The photomultiplier tube generally has a photocathode in either
a side-on or a head-on configuration. The side-on type receives
incident light through the side of the glass bulb, while in
the head-on type, it is received through the end of the glass bulb.
In general, the side-on type photomultiplier tube is relatively low
priced and widely used for spectrophotometers and general photometric
systems. Most of the side-on types employ an opaque
photocathode (reflection-mode photocathode) and a circularcage
structure electron multiplier which has good sensitivity and
high amplification at a relatively low supply voltage.
The head-on type (or the end-on type) has a semitransparent
photocathode (transmission-mode photocathode) deposited
upon the inner surface of the entrance window. The head-on
type provides better spatial uniformity (see page 7) than the
side-on type having a reflection-mode photocathode. Other features
of head-on types include a choice of photosensitive areas
from tens of square millimeters to hundreds of square centimeters