Film-Tech Forum ARCHIVE: Light meter

This is topic Light meter in forum Large Format Forum at Film-Tech Forum ARCHIVE.

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Posted by Ivan Plamondon (Member # 7637) on 01-27-2014, 12:08 PM:

Can anyone suggest a decent low cost light meter?

Posted by Mark Gulbrandsen (Member # 72) on 01-27-2014, 04:17 PM:

Harkness makes a screen brightness checker that's ok. You should really compare it to a real light meter before you use it just to verify where it reads.

Posted by Ivan Plamondon (Member # 7637) on 01-27-2014, 04:38 PM:

Thanks Mark

Posted by Ivan Plamondon (Member # 7637) on 01-28-2014, 09:04 AM:

Harkness is still a bit too pricey for our theatre.

Posted by Mark Gulbrandsen (Member # 72) on 01-28-2014, 10:47 AM:

It's probably the least expensive option available thats made for the job. Even a used Spectra Screen Brightness meter in working condition will set ya back more than the new Harkness.

Posted by Scott Norwood (Member # 30) on 01-28-2014, 11:25 AM:

Just the calibration on the Spectra SC-600 is in excess of $200, but it is a very nice unit. You don't want to see the price for a new one, though.

Posted by Paul Mayer (Member # 355) on 01-28-2014, 01:50 PM:

I suppose one will not like the price of the meter I was recently using at Callahan and Christie: the Photo Research Cinebrate. Specifically designed for use with digital cinema projectors, it reads screen luminance and color coordinates. New, it's priced at under $10,000 USD. Still cheaper than a calibrated, used Minolta...

http://www.photoresearch.com/current/cinebrate.asp

Posted by Ivan Plamondon (Member # 7637) on 01-28-2014, 02:09 PM:

I guess the Harkness unit doesn't seem so bad at $750. Still way out of our price range however.

Posted by Carsten Kurz (Member # 5396) on 01-28-2014, 02:50 PM:

I think the Harkness digital screen checker can be bought from other suppliers as well. I also remember there were offers for collective purchases.

Cinema Technology Magazine had an article on the digital screen checker in a past issue.

If you want to go cheaper, a second hand photographic spot meter can used as well after some training.

- Carsten

Posted by Ivan Plamondon (Member # 7637) on 01-29-2014, 09:17 AM:

Carsten, can you steer me towards a few brands of spot meters? We utilize 7kW Superior lamps.

Posted by Carsten Kurz (Member # 5396) on 01-29-2014, 04:12 PM:

Depends a bit on what you are actually after. A proper screen light meter will give you familiar units like fL or cd/sqm. A photographic spot meter will usually have a coarser readout, as photographic units are usually f-stops or EV, with 1/2 or 1/3 divisions at best. You will need to do some easy calculations to get fL out of these numbers, and the error may be large. In fact, you could even use an existing SLR or DSLR with a spot meter.

Check e.g. Minolta Spot Meters on ebay. You could even use an incident light meter - always depending on what you are trying to achieve. If you have a proper measurement from an installer and don't change screen, relative measurements are possible.

However, this is the Large Format Forum, so I assume you want this for film projection - you can not be sure that any of these cheaper alternatives do well with the 48Hz flicker of film projectors. My incident light meter does well with it, my SLR does not. Both do well with digital projectors.

- Carsten

Posted by Ivan Plamondon (Member # 7637) on 01-30-2014, 10:45 AM:

You are correct Carsten. Trying to measure light from a 7kW Superior lamp projected on a 60 by 80 foot screen. Our system is a 70 mm 24 frames per second GT IMAX rolling loop projector.

Posted by Mark Gulbrandsen (Member # 72) on 01-30-2014, 01:00 PM:

I'll second the Minolta idea. I use a Minolta color space meter and it's been very good. For plain old white light measurements on the fly I use my Spectra FL spot meter is what I use, I also have a Spectra that measures candellas.

Mark

Posted by Bobby Henderson (Member # 840) on 01-30-2014, 01:41 PM:

Is it possible to rent something like a high quality cinema screen light meter for a couple days, or does that piece of gear usually include a human technician, airfare, hotel lodging, etc.?

