Confocal optical detection channel

Figure 3. Confocal optical detection channel demonstrating the concept of spatial filtering. A microscope objective lens collect the light emitted from a point light source or a single molecule. The image appears as a diffraction pattern (Airy pattern, see insert). The diameter of the pinhole placed in the image plane is such that only light from the bright central spot can pass onto the detector. Radiation from an out-of-focus light source in the sample is efficiently discriminated.

Measurements at low light levels are routinely performed with photon-counting techniques. The development of ultrasensitive optical detectors has made great progress in the last couple of years. Integrated photon-counting modules with cooled avalanche photodiodes (APD) have been available for some years [31]. These detectors can have quantum efficiencies of 50% with less than 10 dark counts per second. The light sensitive area of such a device has a diameter of about 200 (im and can serve directly as a pinhole in a confocal detection channel. 

FIGURE 45.9 Schematic of a confocal LIP detection system with source, optics, and detector shown. The optics include mirrors (M), laser line filter (LF), half-wave plate (X/2), polarizer (pol), dichroic beamsplitter (DB), microscope objective (MO), pinhole (ph), filter, and achromat lenses (achr). The source shown is an argon ion laser, and the detector is an avalanche photodiode (APD). While the electrophoresis channel shown here is in a capillary (CE), the system could be readily applied to a microchip. (Reprinted from Johnson, M.E. and Landers, J.P., Electrophoresis, 25, 3515, 2004. With permission.)... 

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