Film-Tech Forum ARCHIVE: What the heck is a lampere????

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Posted by Eliza Plumlee (Member # 4242) on 04-01-2007, 04:58 AM:

I have a psychotic director who wants to do a screening at my theater and demands to know how many lamperes (lampheres???) my projector has or is, or something.
I don't know how it is spelled, but cannot find out what it is anywhere.
an ideas?
perhaps a measurement of light or energy?
how do you find out how many you have?
I work on a dual projection system. Century projectors.
thanks.

Posted by Monte L Fullmer (Member # 2797) on 04-01-2007, 05:03 AM:

the term is footlampberts - fl .. measures of brightness from a projector measured on a screen by a light meter. (courtesy of this famous link ..lots of info is obtained here). I'm sure that the director there wants the recommended 16fl measured on that screen for his film was probably produced with this light requirement. Anything different would make his film look terrible

good luck - Monte

Posted by Frank Angel (Member # 248) on 04-01-2007, 07:39 AM:

Find out what size lamp is in your lamphouse and then tell this genius director that your projector puts out a whopping 2000, 4000 (whatever it is) of pure white voltamps of energy. Then ask him what kelvolumens his film was shot at.

Posted by Richard Fowler (Member # 893) on 04-01-2007, 09:43 AM:

He is probably talking about lumen output of projector which can be converted to foot lamberts or candela after striking a screen surface, factoring screen fabric gain in the measurement.

Posted by Tim Reed (Member # 137) on 04-01-2007, 07:31 PM:

Sounds to me like he's confusing LUMENS with AMPERES.

Posted by John Hawkinson (Member # 1135) on 04-01-2007, 09:05 PM:

Hi, Eliza. Everybody's told you true, but let's try to put it on context.

If this guy really knows his stuff, he'd want to know how bright your screen is. And we measure that in foot-lamberts reflected off the screen.

Unfortunately, if he really knew his stuff, he'd know that almost no projectionists actually know how bright their theatres are in foot-lamberts, because the proper tool to measure it (a spot photometer) is really expensive and specialized. Ideally your theatre technician has one, but probably that doesn't help you.

He might just be asking how bright your lamphouse is, to get an idea of how bright your screen might be. That would be the type of lamp you have, like 2K (2 kilowatt), 4k, etc. etc. That you should know (or be able to easily check; hopefully it's written on the bulb maintenance card for your lamphouse, but you can check the current and voltage to determine it). There are a lot of factors that affect how bright your screen will be for a given lamp wattage (size and gain of screen, type of lens, type of shutter, quality of lamphouse reflector, etc., etc.) but absent all other things, wattage of the lamp gives a general indication.

He might also be asking about lumens (lm), which is a measure of light output (luminous flux). It's typically used when talking about video projectors, and generally not used when talking about film projectors. Lumens are usually measured at the output of a video projector (but not including a screen), and are generally numbers from a manufacturer's datasheet. If he's asking for lumens, well, remind him this is not a video projector. You could check the spec sheet for your lamp (e.g. for an OSRAM XBO 2000W/H OFR, it is 80,000 lumens). But it's not a meaningful number because of projector/lamphouse differences (as well as screen and lens differences).

It's possible he's asking about amperes, but it seems unlikely. The ampere is a measure of current, and it is mostly dictated by bulb size. You should be able to check the ammeter on your lamphouse to see what it is. For each bulb size there's a range, for instance, a 2k bulb has a nominal current of 70A, but an acceptable current range of 50A-85A, for varying the brightness.

It's also possible he's asking about an obscure unit like the candela, the measure of luminous intensity. Again, you can look that up from your lamp's spec sheet (7500cd for a 2KW lamp), but it's not meaninful.

Frank was joking when he mentioned "kelvolumens," of course. A "volt-amp" is almost the same as a watt, but not quite--usually volt-amps are measured on the input side of a device, and watts on the output, the difference being in the efficiency of the device.

Hope this is helpful.

--jhawk

Posted by Eliza Plumlee (Member # 4242) on 04-02-2007, 02:27 AM:

Hey there film Gods,
you all have been very helpful.
I am now sure that he was asking about foot lamberts. I have since found out that my projectors produce 16 foot-lamberts, just like they are supposed to!
However, I am soooooo tempted to ask him about his kelvolumens!
I offered to project a trailer for him, so that he could see the quality for himself, but I guess that he was too busy for something so practical.

Our projectors generally run @ 75 amperes, but i didn't think that was what he was referring to.

Anywho, thanks a lot. You guys are radddddd!!!!!!!

Posted by Randy Stankey (Member # 64) on 04-02-2007, 01:10 PM:

Tell him that you had to adjust the fratistat almost three full turns before you could get the recommended 16 Kelvolumens out of your projector!

[Big Grin]

Posted by Mike Blakesley (Member # 26) on 04-02-2007, 01:22 PM:

And then tell him you're going to maintain this bourdon hold until you receive portal magnus clearance from Melnix.*

(* courtesy of Dean Martin, in the original "Airport")

Posted by Will Kutler (Member # 770) on 04-04-2007, 01:15 AM:

What should you do?

Open up a book like the Machinist's Ready Reference that has a huge section on scientific conversion values. Convert his requirements and mangle it up with the most ridiculus-odd-obscure measurement units you can find, and then let the genius unravel it!! [Big Grin]

Posted by John Pytlak (Member # 331) on 04-04-2007, 09:47 AM:

Standard SMPTE 196M-2003 specifies the viewing conditions. The aim is indeed 16 footlamberts:

http://store.smpte.org/product-p/smpte%20196m-2003.htm

quote:
Motion -Picture Film -Indoor Theater and Review Room Projection - Screen
Luminance and Viewing Conditions

quote:
5.2 Theater nominal luminance
Theater screen luminance shall be nominally 55
cd/m2 (16 fL) measured at the screen center. The
luminance of the screen sides and corners shall be
measured at a distance of 5% of the screen width from
the screen edges. The readings shall be taken from
each location specified in 5.1.

5.3 Theater luminance limits
Theater screen luminance at the screen center shall
be between 41 cd/m2 (12 fL) and 75 cd/m2 (22 fL).
Luminance at the screen sides shall be 75% to 90%
of the screen center luminance, but not less than 34
cd/m2 (10 fL).