In physics,
spectrophotometry is the
quantifiable study of
electromagnetic spectra. It
is more specific than the
general term electromagnetic
spectroscopy in that
spectrophotometry deals with
visible light,
near-ultraviolet, and
near-infrared. Also, the
term does not cover
time-resolved spectroscopic
techniques.
Spectrophotometry
involves the use of a
spectrophotometer. A
spectrophotometer is a
photometer (a device for
measuring light intensity)
that can measure intensity
as a function of the color,
or more specifically, the
wavelength of light. There
are many kinds of
spectrophotometers. Among
the most important
distinctions used to
classify them are the
wavelengths they work with,
the measurement techniques
they use, how they acquire a
spectrum, and the sources of
intensity variation they are
designed to measure. Other
important features of
spectrophotometers include
the spectral bandwidth and
linear range.
Perhaps the most common
application of
spectrophotometers is the
measurement of light
absorption, but they can be
designed to measure diffuse
or specular reflectance.
Strictly, even the emission
half of a luminescence
instrument is a kind of
spectrophotometer.
The use of
spectrophotometers is not
limited to studies in
physics. They are also
commonly used in other
scientific fields such as
chemistry, biochemistry, and
molecular biology.
They are widely used
in many industries including
printing and forensic
examination.
Design
There are two major
classes of
spectrophotometers; single
beam and double beam. A
double beam
spectrophotometer measures
the ratio of the light
intensity on two different
light paths, and a single
beam spectrophotometer
measures the absolute light
intensity. Although ratio
measurements are easier, and
generally more stable,
single beam instruments have
advantages; for instance,
they can have a larger
dynamic range, and they can
be more compact.
Historically,
spectrophotometers use a
monochromator to analyze the
spectrum, but there are also
spectrophotometers that use
arrays of photosensors.
Especially for infrared
spectrophotometers, there
are spectrophotometers that
use a Fourier transform
technique to acquire the
spectral information quicker
in a technique called
Fourier Transform InfraRed...
The spectrophotometer
quantitatively measures the
fraction of light that
passes through a given
solution. In a
spectrophotometer, a light
from the lamp is guided
through a monochromator,
which picks light of one
particular wavelength out of
the continuous spectrum.
This light passes through
the sample that is being
measured. After the sample,
the intensity of the
remaining light is measured
with a photodiode or other
light sensor, and the
transmittance for this
wavelength is then
calculated.
In short,
the sequence
of events in
a
spectrophotometer
is as
follows:
- The
light
source
shines
through
the
sample.
- The
sample
absorbs
light.
- The
detector
detects
how much
light
the
sample
has
absorbed.
- The
detector
then
converts
how much
light
the
sample
absorbed
into a
number.
- The
numbers
are
either
plotted
straight
away, or
are
transmitted
to a
computer
to be
further
manipulated
(e.g.
curve
smoothing,
baseline
correction)
