Scientech S200 Vector - Manual (Page 21)

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.
Calorimeter Damage Considerations
1. Surface Absorbers
Surface absorbing calorimeters have been found to safely withstand 200 W/cm
2 . This heat input is diffused
across the thermopile surface so that the local surface temperature is acceptable. Experience has indicated
the damage threshold for a single pulse to be 1 joule/cm
2 in a 1 microsecond guassian pulse. The
recommended operating limit is illustrated in the chart on page 1, please review the energy density ratings
given.
Heat is produced in the thin pigment layer on the thermopile directly heating the thermopile. Lateral diffusion
of heat across the thermopile is quite effective so that the hot side of the thermopile does not have gross hot
spots. The structure of the thermoelectric junctions is such that each shares the heat flow in parallel and
sums the thermoelectric potential in series. Thus the response is linear with the total heat flow and
independent of heat distribution. The thermopile conductance is about 0.2 watts per degree celsius and the
maximum recommended power of 10 watts raises the average temperature of the absorbing surface about
50
°
C above ambient.
2. Volume Absorbers
See the sensor specification chart for maximum pulse energy densities. The surface temperature will rise to
100
°
C for the briefest instant at these fluency levels. The initial exponential temperature variation with
penetration would rapidly decay as the heat flows through the absorber to the thermopile surface which
remained at ambient temperature during the laser pulse.
If a CW laser beam is being absorbed, a considerably different situation occurs. The continuous supply of
laser power produces steady state temperature distribution from front to back and across the absorber. The
absorption region is small compared to the absorber thickness so the temperature drop is substantially linear
18