New measurement and control technology in tunnelling - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 252

Scanning technology with periodic beam deflection. Scanning is used to define a plane
in space and exploit it for surveying purposes. The beam covers a plane with high frequen-
cy. Fig. 6-7 (bottom) shows an application of scanning. The deflection of an oscillating
reflector is mostly used but the sideways movement of lenses can also be used. Crossed
scanning in a horizontal and a vertical plane also creates a point in addition to the planes
at the intersection line of the planes. Suitable detectors can then determine not only the
position of a machine as the distance from the beam but also all three coordinates.
Similarly to scanning, cylinder lenses can also be used to spread a beam in a plane. In this
case the energy is however greatly reduced so this is seldom used with optical-electrical
receivers.
6.2.4 Optical receiver devices
Photodiodes are light receivers, which are operated with applied voltage; the entry of
light increases the current flow. Photo elements are components, which are operated with-
out voltage and produce a photocurrent when light enters. Photodiodes and elements are
fitted with filters, which only admit the wavelength of the desired signal (typically the red
light of a helium-neon laser).
A number of photodiodes in a row form a photodiode array. This can be used, for example,
with a light plane emitted by a rotating laser to determine position. Photodiodes arranged
over a surface form an active target, which can detect the light of an impinging laser beam.
If the data is appropriately recorded, the contact point can be made visible on the screen or
also calculated and given as coordinates or used as a control impulse.
CCD cameras. While in the processes mentioned so far, the receiver is moved equivalent
surveying is also possible with stationary receivers. Particularly CCD (Charge Coupled
Devices) cameras are used for this purpose. The light from a (moving) source is imaged
with a lens on the approx. 1 cm² sensor, which is formed of a matrix of pixels (picture-
elements) in arrays. The arrays are arranged horizontally and consist of isolated semicon-
ductor elements and electrodes. Through changing voltage to the electrodes adjacent to the
semiconductors, the electrons liberated by incoming light are transferred vertically into
arrayed storage sections, which are not exposed to light.
An appropriate voltage change then transfers the charges horizontally in rows to an out-
put section, where they are amplified and outputted as standardised voltage-modulated
video output. The sequence of this image sampling process corresponds to the scanning of
tube cameras and television screens. The refresh rates range from 50 images per second
(television-compatible) to 300 images per second for special applications. The resolution
of CCD cameras can exceed that of tube cameras, but a CCD camera cannot compete re-
garding grey-scale and light sensitivity. While the functioning of tube cameras can result
in distorted images, an image from a CCD camera is geometrically stable due to the fixed
arrangement of the sensors on the silicon base material, which makes the principle useful
for surveying applications.
The data can be fed to a computer, which can calculate the light areas from the coordinates
and brightness values of the illuminated pixels. In addition to the CCD camera, there are
other types of semiconductor cameras, which in principle could give comparable results
in surveying.