Scientech H410 Vector - Manual (Page 32)

32
Initial Voltage Interpolation:
A method used to eliminate the tedious numerical integration task is to project the thermal decay envelope on to
the voltage axis, determine the 1/e decay time constant T, and estimate the total energy value (E):
E = (V
o
/S) x T
The change from thermal absorption to thermal transport phenomena near the peak causes difficulty in
accurately projecting the envelope on to the voltage axis introducing an error, dV
o
. Further, the determination of
the time constant T, introduces another error, dT. The total error is the sum of the two errors.
dE = (V
o
/S)dT + (T/S)dV
o
The difficulty in eliminating the potential error makes this method typically less accurate than numerical
integration, but much faster in application.
Peak Voltage Estimate:
The peak voltage method requires using an independent determination of total energy and referencing it back to
the peak voltage value, V
p
.
For a given pulse, use the numerical integration method to obtain E. Note the peak voltage, V
p
. Compute the
value, F
F = E/V
p
For the next pulse compute the total energy: E = F x V
p
The error in using this method yields: dE = FdV
p
+ V
p
dF
The accuracy of this measurement depends upon the error in the original calibration, dF, and the error in the
peak voltage dV
p
. A careful numerical integration yields a value for dF near zero. The value of dV
p
can be
minimized by maintaining the geometry of the system (i.e. beam intensity, beam profile, wavelength and
environment) during operation to be the same as during calibration. Under controlled circumstances, the peak
method accuracy usually falls between the numerical integration and initial voltage interpolation methods.
Astral
TM
Photodiode Detectors:
Scientech does not recommend that Astral photodiode detectors be used without a H410 indicator. The crystals
used in the detectors are wavelength dependent and the absorption characteristics vary from batch to batch as
they are manufactured. The H410 indicator’s software contains the correction factors necessary to get accurate
readings.
Cable Requirements:
Astral photodiode detectors are powered up by the indicators. To use an Astral photodiode detector without a
Scientech indicator you must construct a cable as described on page 30 and shown in Figure 6.
Operation of Astral  Photodiode Detectors with a Digital Volt Meter:
The detectors may be used with any digital volt meter (DVM) capable of reading 5 volts full scale only at the
632 nm wavelength. For use at any wavelength other than 632 nm you must contact Scientech to determine the
correct sensitivity for that wavelength.
A. Connect the output of the detector to the DVM.
B. Select the DC volts mode.
C. Direct the laser beam on to the absorbing surface of the detector.
D. When the display of the DVM has stabilized calculate the laser power using the formula: W = V/S
where:
W = Laser power in watts
V = Voltage reading of the DVM in volts
S = Sensitivity of the calorimeter from page 2.