American Cinematographer - August 2017 - 22

Cinematographer Kaity Williams poses for a battery of tests of current LED fixtures. The top-left image
provides a skin-tone reference under tungsten light; there are substantial variances in each subsequent
image, despite all of the LEDs' high CRI ratings.

This is a challenging concept for
many people to understand because our
brains have wonderful color memory. Even
though our eyes don't see the red, our
brains remember that the strawberry is
red, and we imagine that we still see that
color. Film and digital sensors, however,
don't have such memory. They can only
show you what is actually there. When the
strawberry doesn't get red light, it looks
black to the camera. So in order for us to
see real colors as they appear in the real
world, the light that we're using has to
contain those colors; if it doesn't, then
we'll never be able to record the true color
of the object we're looking at.
Red, as it happens, is a crucial color
for most applications, as it is the primary
hue in skin tone - and most of us are
shooting human faces on a regular basis.
That means it's very important to have the
red wavelength when we're lighting.
The extended CRI tries to take this
22

August 2017

into account by incorporating the saturated red and yellow-pink chips. That
helps, but it doesn't solve the problem for
the rest of the color spectrum.
There have been several proposed
alternates to CRI, such as the Television
Lighting Consistency Index created by
color-science expert Alan Roberts; the TLCI
employs the Macbeth color chart's 24 color
patches and incorporates the opinions of
colorists on whether or not the light from
a specific luminary could be timed to look
proper. This system, however, was based
on a theoretical digital camera in Rec 709
and is only somewhat useful for the abundance of digital-sensor color matrices out
there. Other concepts such as the Color
Quality Scale (CQS) and the Gamut Area
Index (GAI) were proposed but never
widely adopted.
The Academy of Motion Picture
Arts and Sciences has been aware of the
limitations of CRI and the problems of LED
American Cinematographer

lighting for a number of years, and the
organization has been thoroughly
researching and testing. Late last year the
Academy published a paper in the SMPTE
Motion Imaging Journal describing the
Spectral Similarity Index (SSI), a new
methodology for measuring color fidelity
from non-continuous-spectrum light
sources. Due to the work of Academy
members Jack Holm, Tom Maier, Paul
Debevec, Chloe LeGendre, Joshua Pines,
Jonathan Erland, George Joblove, Scott
Dyer, Blake Sloan, Joe di Gennaro and Dan
Sherlock, the SSI stands to replace CRI as
the primary methodology for measuring
color fidelity for the motion-picture industry. Pines, Erland and Joblove are also ASC
associate members.
"A new color index is proposed that
is based upon the similarity of a luminaire's
spectrum to a reference spectrum that
eliminates the need for any assumption of
a specific observer or camera spectral
sensitivity," reads the SSI report's abstract.
"The index yields a 'confidence factor,'
where a high score implies predictable
color rendition for cinematography, and a
moderate score implies possible color
rendition challenges."
With just a spectrometer, a lighting
manufacturer can measure the spectral
output of a given fixture and plug the
results into a look-up table to find its score,
which relates the spectral output of the
light to a reference illuminant: sun, tungsten, etc. The resulting number is a "confidence factor." If it is 90 or above, then you
can know the light source will result in
colors that nearly mimic those from the
specified reference source. If it is 70 or
below, you know that there will be problems and some colors will not reflect accurately. Employing this system removes the
bias of human vision and eliminates any
direct comparison to a specific camera or
color matrix. You simply get a result that
relates the spectral curve that is being
tested to that of a reference spectrum.
For the time being, though, manufacturers continue to employ CRI. So when
you read a data sheet and you notice
what's sure to be a high CRI, remember to
take it with a grain of salt.
➣



Table of Contents for the Digital Edition of American Cinematographer - August 2017

Contents
American Cinematographer - August 2017 - Intro
American Cinematographer - August 2017 - Cover1
American Cinematographer - August 2017 - Cover2
American Cinematographer - August 2017 - 1
American Cinematographer - August 2017 - 2
American Cinematographer - August 2017 - Contents
American Cinematographer - August 2017 - 4
American Cinematographer - August 2017 - 5
American Cinematographer - August 2017 - 6
American Cinematographer - August 2017 - 7
American Cinematographer - August 2017 - 8
American Cinematographer - August 2017 - 9
American Cinematographer - August 2017 - 10
American Cinematographer - August 2017 - 11
American Cinematographer - August 2017 - 12
American Cinematographer - August 2017 - 13
American Cinematographer - August 2017 - 14
American Cinematographer - August 2017 - 15
American Cinematographer - August 2017 - 16
American Cinematographer - August 2017 - 17
American Cinematographer - August 2017 - 18
American Cinematographer - August 2017 - 19
American Cinematographer - August 2017 - 20
American Cinematographer - August 2017 - 21
American Cinematographer - August 2017 - 22
American Cinematographer - August 2017 - 23
American Cinematographer - August 2017 - 24
American Cinematographer - August 2017 - 25
American Cinematographer - August 2017 - 26
American Cinematographer - August 2017 - 27
American Cinematographer - August 2017 - 28
American Cinematographer - August 2017 - 29
American Cinematographer - August 2017 - 30
American Cinematographer - August 2017 - 31
American Cinematographer - August 2017 - 32
American Cinematographer - August 2017 - 32a
American Cinematographer - August 2017 - 32b
American Cinematographer - August 2017 - 32c
American Cinematographer - August 2017 - 32d
American Cinematographer - August 2017 - 33
American Cinematographer - August 2017 - 34
American Cinematographer - August 2017 - 35
American Cinematographer - August 2017 - 36
American Cinematographer - August 2017 - 37
American Cinematographer - August 2017 - 38
American Cinematographer - August 2017 - 39
American Cinematographer - August 2017 - 40
American Cinematographer - August 2017 - 41
American Cinematographer - August 2017 - 42
American Cinematographer - August 2017 - 43
American Cinematographer - August 2017 - 44
American Cinematographer - August 2017 - 45
American Cinematographer - August 2017 - 46
American Cinematographer - August 2017 - 47
American Cinematographer - August 2017 - 48
American Cinematographer - August 2017 - 49
American Cinematographer - August 2017 - 50
American Cinematographer - August 2017 - 51
American Cinematographer - August 2017 - 52
American Cinematographer - August 2017 - 53
American Cinematographer - August 2017 - 54
American Cinematographer - August 2017 - 55
American Cinematographer - August 2017 - 56
American Cinematographer - August 2017 - 57
American Cinematographer - August 2017 - 58
American Cinematographer - August 2017 - 59
American Cinematographer - August 2017 - 60
American Cinematographer - August 2017 - 61
American Cinematographer - August 2017 - 62
American Cinematographer - August 2017 - 63
American Cinematographer - August 2017 - 64
American Cinematographer - August 2017 - 65
American Cinematographer - August 2017 - 66
American Cinematographer - August 2017 - 67
American Cinematographer - August 2017 - 68
American Cinematographer - August 2017 - 69
American Cinematographer - August 2017 - 70
American Cinematographer - August 2017 - 71
American Cinematographer - August 2017 - 72
American Cinematographer - August 2017 - 73
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American Cinematographer - August 2017 - 76
American Cinematographer - August 2017 - 77
American Cinematographer - August 2017 - 78
American Cinematographer - August 2017 - 79
American Cinematographer - August 2017 - 80
American Cinematographer - August 2017 - Cover3
American Cinematographer - August 2017 - Cover4
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