Evaluating System
Performance:
Between a
“ROC” and a
Hard Place
Brett D. Guenther
Thinkstock
How can engineers use a receiver operating characteristics curve
to assess the functioning of a complex sensor system?
When Emerson wrote, “Build a bet- ter mousetrap, and the world will
beat a path to your door,” he likely had
no idea how complicated that simple idea
could become. What constitutes “better”
to today’s engineer is often a matrix of
conflicting parameters and concerns
ranging from functionality to marketing. The engineer must sift through this
complexity and decide how well any new
device can actually solve a problem.
This article discusses a complex sensor
system that captures medical prescription
errors using Raman spectroscopy. The
“how well” question is answered with an
applied analysis—the receiver operating
characteristics (ROC) curve.
Raman spectroscopy
When monochromatic light passes
through a material, it is scattered mostly
at the same wavelength as the source (
elastic scattering). However, depending on the
medium, a small portion (about 1 photon
per 10 million) of the light is also scattered
into wavelengths shorter and longer than
the source (inelastic scattering).
Raman scattering, which is a form
of inelastic scattering, occurs when a
photon interacts with the vibrational
or rotational energy of a molecule. The
photon exchanges energy with the
molecule in one of two ways. When
the photon transfers some energy to
the molecule, it is scattered at longer
One of the more interesting
developments for Raman
spectrometers in recent
years comes from
embedded algorithms
that automate the analysis
and interpretation of the
spectroscopic information.
wavelengths (Stokes), and, when it gains
energy from the molecule, it is scattered
at shorter (anti-Stokes) wavelengths.
Since Raman scattering relies on the
interaction between the photon and a
molecule’s polarization, it is rapid and its
wavelength shift is not dependent on the
excitation wavelength.
Raman spectra can be used as a
fingerprint for a molecule, since each
molecule scatters differently based on its
particular structure and bonds. Ana-
lytical chemists use Raman to probe
molecular structure, concentration and
reactions, but the potential applications
are endless, and usually only limited by
cost, convenience and the knowledge
required to analyze spectra.
14 | OPN Optics & Photonics News
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