Principle of Reflectometry:
The main difference between reflectance spectrometry
and conventional spectrometry can be briefly described as follows:
When using reflectance spectrometry the surface absorption of a part of the
electromagnetic spectrum is measured as the difference between the incident
light and the light reflected from the surface. Therefore to avoid
confusion, the incident light must be at an angle to the reflected light and in
measuring the surface absorption, scattering, concentration and absorption must
all be taken into consideration. This is represented by the Kubelka-Munk
equation.
Practical examples are the instruments for reading of test strips for Blood
Glucose.
Instrumentation:
The twin-beam photometer works on the compensation principle. Two neon
lamps light up alternatively and are controlled by a frequency of 50 Hz. There
will be two square wave generators, one of which will provide a
constant-amplitude output which is fed to the measurement neon. The light
produced by this neon strikes the reagent patch on the test strip and is
partially reflected onto the photocell by diffuse reflection. Square wave
generator -second, provides a variable amplitude output which is fed to the
reference neon. The light from this neon is reflected directly onto the
photocell by a mirror. The intensity of the light from the reference lamp
is regulated manually by a potentiometer. This potentiometer is scaled. During
measurement, the potentiometer is adjusted so that the discriminator registers
the same intensity for both lamps and a zero indication is seen on the
indicator. There is therefore, a unique potentiometer setting for every degree
of colour intensity that may be exhibited from the test strip and the measured
value can be read off directly from the potentiometer scale. These
instruments have been developed into a variety of models using light diodes as
light sources and advanced electronics for evaluation of the signal.