What's Your Screen Luminance? : Kodak

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Don't be in the Dark about Measuring Screen
Luminance
What's Your Screen Luminance?
If you ask most projectionists or theatre managers whether their screens have
the proper screen luminance, they'll reply that they are "on standard" or "it
looks okay," and quickly try to change the subject. When you ask what "on
standard" is, they usually answer with a question "Isn't the standard 16 foot -
something's or other?" and sheepishly add "maybe we're a bit low on a few of
the screens," as they furtively look to see if you have a meter to measure it.
Boldly asking when the screens were last measured, the reply might be,
"When the service technician was here last summer," or "When the theatre
was built." Ask, "Do you own a meter to check screen luminance?" and the
answer might be "Why do we need one, when the service tech checks it
several times a year?"
Why Measuring Screen Luminance is Important
The above scenario plays out in too many theatres. All too many screens are
poorly illuminated. The importance of proper screen luminance is not fully
appreciated and the infrequent measurement is left to "the experts." Theatre
personnel are familiar with some of the terminology, but often don't fully
understand what it means or why it is so important. They rarely have access
to an accurate photometer for measuring screen luminance, which should be
done routinely on a regular basis and especially after maintenance such as
changing a xenon lamp or adjusting lamphouse alignment.
What is Screen Luminance?
Simply put, screen luminance is the amount of light reflected from the screen,
as seen by the audience. Every theatre should have a copy of standard
ANSI/SMPTE 196M, "Indoor Theatre and Review Room Projection - Screen
Luminance and Viewing Conditions." It specifies the "screen luminance level,
luminance distribution, and spectral distribution (color temperature) of the
projection light . . . to achieve the tone scale, contrast, and pictorial quality of
the projected print that will be of the quality intended during its production."
Most people remember the figure of 16 footlamberts, which is the nominal aim
screen luminance. But ANSI/SMPTE 196M specifies much more and describes
the measurement procedure.
First, the standard specifies the operating conditions for measuring screen
luminance: the projector should be in normal operation with the shutter
running, lens at normal focus position, and with no film in the gate.
Measurements should be made for all formats (e.g., scope and flat).
The standard specifies a spot photometer with an acceptance angle of two
degrees or less, the spectral luminance response of the standard observer, a
capacity for integrating over the shutter flicker. In other words, the meter
should measure reflected light from a small portion of the screen as the
human eye would, and not be fooled by the 48 or 72 cycle flicker of the
shutter.
The standard specifies that the primary screen luminance measurement should
be taken from the center of the seating area at approximately the eye level of
a seated person. Additional measurements should be taken from the center
and from each end of the middle row in the theatre. For theatres with wide
viewing angles or stadium seating, even more measurements are
recommended.
The nominal screen luminance at the center of the screen should ideally be 16
footlamberts (55 cd/m_). The standard allows a range of 12 to 22
footlamberts (41 to 75 cd/m_) for theatres. Luminance at the sides of the
screen should be between 75 to 90 percent of the center luminance, and never
less than 10 footlamberts (34 cd/m_).
Obtaining Copies of SMPTE Standards
Copies of standards like ANSI/SMPTE 196M and test films can be ordered from
the Society of Motion Picture and Television Engineers. The web site is
www.smpte.org/smpte_store/standards/.
Screen Luminance Meters
In the next issue, we'll discuss the professional screen luminance meters that
are available, including the Minolta Luminance Meter LS-100, the SpectraCine
CineSpot Spotmeter Model SC-600, and the Ultra Stereo Labs PSA-200
Projection Screen Analyzer. We'll also look at how less expensive meters or
even an adjustable camera can be used to check if screen luminance is
correct.