Film-Tech Forum ARCHIVE: Measuring screen brightness with F/c meter

This is topic Measuring screen brightness with F/c meter in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

To visit this topic, use this URL:
https://ft-forum.com/ft/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=1;t=011472

Posted by Rob J. Buskop (Member # 3338) on 06-08-2013, 02:56 PM:

If one is not in the possession of a designated F/Lambert meter whilst having a nice Ft/Candle one available, what, with a matte white screen, would be the error when you would take an average reading of the incident light to see if it coincides with SMPTE standards given in F/L's?
Theoretically with an assumed 1to 1 reflection of the matte white the f/c reading could be interpreted as a f/l reading, right? Or....not quite right?
Is a F/C meter a tool that could be used in your opinion?
Looking forward to your insight.

Posted by Jeffry L. Johnson (Member # 453) on 06-09-2013, 01:24 PM:

if all you have is a hammer, everything looks like a nail

It appears that you want to force an incident-light meter to function as a reflected-light meter, specifically a spot photometer with an acceptance angle of two (2) degrees or less.

Posted by Rob J. Buskop (Member # 3338) on 06-09-2013, 03:28 PM:

Well Jeffry, you seem to have distilled something of my question that enabled you to give this reply.
What you said is thoroughly familiar to me, but I had hoped for a somewhat more academic response...
With the incident light meter with the separate metering head I can also measure reflected light, albeit not within the narrow
angle as generally described.
But apart from that,it should be possible to work out a rough
mathematical approach to come to a physically acceptable formula that would give a reasonable and useable approximation of the relation regarding the parameters at hand.
After all, this is dark room measuring,on a matte white which eliminates lots of variables.
If one assumes the gain of the screen to be 1, then incident light equals refected light and that only leaves the narrow angle
measurement versus the wider angle of the more common meter as
the to be interpolated entities.

Posted by Louis Bornwasser (Member # 3063) on 06-14-2013, 05:52 AM:

I'll jump into the fray. The 2 degree spot meter is a standard that can be used in any cinema. There are essentially no standards for the other meter. If you have both meters at the same time, you could almost make a conversion chart, but it would only be good on that screen, at that seat.

Posted by Steve Guttag (Member # 268) on 06-14-2013, 06:04 AM:

Exactly...assuming the screen gain (regardless of its proclaimed gain) is a foolish assumption to begin with.

The short answer is no...and incident meter just won't give a meaningful information as to reflected screen light...particularly if the the exact characteristics of the screen, as it is installed, is not completely known.

Posted by Carsten Kurz (Member # 5396) on 06-14-2013, 06:29 AM:

What does 'a nice Ft/Candle one' actually mean? Hope it's at least a digital meter, not one of those green/yellow/red plant grow needle indicators ;-)

If it is just to control your known own installations, an ambient light meter could at least serve as a relative measure IF the brightness has once been properly measured during e.g. the installation of a new projector. Without such a reference measurement, it's useless.

You let them take a screen brightness reading with full white, and then take an incident light metering facing towards the projector (not the screen), at a position that can easily be reproduced/marked

Then you have a lux vs fL or cd/sqm relation that remains constant as long as screen and lens setting remain unchanged. That at least serves to control lamp aging or changes/adjustment. And it also serves for a decent calculation of your real screen gain.

Of course this is only for the projectionist to have SOME idea about the lamp status developing. In the long run, buying a used spot meter is probably the better idea. Depends on how many screens you have and what budget is there. With digital, usually lamp warranty hours is all you need to know if the system has been setup properly.

A good SLR/DSLR can also be used as a brightness meter, but it needs some more calculations and the error is usually larger because you can't expect more than 1/3 stop accuracy depending on the model.

The more complicated it get's, the less sense it makes.

The harkness digital screen checker is a good option and should be in everybodys budget.

