Alternative Light Meters

[Lucas Iaccarino]

Hello everyone
I´m writing you all because I´m looking to get an affordable light meter to be able to measure luminance at screens.
A Colorimeter or a Spectra Meter are out of budget. I used to have a Sekonic L758 Cine that was very accurate, but actually It´s discontinued.
Do you have any suggestions?

Thanks!

[Mark Gulbrandsen]

Almost any meter that reads foot lamberts should work... I knew a guy that used a Minolta meter that read footlamberts. You should look for a used meter. Harkness also made a screen brightness meter that was inexpensive. Never tried one, so not sure how accurate it is.

[Mitchell Dvoskin]

Years ago, Harkness had a $30 IOS app called Harkness Verifier for this purpose and it was surprisingly accurate.

Harkness Screen Verifier

Still works on my iPhone SE (2016)

Unfortunately, it is no longer available.

[Pete Naples]

Digital Spotmeter (Sekonic etc) can give false readings, it's down to intergration time vs projector dark time.

I've a Sekonic with modified firmware on it, I cannot remember the supplier, it came from California. Anyway it's bang on when compared with my Jeti 1511

[Carsten Kurz]

Comes down to wether you need the light meter for classic film work as well. For digital, integration time doesn't matter. As such, the L-858 follow-up to the L-758 should be a suitable replacement.

[Harold Hallikainen]

Which makes me wonder, what is the flash duty cycle on a digital cinema projector? I THINK the flash rate is twice the frame rate, but is the duty cycle 50%?

[Randy Stankey]

I always wanted to experiment with an extinction style light meter to see how well it would work.

An extinction meter is like a small, handheld telescope or viewfinder that has a graduated, neutral density filter which goes from perfectly clear to totally dark. Superimposed over that filter is a graticule with markings that indicate zones 1 through 10. The user looks through the scope and finds the last zone where he can see an image showing through. That is the brightness of the subject being measured.

Obviously, this would not be a high precision measurement but I think it would be good enough for theater personnel in daily use.

My idea is that a technician would set up the projector in the normal way, using a proper light meter, then when he has it working the way he wants it to be, he uses the extinction meter to get a reading. Let's say that the extinction meter reads "Zone-6." (I just made that up as an example.) He could give that extinction meter to the people at the theater and tell them that they should be getting a reading between 5 and 7.

When a new lamp is installed, take a reading and adjust the lamp power until it is at Zone-6, plus or minus one zone.
If there is any question as to whether the picture is too dark or bright, grab the extinction meter and take a reading.

If, ever, the theater personnel can't get a picture that reads Zone-6 ±1 they will need to call a tech to figure out what's wrong.

The idea is not to get a precise reading but to simplify things so that people at a theater have an quick and easy way to judge picture brightness and to relay pertinent information to a technician in a way that is easy to describe and that the technician can translate into a meaningful solution.

If we wanted to make a custom device for use in movie theaters, it doesn't have to be graduated in zones like photographers use. We could design a graduated filter and graticule to indicate whatever we wanted. We might be able to make an extinction meter to read in approximate ft-L or just in some generic brightness levels that we find convenient.

We could also design a custom test target that has a grid of squares from 1 to 5, horizontally and vertically. 3-3 being the exact center, 1-1 being the top-left and 5-5 being the bottom-right. The center (3-3) would have to read between certain numbers, the corners (1-1, 1-5, 5-1 and 5-5) would need to read between another level and the edges (1-3, 3-1, 3-5 and 5-3) would need to have their own reading. If there is a question, somebody could put up the test pattern, and use the scope to make nine readings then report them to the tech.

Maybe that's too complicated. Maybe just one reading is all that's necessary but I'm sure that you understand my idea and what I'm aiming at. Right?

[Carsten Kurz]

Harold - except for time-multiplexed 3D, there is no flashing or blanking on digital cinema projectors that I know of.

[Harold Hallikainen]

Seems like there would be visible flicker if there is not something like 48 Hz flash with dark periods between flashes.

On the extinction meter, it seems like there would be too much variation person to person and whether the eye is light or dark adapted. I have heard of meters with a light where you adjust the brightness of the light until it matches the screen, then read the luminance. I don't remember if the light source was adjusted or neutral filters were put in the path. I read about this many years ago while researching luminance measurement for a SMPTE project.

[Randy Stankey]

An extinction meter is accurate enough for a person with average vision to take photographs with under varying light conditions.

Some types of extinction meters have a dial to turn until the image in the viewfinder fades. The user looks through the viewfinder, turns the dial then reads a number from a scale at the point where he can’t see an image.

If such a thing was used with a specially designed test pattern with high contrast markings there shouldn’t need to be a graticule inside the scope. Suppose there was a grid of numbers in a large boldfaced font. The user looks through the scope, points it at the screen then turns the dial until the number(s) on the screen can’t be seen.

Read the number on the dial of the scope and you have your answer.

[Jim Cassedy]

In the mid-to-late 1960's Polaroid put out a cheap, black and white only, camera called "The Swinger". I remember getting one for Christmas or maybe it was my birthday. It had some sort of extinction meter arrangement in the viewfinder, where as you turned a small knob on top of the camera, the words "YES" or "NO" appeared superimposed below viewfinder image. You turned the knob until you saw the word "YES" and that would set the proper exposure. Harold's concerns about variations in people's visual perception possibly making the extinction meter for measuring screen brightness unreliable might be a bit overblown, as "The Swinger" was one of Polaroids most popular selling cameras at the time, so the extinction meter process apparently worked well enough to be successfully used by millions of people. I like Randy's idea.

[Harold Hallikainen]

I learn something every day! I was no aware of extinction meters. As I mentioned before, I had read about comparison meters. Various instruments are described at http://www.earlyphotography.co.uk/site/meters.html . The first comparison meters used a candle as the reference light source that the incoming light was compared to.

Harold

[Carsten Kurz]

Harold - there is no dark period for DLP or (Sony) SXRD projectors. It's a constant luminous flux, only modulated by the individual imager pixels. Only if you configure the DLP projector for time multiplexed 3D, there is a dark time needed for slow polarizers, wheel transitions, shutter glasses, etc.

I usually check our screen brightness with a common cheap (incident) digital light photometer ('luxmeter'). You can buy these on ebay etc. for very little money (from 20 US$ up). It's the same type of device you would e.g. use to check proper office or workplace lighting conditions. Yes, you would need to expect the device went through some basic factory calibration. But that is the same for any instrument.

They don't measure screen brightness, but the light level falling onto the meter/screen. But it's easy to calculate cd/sqm or fL from it if you know your screen gain characteristics. And of course it is helpful if you get a reference measurement on a tech visit from time to time. Screen reflectance changes very little over time, and the typical luminous flux variations come from ageing bulbs or preset changes. Which is what you typically want to track. Of course, this type of device is not sufficient for a travelling cinema tech.

- Carsten

[Gordon McLeod]

I still rely on my PSA USL light meter when the gods of microsoft permit the drivers to load

[Jim Cassedy]

I found a picture & description of the extinction meter system in the viewfinder of
the old Polaroid "Swinger" camera I mentioned in my earlier post. There was a
checkerboarded area superimposed just below the image in the viewfinder.
You turned the red knob until the word "YES" appeared clearly, and that set
the exposure. The model I had was one of the earlier ones, which alternated
between "YES" and "NO". Later models just had "YES" which faded in & out.
SwingerViewfinder.jpg