Laser Equipment for Alignment Testing (Metrology)

7.6.
The use of laser permits alignment test to be carried over long distances compared to
other conventional equipment like alignment telescope. The advantage of laser equipment is
that it produces a real straight line that can be seen in space but the telescope provides only
an imaginary line. Laser equipment for alignment testing is particularly suitable in applica-
tions like aircraft production, ship building, etc. where a number of Components, spaced
relatively long distances apart, have to be checked to a predetermined straight line. Other uses
of laser equipment are testing of flatness of machined surfaces, checking squareness with the
help of optical square, etc.
Laser equipment for alignment testing basically consists of a laser tube which produces
a cylindrical beam of laster of about 10 mm diameter and an autoreflector (centring detector)
which locates the centre of the cylindrical laser beam at any distance with a high degree of
accuracy. Laser tube consists of a helium-neon plasma tube in a heavy aluminium co-axial
cylindrical housing. Arrangements are made to support this housing on a stand. Sophisticated
arrangements are carried out inside the tube to reduce transmission loss and a flat mirror is
provided to provide high reflectivity. The laser beam comes out of the housing exactly from its
centre (within 0.02 mm) and parallel to the housing within 10″ of arc. The alignment stability
of such devices is of the order of 0.2″ of arc per hour.
Autoreflector consists of a detector head and a readout unit. The detector incorporates
a number of photocells arranged to compare the intensity of laser beam in each half—vertically
and horizontally. The panel meter (having two indicators) immediately detects any offset
between the centroid of the beam and centre of the centering detector. The centering detector
is housed on a stand which has two adjustments to translate the detector in its two orthogonal
measuring directions perpendicular to the laser beam. The device thus can detect the align-
ment of flat surfaces perpendicular to a reference line of sight. The laser autoreflector projects
the collimated laser beam to a flat target mirror and the perfect alignment of the mirror causes
the beam to return directly superimposed on the projected beam. Misalignment of the mirror
causes an angular offset of the return beam and it is detected by the two-axis angular position
sensitive detector and data displayed on two panel meters.
Thus a reference straight line is established first and then using a second centering
head, any number of intermediate parts can be aligned. Using optical square or pentaprism,
which reflects the laser beam by exactly 90° within 1″ of arc it is possible to measure squareness
of surface.
The surface plate can be checked for deviation from true planes by moving the laser to
a number of positions and establishing as many intersecting lines as required. The level of
surface can also be checked by using a levelling mirror.