Posted by Carsten Kurz (Member # 5396) on 01-30-2014, 02:28 PM:

Forgive me ;-) - shouldn't a place with an IMAX GT projector and a 60*80ft screen be able to afford 750US$ for a light meter?

I may be able to test a few dedicated spot meters on a 35mm machine soon.

- Carsten

Posted by Harold Hallikainen (Member # 5405) on 01-30-2014, 08:26 PM:

For QC checking, there's always the USL LSS-100. It is permanently mounted in the auditorium. You run a test DCP daily and get reports on luminance, color, and SPL from each speaker. See http://www.uslinc.com/index.php?page=shop.product_details&flypage=flypage.tpl&product_id=58&category_id=18&option=com_virtuemart&Itemid=103&vmcchk=1&Itemid=103

Harold
LSS-100 designer

Posted by Scott Norwood (Member # 30) on 01-30-2014, 08:31 PM:

I have no idea where one would rent a screen luminance meter, but the device itself is easy to use and does not require training. The tough part is ensuring that the unit is in calibration and in good working order, since these are very sensitive devices and are also somewhat fragile (dropping one would probably an expensive mistake).

How much is the LSS-100? It looks like a neat device. Does the screen luminance measurement work with film projectors, too?

Posted by Phil Ranucci (Member # 3820) on 01-31-2014, 12:30 AM:

I spoke with Photo research about their meters and was told they do rent them, no idea the cost/terms.

Posted by Carsten Kurz (Member # 5396) on 01-31-2014, 07:13 AM:

Renting may be an option for color calibration meters, but for screen luminance checks and lamp adjustments on such a large screen venue, a brightness meter should be on location all the time IMHO.

- Carsten

Posted by Gordon McLeod (Member # 33) on 01-31-2014, 10:10 AM:

I thought they had the aircooled classic machine not a GT
I remember one imax booth where they had taken a piece of plywood that covered the port and had solar cells mounted on it where each of the holes in the punched test film were and had a voltmeter beside it so that they could balance the light repeatedly and they had a chart of what the voltage was in relation to a test with a calibrated meter

Posted by Ivan Plamondon (Member # 7637) on 02-03-2014, 01:20 PM:

Harold, any idea on the cost of a LSS-100?

Posted by Ivan Plamondon (Member # 7637) on 02-03-2014, 03:14 PM:

Gordon, our IMAX projector is a GT and also uses an air-cooled 7kW superior lamp that is installed horizontally.

Casrten, in regards to your comment about affording our own light meter, we as a business have been affected like all others by the recession. Expenses are always a hot button issue. Our last meter broke back in 2004 I believe.

Posted by Carsten Kurz (Member # 5396) on 02-03-2014, 06:35 PM:

Ivan, didn't want to insult you. Just thought that on such a projection system, a light meter should easily pay for itself by being able to utilize these expensive lamps to the max.

That said - I just brought home two spot meters from my friend - a Gossen Spot-Master and a SEKONIC L-558. Let's see how they deal with our Ernemann flicker frequency. There is also a Minolta I can tryout later.

- Carsten

Posted by Harold Hallikainen (Member # 5405) on 02-03-2014, 07:43 PM:

I'm told the LSS-100 lists for $750US. Dealers may discountbit.

Harold

Posted by Carsten Kurz (Member # 5396) on 02-05-2014, 09:28 AM:

Ivan - I checked the two spot meters, but they can't be used for film projection, they do not filter the flicker frequency properly. The Sekonic tries hard and delivers near realtime measurements, but it still jumps up and down. The Spotmaster is completely erratic, as it only does short time measurements and they jump all over the place.

They are good for digital projection, but not film.

Harold - the LSS-100 is certainly aimed at controlling digital installs - do you know if the luminance meter can cope with film flicker?

- Carsten

Posted by Harold Hallikainen (Member # 5405) on 02-06-2014, 07:40 AM:

The LSS-100 integrates over 400ms, so variation due to flicker is minimal (I'll see if I can get some numbers). I've proposed a variation of 1% or less with a flicker frequncy of 48 Hz or higher and duty cycle of 50% or higher to the SMPTE 20F group on luminance measurement.

