3.3.1 Integration Zero
A
‘ZERO’
state for integration is when the first
derivative of the output is zero. In other words, a non-
changing output . To get this state, the integral of any
unwanted input signal must be measured and subtracted from
the integral of the total signal.
‘Unwanted ’
can mean ambient
light or some detector dark current contribution. An example
may be the best way to shed some
‘light’
on the subject.
Suppose we are in a room with two table lamps in it, called
‘
A
’
and
‘B’. ‘
A
’
is left on so you can see what you are doing,
and
‘B’
is the sample lamp to be measured. With
‘B’
off, you
would press
‘ZERO’
so the signal dose from
‘
A
’
will be
ignored. The ILT1700 must measure this incoming signal
from
‘
A
’, whic
h will subsequently be removed from the total
integral. This requires subtracting the product of
‘
A
’
times the
integral time from the total integral produced by
‘
A
’
and
‘B’
together. Fortunately, this is all done for you by the internal
computer. Another thing that is done automatically is the
zeroing of the dose accumulation registers at the time you
press the
‘INT’
button. If you zeroed on
‘
A
’
and never
turned
‘B’
on, the accumulated dose should be essentially
zero, or equal to any small change in
‘
A
’
as compared to its
value when the
‘ZERO’
button was pressed. This change
can be positive or negative, depending upon whether
‘
A
’
got
a little brighter or dimmer, respectively. It is reasonable to
make sure your sample lamps dose is much greater than
any zeroed dose, to be sure that small changes in the
ambient will not effect the accuracy of the final result. A
good rule of thumb is to try to keep your signal
contribution at least 10 times larger than the ambient
contribution. This may require workin g in the dark,
which can be done by either remote control from another
room or by pressing the hold button while still in the dark,
and then taking the data with the lights turned on (See
section 3.3.6 for more on this).
3.3.2 Long Term Integration
Any integration longer than one second is considered
long term integration. In this state, the ILT1700 keeps up
with the incoming signals, and presents an updated readout
of the progress every 1/2 second. A proper zero should be
done prior to use of integration.
As soon as the
‘INT’
button is pressed, the integration begins. When the
‘HOLD’
button is pressed the integration ends and reads out the final
integral. If you wish to continue integration without
restarting, adding the new dose to the old dose, just press
‘INT’
again,
and ‘HOLD’
when you want to stop. If you
want to start over again from zero, then press
‘D.C.’
or
‘ZERO’
before
the ‘INT’
button. If you press
‘INT’
two
times in a row, the system will start a new integration at the
time of the second press. This can be useful for multiple
exposures, assuming the light is momentarily off to permit
time for readout or transfer to a computer or printer. A press
of
‘INT’
starts the next cycle without any other buttons
being pressed.
There is no limit to how long you can integrate. The
system will maintain perfect accuracy for over 18 years even
if the signal is the maximum permitted into the ILT1700.
There is no loss of accuracy for extended integrations, and
the auto ranging keeps track of the magnitude no matter how
large it gets.
3.3.3 Flash measurements
The procedure for making flash measurements requires
the use of three other function buttons. We start with the
‘5
V BIAS
’
button. IT IS NECESSARY to reverse bias the
detector to make the detector respond faster, as well as to be
able to handle the high peak currents without saturating and
also to provide bias compliance to an input charge circuit
which temporarily holds the charge long enough for the
precision charge measurement to be performed. In other
words the red
‘5
V BIAS
’
LED should be lit when making
any flash measurement at speeds faster than one second.
The
‘ZERO’
button is used to cancel out any unwanted
ambient contributio n that is present before and during the
flash (see previous section). Next, press the
‘INT’
button
just before the flash is to occur, or if you wish, you may use
the
‘FLASH not ’
output line (pin 9) to activate the flash
event (see section 6.1 for details). The summation registers
will be instantly zeroed and the integration will commence.
Two and a half (2.5) seconds after the flash, press the
‘HOLD’
button to freeze the final result. This extra time is
required to fully dump the charge from a temporary holding
circuit on the front end of the input system and be displayed
by the computer. This permits high speed measurements to
occur faster than the computer can keep up with the input.
This feature allows the system to handle 6 decades of high
speed charge ranges at a sacrifice of summation speed. In
fact, it can take up to 2.5 seconds to produce a final answer
to 0.1% resolution. This does not pose any difficulty in any
application that can operate in
a ‘one
-shot
’
mode. However,
if there are multiple flashes, you must use a shutter to
capture one flash, or integrate for a known number of
flashes, and divide the answer by that number. Often it is
possible to turn off the power supply to the flash lamp after
the required number of flashes has occurred. This will
permit the summation time (2.5 seconds) to occur.
3.3.4 Averaging
As mentioned above on the subject of multiple flashes,
the system can be used to find the average of many flashes.
This is done by integrating throughout many flashes. By
maintaining a count of the flashes and dividing by that
number, you will arrive at the average output from one flash.
Another form of averaging is that which occurs as the result
of a very irregular light output. An example of such a
source is an arc welding process. If you wished to find the
average ultraviolet dose the welder might receive during a
work day, you could leave the Radiometer running in the
integrate mode all day long, or you could integrate for a
reasonable length of time to determine the dose for part of a
day, then multiply by the number of daily portions
representative in the work day to find the total accumulation
13
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