Sensor - The Electronic Nose Artificial Olfaction Technology - page 54

2.5 Optical Sensor

Optical methods are best-recognized techniques for sensing biochemical analytes.
Instrumentation for optical measurements generally consists of a light source, a
number of optical components to generate a light beam with specific character-
istics and to direct this light to some modulating agent, and a photo-detector for
processing the optical signal. The central part of an optical sensor is the modu-
lating component. Figure 2.14 displays the basic principle of optical sensor.
Optical displacement sensors work on the basic principle that the intensity of light
decreases with distance. So if the source and detector are fixed, the amount of light
reflected from a moving surface will depend on the distance of the moving surface
from the fixed ones. Measurement using this principle requires proper calibration
since the amount of light received depends upon the reflectivity of the surface,
intensity of the source, etc. Yet it can provide a simple method for displacement
measurement. Optical fibers are often used to transmit light to and from the mea-
suring zone.
Types of optical sensors:

• Intrinsic
• Extrinsic
• Distributed

Optical sensors are usually based on optical fibers or on planar waveguides.
Generally, there are three distinctive methods for quantitative optical sensing at
surfaces:

1. The analyte directly affects the optical properties of a waveguide, such as
evanescent waves (electromagnetic waves generated in the medium outside the
optical waveguide when light is reflected from within) or surface plasmons
(resonances induced by an evanescent wave in a thin film deposited on a
waveguide surface).
2. An optical fiber is used as a plain transducer to guide light to a remote sample
and return light from the sample to the detection system. Changes in the
intrinsic optical properties of the medium itself are sensed by an external
spectrophotometer.

Fig. 2.14 Optical sensor basic principle