This is topic Light Meter in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.


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Posted by Jay Glaus (Member # 6011) on 04-26-2015, 02:04 AM:
 
Just curious if anyone knows…

I've always wanted a spot light meter, but they always seem to get pricey quick. Does anyone know where I might be able to find one that is good but won't break the bank?

Thanks!

Jay
 

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Posted by Gary Lightfoot (Member # 8842) on 04-26-2015, 06:22 AM:
 
I use a Robin RT24 and an Extech (no longer made I believe), but the Tepel 531 is much the same and has a CIE photopic response which I believe is preferred for taking measurements of this kind. I think they all can be NIST registered for calibration as well. It was around $90 IIRC:

http://www.tecpel.net/DLM-531.html

There might be others that look identical except for the colour (the two I have and the Tecpel are). These meters do lux and foot candles, so you can calculate reflectance levels as foot Lamberts, and the effective lumen output of the projector if you want.

Gary
 

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Posted by Dave Macaulay (Member # 813) on 04-26-2015, 10:25 AM:
 
For photography or cinema screen brightness?
Cinema ones tend to be photography models modified to read ft-L directly.
The good ones are never cheap.
A photography one reads in different units which can be converted to Ft-L - only roughly though since the internal optics should be different to read in the correct color space. For general check-up use that isn't going to be enough of a difference to matter. You could have a tech set the system to 16 ft-L and then read the same screen spot using any spot meter, and try to match that in subsequent do-it-yourself tune ups.
To do a decent cinema projector setup you need a color/chromaticity meter to calibrate the light engine so "white" projects as the proper x=0.314, y=0.351 DCI white. Unless this is ridiculously bad nobody will ever notice of course: film projectors are all over the place on color temperature. Since D-cinema projectors can adjust color, now we're expected to make it right.
 

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Posted by Jay Glaus (Member # 6011) on 04-26-2015, 12:47 PM:
 
Thanks Gary.

And Dave, I was looking for something to measure screen brightness for D cinema. At the drive in I have a pretty long throw to the screen, I was looking for one of the guns you can look through and see the Ft/L on any given spot of the screen
 

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Posted by Carsten Kurz (Member # 5396) on 04-26-2015, 02:15 PM:
 
Depends on what you are after. Spot meters are usually a lot more expensive then incident light meters. There are different views on the usability of incident light meters for screen brightness measurements.

Some photographic spot meters allow to make readings in cd/sqm or foot candles, like the Sekonic L-558/L-758 Cine models. Sometimes you can get a second hand one for 300-400US$.

The Harkness digital screen checker is around 500€.

You can get away with different, cheaper solutions. Again, depends on what you want to achieve and in how far you will compromise convenience for money...

For a fixed installation, your screen reflectivity will usually change little over time and what you may want to measure is bulb aging. You can do that with a cheap incident light meter, but you need a solid reference measurement from time to time, e.g. when a tech is there. However, for a drive-in, a hand-held incident light meter seems to be out of the question...

You can even use a good SLR or DSLR camera with spot metering function - of course, only if you own one already and like to do some decent number sorting.

USLs LSS-100P may be another option, if you want automated operation in the form of a QoS control device. It is in the price range of a new Sekonic 758Cine, but has more functions targeted for cinema quality checks, including brightness, color, audio. It is not a 'portable device' in the strict sense, although it could be used handheld if needed.

We had a similar discussion here:

http://www.film-tech.com/ubb/f14/t000256.html

with a side-line into film/flicker tolerant meters for film projection.

- Carsten
 

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Posted by Jay Glaus (Member # 6011) on 04-26-2015, 07:48 PM:
 
I'm new to light meters so forgive me, but all of the spot light meters I find have settings for shutter speed and a few other things. When you're throwing a white test pattern up on the screen, would those type of meters still be able to give an accurate reading? It seems different than checking it on a film projector with the shutter.
 

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Posted by Carsten Kurz (Member # 5396) on 04-27-2015, 06:33 AM:
 
Most spot meters will only show EV, and compute f-Stop and shutter from this for photographic use. Some few models, e.g. the Sekonic L-558/758 CINE models also display cd/sqm or foot-candle. You can calculate cd/sqm or foot-candle from other values, like f-stop/aperture combinations. So you can use cheaper or existing devices, but need to do some number crunching before you get to a useful number,

When you still use film and want to measure screen brightness for film projectors, you need a special (=more expensive) device with longer integration times. I think the Harkness Screen checker is the cheapest dedicated screen brightness checker with shutter/film eligibility.

They also offer an iOS app now for measuring screen brightness, but I have no idea how consistent these smartphone cameras are.

A tech traveling to different locations and doing base installs definitely needs a precision device. For your local needs as projectionist or operator, you may be ok with relative measurements that will tell you 'I lost 30% brightness since the last bulb change'.

- Carsten
 

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Posted by Gary Lightfoot (Member # 8842) on 04-27-2015, 08:59 AM:
 
The ones I mention I fine for measuring fL IMHO. Because they are photopic and conform to "the standard observer" - they see light similar to the human eye. Ideally you will take readings after the projector has been calibrated.

