Luna 4600 - Calibration Files; Conditions that Require Calibration; Consideration Prior to Calibrating

36 Chapter 4
Performing Measurements
4
Calibration Files
The most recent calibration is stored in a file which is loaded by the control software
upon start-up. If the software is unable to locate a calibration file, a dialog box will
appear to alert the user that a calibration must be performed before a measurement
can take place. In this event, the Scan button will be unavailable (gray) until a
calibration has been performed.
Conditions that Require Calibration
In a typical laboratory or manufacturing environment, calibration should be
performed at least once every 24 hours. The need to recalibrate is largely determined
by the surrounding environmental conditions. If the temperature is fluctuating
rapidly, it may be necessary to calibrate more than every 24 hours. If the surrounding
temperature is very stable, a longer interval may suffice.
Calibration should be performed whenever the surrounding temperature has changed
by more than ±5 degrees C. And as already mentioned, the OBR must be calibrated
after an alignment.
In certain cases the control software will detect that the internal optics of the
instrument have drifted, and it will alert the user via the System Status Bar indicator
. When this occurs performing a calibration will typically suffice.
Consideration Prior to Calibrating
The maximum spatial resolution that can be achieved with the OBR is related to
the wavelength range of the measurement according to
λ1 λ2
=
---------------
,
∆z
n eff ∆λ
where
∆z is the spatial resolution, n
ef f
is the effective index of refraction of the
device under test,
λ
1
and λ
2
are the start and end wavelengths of the scan, and
∆λ = λ
1
– λ
2
. Thus, calibration over larger wavelength ranges will allow for the
highest spatial resolution
measurements.