Other detector technologies
have also made a tremendous impact
on optical spectrometers. Traditional
technologies such as CCD and CMOS cannot
extend beyond about 1,050 nm. Better manufacturing
processes of indium gallium arsenide (InGaAs)-based detectors have led to
significant cost reduction in NIR spectroscopy, as well as SWIR up to 2,200
nm. Mercury cadmium telluride (MCT) can further extend the spectral
range to 2,500 nm, allowing identification of materials which were traditionally difficult to analyze using visible spectroscopy.
The introduction of 2-D area detector-based spectrometers for
non-laboratory use has also greatly improved the sensitivity of portable
and industrial systems. A 12-mm slit for example, oriented along the
height of the detector by binning the columns of the pixels vertically, can
be used in some systems to collect signal, improving the SNR by a factor
of almost five as compared to the linear detector arrays, even if everything else is identical. This stems from the fact that SNR improves by a
square root of available measurement elements (in this case 0.5-mm-high
detector rows). A high-performance optical spectrometer can now reach
an SNR of well above 100,000:1 in the same acquisition time as typical
systems achieving an SNR of 1000:1. Such a spectrometer can therefore
detect materials which have 100 times smaller concentrations, or take
1/100th of the time to acquire identical spectrum as its less capable cousin.
Large-slit 2-D area detector-based spectrometers can also be used as
multichannel systems or hyperspectral imagers with high spectral reso-
lution of hundreds of color bands instead of traditionally tens of bands
available with other imaging technologies. It’s now possible to mount a
slit-based system above a fast-moving conveyor belt, or even on an air-
plane to inspect multiple objects at the same time. Thus, in any applica-
tion which requires chemical differentiation of substances, a single system
can be used continuously to inspect the entire process. This is different
from the traditional use of spectrometers as point-measuring devices only.
The final development which will continue to impact the growth of
optical spectroscopy is the constant evolution of processing algorithms
used for spectral analysis. Chemometrics allows the transformation of
large amounts of spectral information obtained from spectrometers into
information used for decision making, such as actual chemical composi-
tion or other properties of interest. Methods such as principal component
analysis (PCA) and partial least square (PLS) regression are now sup-
plemented with new methodologies, such as genetic algorithms, wavelet
analysis and even sparse data methods for spectral pattern recognition.
President and CEO
P&P Optica Inc.
Spectrometers are no longer confined to laboratories;
the decrease in the size of optical spectrometers has
resulted in producing smaller, robust, portable sys-
tems including some handheld units.