There may be issues with film projection due to the shutter (I only use mine for home theatre digital projectors), but I would think it would be possible to average out readings if they were used for film, but that's purely a guess.

To calculate fL from a lux reading, you just divide the reading by 10.76, and then multiply by the gain of the screen (check the manufacturers figures). If you want to work out the lumen output from the projector, just multiply the lux reading by the area of the screen in square meters. 151 lux in the center of your screen would be around 14fL, assuming a unity gain screen. Perf would perhaps be around 0.8. I only use the meter facing the projector and calculate the results, but I guess it would be possible to take a direct reflectance reading ensuring the meter head wasn't reading from it's own shadow.

The meter also does foot candles which I think is the same as foot Lamberts, but I've not measured using fc so I've never compared. I might try it some time just to see. Don't know why I never tried doing that before - would certainly be easier

Unless you can compare an Iphone (or Android) to a known meter, I've no idea how accurate they would be, and I doubt they'd have a photopic response. Would be great (cheap and convenient) if they were suitable though.

I believe the Tecpel is accurate to plus or minus 3%, so if you were aiming for the DCI spec of 14fL, and that is what you were reading (151 Lux), you may actually have 13.6 or 14.4fL, so well within the tighter tolerances for screening rooms which I believe are around +- 0.7fL.

I've found the meters to be reliable and comparable to the readings from more expensive meters and will get you in the ball park. If you get them regularly calibrated then you will know that the readings you will get are reliable to within the meters capabilities.

Gary
 

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Posted by Carsten Kurz (Member # 5396) on 04-27-2015, 10:51 AM:
 
The question is, where/how do you actually measure incident light at a drive in in order to compute screen brightness...

- Carsten
 

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Posted by Steve Guttag (Member # 268) on 04-27-2015, 12:55 PM:
 
The other problem is that one cannot use manufacturer's data to "calculate" gain. Most are not very accurate for that and it doesn't take into account any aging or environment artifacts.

Note, too, one should take the reading from at least dead-center screen and ideally at 9 points and an incident meter is worthless for that.
 

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Posted by Gary Lightfoot (Member # 8842) on 04-29-2015, 11:27 AM:
 
Unfortunately, you're dead right about the screen.

The screen gain could be anything - even if the advertised gain was accurate, the weathering/dirt etc from being outside would probably reduce the gain and make it an unknown.

If currently the reflectance level is unknown, the lumen output could still be measured and calculated and that could be useful.

I know someone with a Tecpel 531 and will ask them to try a few things for me to see if the meter can be used for the purpose of adjusting screen reflectance levels directly. I can't do that right now myself, otherwise I would, but it would be handy to find a more affordable meter that could be used for this task.

Gary
 

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Posted by Harold Hallikainen (Member # 5405) on 04-29-2015, 08:41 PM:
 
By the way, I wrote a paper on color measurement a few months ago (with respect to the USL LSS-100P). I'd be interested in hearing any comments. It's at http://ftp.uslinc.com/ftp/Products/LSS-100P/Documents/Technical/ColorMeasurement/ColorMeasurementLSS_150130.pdf .

THANKS!

Harold
 

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Posted by Gary Lightfoot (Member # 8842) on 05-04-2015, 06:39 AM:
 
Interesting article - thanks for posting.

What is the cost of the USL LSS-100P over the Sekonic L-558/758 CINE which seems comparatively cheap and affordable in the scheme of things.

Cheers

Gary
 

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Posted by Mark Gulbrandsen (Member # 72) on 05-05-2015, 02:47 PM:
 
Harkness just released a light meter for eye phone. Not available yet for Android. So you ladies toteing eye phones around can now have a light meter. $39.95 is the cost.

Mark
 

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Posted by Gary Lightfoot (Member # 8842) on 05-05-2015, 03:14 PM:
 
I just received that email from Harkness as well. I wonder how it compares to any of the free apps you can get for the iphone?
 

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Posted by Carsten Kurz (Member # 5396) on 05-05-2015, 03:22 PM:
 
There are quite a few apps for ambient light measurement. The harkness needs to measure luminance. It may work because unlike other smartphones, the iOS devices only use a very limited range of different camera modules, so they may be spec'd tighter and thus be more accurate. Whatever 'accurate' means in this case.

- Carsten
 

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Posted by Gary Lightfoot (Member # 8842) on 05-05-2015, 03:33 PM:
 
Yeah - that makes sense. We've probably no idea if any of the free apps have been checked against a calibrated meter to ensure accuracy to an acceptable level.
 

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Posted by Steve Guttag (Member # 268) on 05-05-2015, 04:33 PM:
 
The claim is that the app is accurate to within +/- 1fL
 

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Posted by Armand Diaz (Member # 8861) on 06-16-2015, 07:21 AM:
 

quote: Jay Glaus

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I was looking for something to measure screen brightness for D cinema. At the drive in I have a pretty long throw to the screen, I was looking for one of the guns you can look through and see the Ft/L on any given spot of the screen

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