Biophotonics_Concepts_to_Applications

activating M4. The resulting output current is related to the capacitor voltage, which
is a measure of the light intensity.
The CMOS device architecture and operating process have several advantages.
The operating voltages required for a CMOS array and the power consumption are
lower than for a CCD array. Digital timing and control signals along with other
digital image processing functions can be located directly on the chip. Because each
pixel is individually selectable, functions such as random access of specific pixels,
data windowing (taking a small subset of a large data set), and having a variable
frame rate within a chosen set of pixels can all be supported. Having additional
circuitry within a pixel element and the operation of this circuitry increases the
complexity of the chip, but allows cameras containing CMOS arrays to be smaller
than cameras that use CCD arrays. However, devices containing CMOS arrays tend
to have lower image quality because more of the chip area is covered with circuitry.

5.5 Photomultiplier Tubes.

Aphotomultiplier tube(PMT) is an optical detection device that uses an external
photoelectric effect, in contrast to semiconductor-based photodiodes that use an
internal photoelectric effect. In an external photoelectric process, when light strikes
a semiconductor-coated metal or glass plate that is located in a vacuum, electrons
(referred to asphotoelectrons) are emitted from the material surface into the vac-
uum. Because PMTs have fast response speeds and superior sensitivity, they are
used widely in biophotonics research, medical equipment, and analytical instru-
ments, particularly when very low light intensities need to be measured.
As Fig.5.10shows, a generic PMT consists of a photocathode (also called a
photoemissive cathode), a series of dynodes, and an anode, all of which are
encapsulated in an evacuated glass enclosure. Adynodeis an electrode in a vacuum
tube that serves as an electron multiplier through a secondary emission process,
which occurs when primary electrons of sufficient energy hit the dynode material.

Primary

photoelectron Dynodes

Secondary

electrons

Anode

Incoming

photon

Photoemissive cathode

PMT housing

Fig. 5.10 Basic structure of a
PMT with a reflection mode
photocathode

136 5 Fundamentals of Optical Detectors