Optical Tomography (OT)

Optical tomography involves projecting a beam of light through a medium from one boundary point and detecting the level of light received at another boundary point (Abdul Rahim, 1996). This optical system can be designed by using a transmitter-receiver pair such as light emitting diode (LED) and photo detector. In general, infra-red and visible lights are used as transmitter in optical sensor. It is an attractive method since it is conceptually straightforward and relatively inexpensive, has a better dynamic response and can be more portable for routine use in process plant than other radiation-based tomographic techniques. The optical sensor provides a sufficiently wide bandwidth that enables measurement to be performed on high-speed flowing particles (Pang, 2004).

The main function of LED is to generate a light in the form of collimated beam that will propagate through the material (transparent), attenuated and received by sensing photo detectors. The multiplexed sensor generates analogue voltage signal (based on the light signal received by sensing photo detector) shows the amount of attenuation in the path of the beam caused by the flow regime. Usually, several groups of transmitter and receiver pairs are employed to obtain image in better resolution and minimize the aliasing that occurs when two particles intercept the same view.

Optical fibres exhibit high linearity when used to measure solid flow rates and show good agreement between predicted and measured values for the spatial filtering effect (Pang et al., 2004). This result demonstrates the suitability of low cost optical fibre sensors for monitoring flow of materials. Optical methods find a very limited application for fluid flow measurement in industrial pipelines due to the problems of the optical surfaces. However, there are useful research tools, especially when one is using lasers, because the parameters defining the collimation of the detected field, spatial bandwidth, etc. can be precisely specified (Chan and Abdul Rahim, 2002). The major problems associated to optical sensors are in obtaining better resolution caused by limitation of space for sensor placement whereby sensor cannot be arranged too close to each other to avoid reflection.