Harold

Posted by Pete Naples (Member # 761) on 02-06-2014, 04:00 PM:

Carsten/Ivan. I have a Sekonic L558 cine that has been modified to properly integrate projector shutter flicker. I suspect it's running a modified firmware. Certainly put it against my SpecBos 1201 and the readings are close enough not to question the instrument.
I don't really use it much these days as I tend to be working on D Cinema so the SpecBos is king. Next time I do have it out I'll see if I can find the details of the mod.

Posted by Carsten Kurz (Member # 5396) on 02-06-2014, 04:49 PM:

That's interesting, Pete. For a second, I even thought I should suggest a modified integration time to Sekonic myself, as the Cine version already outputs cd/qm and fL, and has a dedicated option menu to support special settings like this. But actually I didn't think Sekonic would listen to me, escpecially since film projection will be considered dead now.

Please get back to us about this.

- Carsten

Posted by Gordon McLeod (Member # 33) on 02-06-2014, 06:24 PM:

So you have the twin rotor 3d machine?

Posted by Ivan Plamondon (Member # 7637) on 02-10-2014, 02:13 PM:

No Gordon. Just a 2D projector, single rotor Grand Theatre Unit made in Cambridge, Ontario

Posted by Carsten Kurz (Member # 5396) on 02-14-2014, 09:50 AM:

Here's what I received from Sekonic on the matter of integration time and possible firmware option:

---
Dear Mr. or Ms. Carsten Kurz,

Thank you for your inquiry about L-558CINE.
I am Minoru Oda(Mr) from Sales Operation Sec.
of light meter in Sekonic Corporation in Japan.

The measuring period of luminance measurement of L-558CINE is 33 msec.
Therefore, if there is 48Hz flicker in your measurement, measurement value is affected by this flicker.
And unfortunately, L-558CINE do not have a longer integration time function and any other special update.

Please kindly use it by measuring several times and averaging them.

If you have any question, please kindly let us know.

Sincerely yours,

Minoru Oda
Sales Operation Sec.
---

So, Pete, if you can try to find out about your L-558 Cine...

- Carsten

Posted by Gordon McLeod (Member # 33) on 02-14-2014, 10:37 AM:

The GT projector is a twin rotor design using 2 15K watercooled lamps for 3d with the advantage of two lens very close togather to minimize the 3d distortion
The "classic" was available as a 15K water cooled or a originally 4K later 7K aircooled machine
Later the SR machine which was also aircooled but with a small rotor and then the imfamous MPX machines

Posted by Pete Naples (Member # 761) on 02-18-2014, 03:19 PM:

Oddly enough I just dug it out and put batteries in it. Still works, still gives readings very close to my SpecBos

It's a re-badged Sekonic, sold as 'MRL LightSpot'. Peel the stickers back and it's a Sekonic underneath. I'd wager MRL were the people who modified it. Certainly it's not any kind of official Sekonic modification.

Given that this model is long obsolete and that the company I bought it from here in the UK are no more (I last had it calibrated in 1999 [Roll Eyes]

). I think we're on to a non-starter for the OP.

Looking at the prices of new Sekonic meters, issues of integration time aside, the price of the Harkness gadget really isn't bad at all.

Posted by Carsten Kurz (Member # 5396) on 02-18-2014, 03:41 PM:

You are right, official retail prices do not make a big difference between current Sekonic spotmeters and the Harkness screen checker. I was hoping there would just be a hidden config menu in the Cine model to enable a longer integration time. Maybe MRL commissioned a batch of modified 558 cine models for their clients or technicians.

The 558 cine is still a very nice spot meter and you can get it for modest prices on ebay. Actually it's no problem to use them on digital screens.

- Carsten

Posted by Pete Naples (Member # 761) on 02-19-2014, 10:56 AM:

I used to use it on digital & film screens, as the early software for the SpecBos didn't give Fl readings!

Once the Jeti software was updated to give x,y +Fl, I stopped using the spotmeter, although a colleague used it for a time for Technicolour 3D installations.

Posted by Ivan Plamondon (Member # 7637) on 02-24-2014, 02:07 PM:

Gordon, you are describing the 3D version of the GT projector. Ours here at Niagara Falls is a 2D version GT projector with a SINGLE rotor and a converted 7kW lamphouse (horizontal placement). It was originally a 4.5 kw lamphouse.