Film-Tech Forum ARCHIVE: Projectionist's hand-held FtLb Light Meter??

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Posted by Frank Angel (Member # 248) on 06-21-2005, 04:07 AM:

I know I read a post about someone making a very simple handheld light meter so a projectionist could do routine check on his screen brightness. I did a search here but everything under the sun came up -- lots of good stuff to reread, but nothing on this little meter.

Somehow I have it in my mind that it was a screen company that made this device -- Harkness Hall perhaps? Their website had nothing, and much to my surprise, it looks like they are going mostly into the home theatre business....not a heck of a lot there on commercial cinema.

So anyone remember what that unit was called and who made it? I think it read FL out directly -- no conversion necessary.

Someone give me an "oh yah...NOW I remember" moment.

Posted by Michael Schaffer (Member # 1204) on 06-21-2005, 04:36 AM:

Harkness Hall provides (or provided, I don't know if they still do) the screen checker. Here is a British supplier which also has it: TLS UK. I don't know if theirs is the original or rebranded or the other way around. However, the one I have is completely worthless. Compared to 2 (allegedly) calibrated Minolta spot meters, it was all over the place. There wasn't even a typical offset, like 2fL high across the spectrum or similar. It was just completely off. I actually wanted to contact Harkness Hall because I might have gotten a bad apple. A friend has one too, and if I remember correctly, he said his was pretty accurate when compared to a calibrated meter in a post facility in Hollywood (where you would hope things really are calibrated).

Posted by Ari Saarinen (Member # 3188) on 06-21-2005, 05:18 AM:

We have compared is to Spectra CineSpot and it's fairly accurate for projectionist use... you just need to use it correctly, I think there is even manual in Film-tech manual libary Harkness Screen Cheker name... also note that ambient light disturb it easily.

Posted by John Pytlak (Member # 331) on 06-21-2005, 07:59 AM:

Any meter used should measure light REFLECTED from the screen, and should have a photopic response. Most any spotmeter can be used, as long as it has sufficient sensitivity and accuracy, and is proper calibrated. Here is an article I wrote several years ago:

Screen Luminance Meters

quote:
Shedding Light on Screen Luminance Meters
To Measure Light, You Need a Photometer
Standard ANSI/SMPTE 196M "Screen Luminance and Viewing Conditions"
specifies the "Photometer type" as follows:
"Screen luminance shall be measured with a spot photometer having the
spectral luminance response of the standard observer (photopic vision) as
defined in CIE S002. The acceptance angle of the photometer shall be 2° or
less. The photometer response to the alternation of light and dark on the
screen shall be to integrate over the range of 24 Hz to 72 Hz and display the
arithmetic mean value."
Can You Please Translate That?
Simply put, the meter should "see" the light reflected by the screen as the
human eye does. Some meters have photocells that are more sensitive to
invisible infrared energy, and may give incorrect measurement of visible light.
Meters designed specifically to measure screen luminance use special sensors
and filters to have a "photopic" response just like the human eye.
Second, the photometer used to measure screen luminance in a theatre should
measure light reflected from a small area of the screen, no more that two
degrees in viewing angle. For example, if you take measurements 60 feet from
a screen 20 feet high, a meter with a two degree acceptance angle "sees" an
area on the screen only about two feet in diameter. Having a relatively narrow
viewing angle allows the user to measure luminance in various parts of the
screen, to evaluate the uniformity of illumination across the screen.
Since screen luminance is measured "open gate" with the projector running,
the light on the screen is actually going off and on 48 or 72 times per second.
At the normal 24 frames per second, a two- blade shutter gives 48
interruptions of light every second. A three-blade shutter gives 72
interruptions. A meter used for measuring screen luminance needs to be
properly calibrated for the alternating light/dark cycle of a projector.

Posted by Alan Gouger (Member # 543) on 06-21-2005, 09:58 AM:

I have the Harkness and its very accurate. We measured it against a spectro device. I find its also repeatable and not so critical on placement. Maybe I have a good one of the bunch:)

Posted by Michael Schaffer (Member # 1204) on 06-21-2005, 11:59 AM:

quote: Ari Saarinen
We have compared is to Spectra CineSpot and it's fairly accurate for projectionist use... you just need to use it correctly, I think there is even manual in Film-tech manual libary Harkness Screen Cheker name... also note that ambient light disturb it easily.

