American Cinematographer - August 2017 - 24

Tech Essentials
The Waveform
Although the light meter is still the
best tool for exposure judgments, in the
digital age we have a number of additional
tools available to us. A keen understanding
of how to interpret them is necessary to a
smooth workflow.
Probably the most common is the
waveform, which is built into many production monitors and camera systems today.
Waveforms have been around since the
early days of analog video and have been
used by engineers to monitor the values of
an image for over half a century.
Reading a waveform monitor takes
a little practice, but it is mostly intuitive.
There are three primary display modes on
most waveforms: luminance, luminance
with chrominance, and parade. We'll start
with the luminance function.
The waveform monitor starts with a
grid marked out on the screen. The primary
section of this grid is marked in horizontal
delineations from 0 to 100. This represents
the signal intensities from 0 percent (black)
to 100 percent (white/peak). The markings
actually represent IRE (Institute of Radio
Engineers) values, which translate to
percentages of luminance in the image.
Some waveforms have a scale that goes
beyond the standard 0-100, having values
below 0 (generally to -40) and above 100
(generally to 120). The lower values are for
the sync-pulse signals, or line-blanking
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August 2017

intervals, and are of no importance to us in
judging exposure. The area above 100 is
the "super-white" area; generally any signal
in this area has complete loss of image
detail. In addition, some waveforms have a
marking at the 7.5 IRE for "setup"; this was
required for getting solid blacks in standard
definition (Rec 601), but you can
completely ignore 7.5 for HD and cinema
signals.
Although waveforms do consider
chrominance information, they are primarily
for measuring luminance information. The
vertical IRE scale represents a percentage of
luminance of the image. When you see
areas of the waveform above 100 IRE, you
know those areas are "clipping" - they
have become pure white and cannot be
brought down. If areas of the scene are
below 0, they are pure black, without
detail.
In fact, whites should really sit
between 80 and 100 for a well-exposed
image; if you're shooting a wedding scene,
the bride's dress shouldn't register above 85
IRE or else you'll risk losing detail in the lace.
Medium gray should be set between 45
and 55 IRE. Caucasian faces generally fall
between 60 and 70 IRE.
We can also combine chrominance
(color) information, but this can get kind of
messy and hard to read, so if your waveform has adjustments, it's better to set it to
IRE or "luma" only and ignore the chrominance - unless you're in the parade mode,
which we'll discuss shortly.
American Cinematographer

Waveform image courtesy of Jay Holben.

A typical waveform signal.

While the Y-axis (vertical) scale on the
waveform represents luminance percentage, the X-axis (horizontal) scale represents
the image from left to right. This is where
things can get confusing. If we're
photographing a white piece of paper in the
upper half of the frame and a black piece of
paper in the lower half, each point along
the waveform will register both white and
black. Similarly, if we have a grayscale positioned vertically in the frame, you'll notice
that each point on the waveform has a line
that represents the steps of that scale.
You can use the waveform monitor
like a light meter if you put a gray card out
into your scene where the talent will be, fill
the screen with the gray card, and then
adjust your aperture until the waveform
reads between 45 and 55 IRE. This will give
you a proper exposure for that area.
The waveform is a solid tool for
seeing your overall exposure range and for
maintaining a solid signal-to-noise ratio in
your image. With a low-light scene, you'll
notice that the signal is crowded toward the
bottom of the waveform; in that case,
you're likely to be picking up a lot of noise
in the recorded image, and it's generally
better to open up, expose a little higher on
the waveform scale and, if needed, reduce
the brightness later in post.
With the waveform's parade mode,
we separate out red, green and blue into
their individual components so we can look
at the luminosity of each channel side by
side. It's a quick way to see if you're overexposing your skin tones too much - is the
red channel running hotter than green and
blue? - which can lead to strange clipping
and color solarization.
It's important to note that most
waveform monitors are only going to work
properly with Rec 709 signals. Log and raw
signals are much more difficult to interpret
on a waveform, although new tools are
beginning to come out for just this purpose.
- JH
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Table of Contents for the Digital Edition of American Cinematographer - August 2017