HP 5501A Operating And Service Manual page 56

2.22 Velocity of Light Compensation
The wavelength of the laser in a vacuum i s known to better than 1 part per million. However,
the wavelength in air is somewhat shorter than the vacuum wavelength because the velocity
of light (VOL) in air i s less than in a vacuum. In addition, the velocity of light in air i s not con-
stant but i s a function of air composition, temperature, and barometric pressure. I t is therefore
necessary to accurately determine
all
of these factors in order to define the wavelength of the
of the laser in air. Fortunately the composition of air is well known, and only the relative hu-
midity needs to be measured in order to define air composition. Temperature and pressure,
however, can also significantly affect the laser wavelength in air. As a general guide it can be
stated that an error in wavelength of approximately one part per million is incurred for each
error of
I0C
in ambient temperature; 2.5 mm Hg. in absolute pressure, or 30% in relative hu-
midity. For this reason the Laser Transducer System has provisions for determining baro-
metric pressure, temperature, and relative humidity either via manual input (10756A Manual
Compensator) or automatically using the 5510A Opt 010 Automatic Compensator.
Since the interferometer counts the number of wavelengths of motion traveled, the distance
i s calculated as follows:
0
distance
=
(wavelengths of motion) X
air wavelength
X (vacuum wavelength)
vacuum wavelength
Note that the air wavelength/vacuum wavelength ratio need not be known if the measure-
ment i s made in a vacuum rather than in air. Since the sole purpose of VOL compensation is
to determine the air wavelength/vacuum wavelength ratio, obviously VOL compensation i s
not required for vacuum measurements and the previous relation reduces to:
@
distance (in vacuum)
=
(wavelengths of motion) X (vacuum wavelength)
2.23 Deadpath Error Compensation
The previously defined relatior
a
is
valid only if the air wavelength does not change during
a measurement. If the air wavelength does change during a measurement, additional com-
pensation must be made for deadpath error. In simple terms, deadpath error is an error intro-
duced because of an uncompensated length of laser light between the interferometer and the
retroreflector when the machine is at its 0 position a s defined by the machine coordinate
system. I n Figure 2-29, the deadpath area of the laser measurement path i s the distance be-
tween the interferometer and the 0 position of the machine (L,). At this point the machine
coordinate system indicates that the cube corner is at position L, . Assuming there is no
motion between the interferometer and retroreflector but environmental conditions sur-
rounding the laser beam change, then the wavelength changes over the entire path (L,+L,).
Since relation contains the term "wavelengths of motion" which involves only the dis-
tance L, , it can 111dke no correction for the wavelength change over L, and the interferometer
causes a shift in the
0
position of the machine coordinate system. This
is
known a s deadpath
error and occurs whenever environmental conditions change during a measurement.
Deadpath error can be minimized in two ways:
By mechanically minimizing the distance L,. This is done by mounting the interfero-
meter a s close to the retroreflector as possible when the machine is at 0 position a s
defined by its own coordinate system.