O'Reilly logo

Optical Sources, Detectors, and Systems by Robert H. Kingston

Stay ahead with the world's most comprehensive technology and business learning platform.

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, tutorials, and more.

Start Free Trial

No credit card required

178
Systems 11.: Imaging
image devices, the CCD or charge-coupled device imager. In all of
these approaches, the sensitivity is mainly determined by the read-out
technique and the associated noise.
8.1 Photoemissive Image Tubes
Vacuum tubes have historically been the most important imaging
devices until the recent advances in the CCD. These tubes use either
photoemissive or photoconductive area detectors. We first consider
photoemissive devices and as a representative example, the image
orthicon, one of the more popular and versatile forms. In this, as in all
image tubes, an electron beam is "raster" scanned over a read-out area as
shown in Figure 8.1. In the case of the image orthicon, the scanned
surface is that of a semi-insulating sheet as shown in Figure 8.2. The
optical image is formed on the negative transparent photocathode and
the photoelectrons are accelerated through the mesh striking the read-
out plane. Secondary electrons, 5-5, are collected by the slightly
positive mesh leaving a positive charge distribution of (5 -
1)
charges
per incident electron on the layer, which is a reproduction of the optical
image. The scanned electron beam operates at a low enough voltage
that the secondary emission ratio is less than one. The beam may then
deposit negative charge until the positive charge is cancelled. Any
further deposition would cause the target to become more negative than
Figure 8.1 Raster scan motion of electron beam. During the thinner return path, the
beam moves much faster and the beam current is usually zero.
8.1 Photoemissive Image Tubes
179
Transparent
Photocathode
Semi-insulator
Electron multiplier
Figure 8.2 Image orthicon, simplified diagram.
the electron-beam cathode and thus repel the beam. The remainder of
the beam returns to an electron multiplier similar to that in a photo-
multiplier tube. The
reduction
in return beam current is then a measure
of the incident optical signal on the photocathode or "retina.'' If we
define a "pixel" or picture element width as the product of the electron
beam horizontal velocity and the electrical sampling time r, then one
photon in a pixel results in a (S - 1) ^4
electron
reduction in the return
beam current. If the expected peak signal is 40,000
photoelectrons/pixel,
then the incident beam current must be at least
160,000
electrons per
sampling time. For low light levels, the input to the electron multiplier
thus has an rms fluctuation of v^ = 400 produced by the shot noise in
the electron beam from the thermionic cathode. With this very simple
model of the noise, the number of incident photoelectrons required for a
(SIN)y = 1 becomes 400/4 = 100. This quantity is sometimes defined
as the noise equivalent electron count or NEE, The noise equivalent
photon count, NEO is then NEE/tj, which for
20%
quantun\ efficiency
becomes 500 for this case. The dynamic range at the specified beam
current is then
40,000/100
= 400, A detailed treatment should take into
account the noise in the secondary emission process for both the
photoelectrons and beam electrons, as well as electron losses at the mesh
electrodes. Such calculations and an excellent and extensive review of
image tubes can be found in
(Csorba,
1985), who defines the (S/N) as the

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, interactive tutorials, and more.

Start Free Trial

No credit card required