in "low" mode, you can leave it in "low" lamp mode for calibration. If your iris is adjustable, you can use the iris to help you get 16 fL also. But you have to understand that opening the iris will make the black level brighter... and you may find that 16 fL for 100% white also makes the black level too bright and you may want 14 or 12 fL just to keep the blacks as dark as possible. This will depend on the black level performace of the projector and the gain of the screen. This is something you must understand when deciding what starting points to use for auto-cal. Unfortunately, there is no Auto-Cal process that can make these decisions for you. You must know enough about calibration and images and adjustments and how the iris, contrast control, and brightness control (and gamma) interact with each other so you can select the best possible starting points and the best possible gamma for the projector. Auto-Cal cannot do that for you.

I'll note for Murray's sake, he has a good quality theatre happening here. (from his sig) I was following his build with interest at the time.

home-cinema-build

His main issue has been the typical problem of too large a screen for typical home theatre projector design, however technology is catching up now with the latest crop of projectors.

So when setting luminance, are we supposed to use the constrast to raise or lower the light output to the desired 16Fl? My Epson 5020 seems to be clipping white around 232 and frankly it is too bright.
Also,, I am supposed to se using full fields and not windows?
I have measured between 25 and 30fl using a colormunki, but I am projecting onto a small screen (80")

With a projector you typically have your meeter a foot or 2 from the screen so windows or full fields are fine. Full fields can effect readings of gray scale if the room has colored wall and the colored light reflects back on the screen
With a projector you can reduce light in several ways if it is to bright.
You can lower the contrast
You can close the iris if so equiped
you can use low lamp mode if you are not using it currently
You can install an ND (neutral density)
Add bias lighting to the room to reduce eye strain.

Also you would want to recheck the light output after you have completed the calibration as it will be lower than OB readings.

Hope that helps.

Just a another note about the meter... for measuring light from a projection screen, the meter has to be classified as a reflectance meter. There are meters out there that are only incident meters... meaning the light has to reach the meter directly... those can only be aimed at the projector leaving the screen out of the calibration entirely. That's a terrible idea... the screen has properties and those need to be taken into account when you calibrate a projector. So you have incident and reflectance meters. But there are also meters that can be used as incident OR reflectance meters... those are the most flexible meters and the ones most often used by professional calibrators.

Also, calibrating projectors is more difficult because the light reflected from the screen per unit area is not as bright as light from most direct view TVs. That means if you select a meter that is very slow making measurements at low light levels, it can get really annoying when calibrating projectors. Meters that are slow making measurements at low light levels also tend to get inaccurate at low light levels faster than meters that do a better job with low light levels. You might find some meters (typically inexpensive ones) that can't read levels below 20% white from a projection screen because that's about the low-light measurement level for the meter.