LIGHTING BY CRAIG DILOUIE
Developed by the International Commission on Illumination (CIE), the CRI
expresses how closely a source renders
colors compared to an ideal light source.
While the current version of CRI has
been in use since 1974, it has significant
limitations. The CRI is based on color
science going back to 1937, and many
scientific advances have occurred since
then. Despite its shortcomings and calls
to overhaul it, the CRI has stood the test
of time, apparently due to resistance
based on a belief that the lighting industry wouldn’t accept change.
The advent of solid-state lighting,
however, increased demand for a more
accurate color fidelity metric. The
light- emitting diode (LED) source produces light differently than traditional
sources, exposing the CRI’s limitations
and igniting calls for change.
Since 2006, the CIE has worked
on creating a new metric but hasn’t
reached agreement. In 2013, the Illuminating Engineering Society (IES)
formed the Color Metrics Task Group,
which developed TM- 30-15, IES
Method for Evaluating Light Source
Rendition, to introduce potential new
metrics for industry use. It may result
in revisions and further development
before it becomes a standard. Until
then, the intent is for the new metrics
to be used alongside the CRI. The CIE
is evaluating TM- 30.
Built on the progress researchers have
made over the past two decades and syn-
thesizing many of their concepts, TM- 30
is designed to address many of the CRI’s
limitations, providing more information
with greater accuracy.
The TM- 30-15 method quantifies
color fidelity (closeness to a reference)
through the Fidelity Index (Rf) (0–100
scale), which is analogous to the CRI
but is based on average fidelity across 99
color samples instead of eight to 14.
The higher the score, the more accurately colors will render as they would
under the reference light source. A high
number doesn’t inherently mean the light
source is better for a given application.
For example, suppose we have two light
sources with an equal Rf and CRI, but one
results in reds visually popping because its
emission enhances reds or the other lamp
mutes that color. To predict this, we use
a second color metric, Gamut Index (Rg).
Gamut describes an increase or
decrease in chroma. Rg ranges from
around 60–140 when Rf is higher than
60. To calculate Rg, the 99 color samples
are broken down into 16 bins consisting of multiple color samples, and those
numbers are averaged. The resulting ratio
between the plotted area between the test
source and the reference source is multiplied by 100 to get Rg.
What’s important to know: An Rg
greater than 100 means there is an average increase in saturation, while a value
less than 100 means there’s an average decrease. This is valuable to know
because we might have a light source
with two lamps, each with an Rf of 90
but where one has an Rg of 110, increasing saturation, and the other has an Rg
of 90, which can cause some colors to be
muted. By using this second metric, we
can more accurately predict how objects
and spaces are going to look.
That being said, as with the CRI, Rf
and Rg suffer from a limitation in that they
are averages, which can conceal important information. A source with an Rg of
110 may, on average, enhance saturation,
but only certain colors may be saturated,
while others may not be affected or may
even be muted. To address this, TM- 30-15
offers a method to produce color vector
and distortion graphics providing a visual
depiction of hue and saturation changes.
For example, in the color distortion
graphic, colors outside the white circle
indicates increased saturation, while a
lack of color (black) inside the white circle indicates those colors will be muted.
For those who want to take their
analysis further, TM- 30-15 offers additional indexes, including skin fidelity
(Rf,skin), fidelity by hue (Rf,h#), chroma
shift by hue (Rg,h#) and fidelity by sample
On its face, TM- 30 represents a major
leap forward for predicting, evaluating and talking about color. However,
TM- 30 is not yet a standard, nor is it a
slam dunk to become one. What the
industry ultimately adopts as a standard
may be different than what is being proposed. Stay tuned.
A new look at color
FROM MAKING CLOTHING LOOK VIBRANT IN A RETAIL STORE
to facilitating interaction in offices by properly rendering skin tones, a light
source's color quality is an important specification characteristic. For this, we use
two metrics, correlated color temperature and the color rendering index (CRI). Varying these color qualities can affect how objects, spaces and people appear to the eye.
DILOUIE, L.C., a lighting industry journalist, analyst and marketing consultant, is
principal of ZING Communications. He can be reached at www.zinginc.com. I S T