Characterization techniques - Micromixers Fundamentals Design and Fabrication - page 302

FIGURE 8.6

Schematic concept of a real-time, direct view scanning optical microscope.

8.1.4 Acquisition and processing of digital images
8.1.4.1 Image sensors
Characterization of micromixers requires accurate measurement of the intensity field, which is
equivalent to the concentration field of a fluorescent dye. Besides the requirements for the light
source and the optical system discussed in the previous sections, the optical detector should also
offer good properties, such as linearity, dynamic range, and sensitivity. Fluorescence imaging is
often the main task in many experiments related to micromixers. Because fluorescence signals
coming from fluorescence microscope and confocal microscope are low, high sensitivity and low
noise level are the key parameters for selecting the optical detector for characterization experiments
of micromixers.
Currently, two types of high-resolution solid-state imaging devices are commercially available:
charge-coupled devices (CCDs) and complementary metal oxide semiconductor (CMOS) image
sensors. Both imaging sensors were developed between the early and late 1970s, but CMOS sensors
have only gained attention since the early 1990s due to advances in CMOS technology. Both CCD and
CMOS sensors detect light based on the photoelectric effect where photons interact with silicon to
move electrons from the valence band into the conduction band. These electrons are called photo-
electrons. The number of photoelectrons generated is a function of the wavelength and the light
intensity. Photoelectrons are collected in a potential well until the exposure period is completed. These
charges are then either converted into a voltage (in a CMOS sensor) or transferred to a register (in
a CCD sensor). The sensor's pixel is a photodiode. The pixels are typically organized in an orthogonal
matrix ranging from 128 1024. Both CCD and CMOS sensors

are black-and-white devices, responding only to the light intensity and not to the color of the light.

128 pixels to a more common 1280