Welcome to film-tech, Ari. Do you remember how we met 3 years ago at the Alhambra in Berlin? Instructions on how to use the thing are even printed on the back of itself. The comparison tests I made were under "ideal" conditions with all house lights completely off and carefully chosen positions and measurement points on the screen. It looks like the one I have may be an odd bad egg... [Frown]

Fortunately, it's still under warranty [Smile]

Posted by Dominic Case (Member # 1830) on 06-21-2005, 05:44 PM:

John Pytlak writes:

quote:
The photometer response to the alternation of light and dark on the
screen shall be to integrate over the range of 24 Hz to 72 Hz and display the
arithmetic mean value."
Can You Please Translate That?

A bit more clarification please, John. Does this mean that if you get a meter reading of 16ftL with the projector running (and therefore with the shutter closed half the time) then you would get a peak reading of about 32ftL if you could stop the projector with the shutter open ? I believe this would be so, if the meter is indeed integrating luminance over time rather than reporting a peak value: but surely this would have implications when comparing film projector brightness with a digital system where the screen is not necessarily black for half the time (indeed its brightness is determined by the length of time it is lit for each frame).

I ask this also because the same standard recommends taking color temp or chromaticity readings with the shutter stopped, and the difference in the method could cause confusion.

Dominic

Posted by Steve Guttag (Member # 268) on 06-21-2005, 07:13 PM:

It does and it does! At least that is my findings...that for 16fL on film to look similar in video you need for video to be much brighter...though to the photometer, 12fL on video wit a white raster measures as clear film on a film projector.

Posted by Frank Angel (Member # 248) on 06-21-2005, 11:55 PM:

Thanks guys. Welp, at least my memory is TOTALLY fried. It was Harkness but the meter it's no longer listed on their site. I emailed them; perhaps they just want to make the site look more attractive to the home market and less technical but will sell us one. Perhaps. If not, I will try to search for another dealer, maybe even have it shipped from TLS in the UK. Something that small shouldn't be too expensive to ship.

Posted by Ari Saarinen (Member # 3188) on 06-22-2005, 12:02 AM:

Yes I remember Michael... how's Alhambra doing... and are you coming to CinemaExpo ?

I think it's a good, simple and cheap tester for rapid checking and every booth should have one.

Posted by Michael Schaffer (Member # 1204) on 06-22-2005, 02:28 AM:

I have relocated since then (see left). What happened with that project is a very long and complicated story, not really interesting at all. Basically the owner developed a habit of not paying his bills and then our salaries either, so I took off and sued him.
How are things going with Finnkino? Are you expanding in the Baltic States? Are you moving into Russia? Do you still hang your speakers above the screen?

Posted by John Pytlak (Member # 331) on 06-22-2005, 09:03 AM:

quote: Dominic Case
A bit more clarification please, John. Does this mean that if you get a meter reading of 16ftL with the projector running (and therefore with the shutter closed half the time) then you would get a peak reading of about 32ftL if you could stop the projector with the shutter open ?

That is indeed the case. A meter used for measuring "open gate" screen luminance should be insensitive to whatever frequency the light is getting interrupted by the shutter. So with a two blade shutter with 50% efficiency, you should theoretically measure 16 footlamberts with the shutter running, and 32 footlamberts with the shutter stopped and open. Of course, if you always use the same type of shutter and frame rate, ANY meter could be calibrated at that frequency, as long as you didn't try to use it on another projector with a different type of shutter or frame rate.

Not knowing the particulars of meter circuits, I assume they would use a capacitor on the output of the photocell to integrate any instantaneous changes in light level.

Posted by Brian Guckian (Member # 1678) on 06-23-2005, 09:56 PM:

ScreenChecker was researched and developed by TLS UK...it's a great way of making sure screen illumination is within Standard.

But it is not a substitute for a more precise spotmeter. The idea of ScreenChecker is to let the Projectionist know they are either inside or outside the range allowed, and where they are in relation to 16fL. It is intended to be used in between Engineer's visits as totally accurate calibration must still be done with the spotmeter.