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Author
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Topic: Projectionist's hand-held FtLb Light Meter??
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John Pytlak
Film God
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Posts: 9987
From: Rochester, NY 14650-1922
Registered: Jan 2000
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posted 06-21-2005 07:59 AM
Any meter used should measure light REFLECTED from the screen, and should have a photopic response. Most any spotmeter can be used, as long as it has sufficient sensitivity and accuracy, and is proper calibrated. Here is an article I wrote several years ago:
Screen Luminance Meters
quote: Shedding Light on Screen Luminance Meters To Measure Light, You Need a Photometer Standard ANSI/SMPTE 196M "Screen Luminance and Viewing Conditions" specifies the "Photometer type" as follows: "Screen luminance shall be measured with a spot photometer having the spectral luminance response of the standard observer (photopic vision) as defined in CIE S002. The acceptance angle of the photometer shall be 2° or less. The photometer response to the alternation of light and dark on the screen shall be to integrate over the range of 24 Hz to 72 Hz and display the arithmetic mean value." Can You Please Translate That? Simply put, the meter should "see" the light reflected by the screen as the human eye does. Some meters have photocells that are more sensitive to invisible infrared energy, and may give incorrect measurement of visible light. Meters designed specifically to measure screen luminance use special sensors and filters to have a "photopic" response just like the human eye. Second, the photometer used to measure screen luminance in a theatre should measure light reflected from a small area of the screen, no more that two degrees in viewing angle. For example, if you take measurements 60 feet from a screen 20 feet high, a meter with a two degree acceptance angle "sees" an area on the screen only about two feet in diameter. Having a relatively narrow viewing angle allows the user to measure luminance in various parts of the screen, to evaluate the uniformity of illumination across the screen. Since screen luminance is measured "open gate" with the projector running, the light on the screen is actually going off and on 48 or 72 times per second. At the normal 24 frames per second, a two- blade shutter gives 48 interruptions of light every second. A three-blade shutter gives 72 interruptions. A meter used for measuring screen luminance needs to be properly calibrated for the alternating light/dark cycle of a projector.
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