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Principles of Instrumental Analysis, 6th ed. Chapter 13
 
 6
Phototubes are generally more sensitive and have a greater wavelength range. Photocells 
are in general simpler, cheaper and more rugged. Photocells do not require external 
power supplies. 
(d) A photodiode consists of a photo-sensitive pn-junction diode that is normally 
reverse-biased. An incident beam of photons causes a photocurrent proportional to the 
photon flux. A photomultiplier tube is a vacuum tube consisting of a photoemissive 
cathode, a series of intermediate electrodes called dynodes, and a collection anode. Each 
photoelectron emitted by the photocathode is accelerated in the electric field to the first 
positively charged dynode where it can produce several secondary electrons. These are, 
in turn, attracted to the next positively charge dynode to give rise to multiple electrons. 
The result is a cascade multiplication of 106 or more electrons per emitted photoelectron. 
Photomultipliers are more sensitive than photodiodes, but require a high voltage power 
supply compared to the low voltage supplies required by photodiodes. Photomultipliers 
are larger and require extensive shielding. Photodiodes are better suited for small, 
portable instruments because of their size and ruggedness. 
(e) Both types of spectrophotometers split the beam into two portions. One travels 
through the reference cell and one through the sample cell. With the double-beam-in-
space arrangement, both beams travel at the same time through the two cells. They then 
strike two separate photodetectors where the signals are processed to produce the 
absorbance. With the double-beam-in-time arrangement, the two beams travel at 
different times through the cells. They are later recombined to strike one photodetector at 
different times. The double-beam-in-time arrangement is a little more complicated