- Carsten

Posted by Rob J. Buskop (Member # 3338) on 06-25-2013, 09:52 AM:

Thank you all for your input.
However I made the mistake not to mention that the application of the f/c meter readings were for "use" in a screening room,
where one could hold the sensor against the screen or alternatively do a reflection measurement.
The narrow angle approach does'nt come into the equation here.
As the comparative results show, this method can certainly be
used in this application.
22f/c direct versus about 18 f/l, bright and in accord with
Harkness, thank you.

Posted by Manny Knowles (Member # 1171) on 06-25-2013, 10:21 AM:

Step One - Ask question

Step Two - Refute answer

Step Three - Answer own question to one's own satisfaction

Posted by Pravin Ratnam (Member # 1417) on 06-25-2013, 12:06 PM:

Does every movie theater have access to one of these. THe reason I ask is when I see some auditoriums consistently too dark, is there a way to ask the manager to check the brightness of the picture easily?

Posted by Manny Knowles (Member # 1171) on 06-25-2013, 03:48 PM:

Theatres that care about screen luminance can either purchase a luminance meter, or hire a tech who is equipped with one.

So, yes. In the most basic sense, all theatres have access to luminance meters.

However, if the screens are dark, it's a sign that particular location does NOT have a meter on-site.

Posted by Scott Norwood (Member # 30) on 06-25-2013, 05:14 PM:

Has anyone ever actually worked for a theatre (not a screening room, not a special venue, not a large-format house--I am only asking about commercial theatres here) that actually had a proper light meter?

At most venues, it is the sort of thing that the technician will check at a twice-yearly service call, not something that an individual theatre would normally do on a regular basis.

I am not saying that this _should_ be the case--only that is _is_, in my experience. For most theatres, $3k+ for a light meter would be hard to justify.

Posted by Manny Knowles (Member # 1171) on 06-25-2013, 05:16 PM:

Buy a used one and then send it in for calibration. Doesn't have to cost $3k.

Posted by Carsten Kurz (Member # 5396) on 06-25-2013, 05:49 PM:

The Harkness/TLS meter goes for roughly 750US$ in single quantities.

I don't think such a meter is a must for every cinema. If you have to do a lot of manual lamp alignments, it certainly is a valuable item.

If you stay close to your tech while he is performing the initial measurements during the install, you know the basic parameters of your projector/screen combo.

Our Sony comes with a set of prealigned UHP lamps. There is nothing I can't do about basic brightness or lamp alignment anyway.

- Carsten

Posted by Louis Bornwasser (Member # 3063) on 06-25-2013, 06:21 PM:

Yes, we must have sold 50 to National Amusements. This is enough for key theatres and all districts to have one. I guess owning one is a good place to start, but then again, you still have to give a sh*t. louis

Posted by Rob J. Buskop (Member # 3338) on 06-27-2013, 03:40 PM:

I would like to answer Mr. Knowles' not-very-gracious first reply. At the time I asked the question, it was meant to obtain some insight or experience from all who have the interest, intellect and curiosity to sometimes think "out of the box".
At that moment I did'nt have a lux/Fc meter in my possession.
After the responses that did'nt really help me or prove to me that an on-screen measurement would be worthless and irrelevant, I decided to acquire the meter I had my eye on and take a chance.
After getting the device (excellent value for the money..!) I performed the tests with direct on screen incident light and also with refected light ( the latter measured at some 8 inches from the screen).
Then the more expensive equipment was momentarily brought in for comparison.
The results that were quite close have already been mentioned.
So, all in all, I don't see why I could'nt ask about other peoples experiences first, before buying the equipment.
Hard cheese for Mr. Knowles maybe when the application seemed to work rather better in my specific application?
Dare I possibly suggest that someone at some time might have some
use for a less expensive device in smaller projection environments and find some support here?

Posted by Manny Knowles (Member # 1171) on 06-28-2013, 06:49 PM:

What. Ever.

Would you like to know what you can do with your hard cheese?

[fu]

Posted by Chris Slycord (Member # 4239) on 06-30-2013, 03:09 PM:

I'd go with grating, melting, or slicing it myself.

Posted by Tom McGarrity (Member # 7320) on 06-30-2013, 03:21 PM:

Fondue ?