3.3.5 Readout (Scientific Notation)
The display reads the total dose in scientific notation
(see section 3.2.5 for an explanation). When integrating, the
readout is a summation of light over a period of time. The
units of readout become the product of the Illuminance or
Irradiance times time. As an example, suppose you were
integrating to determine the exposure for UV curing. The
detector would be reading instantaneous irradiance in units
of Watts per Square centimeter (W/cm
2
), but the exposure
would be in
‘
Watt seconds per square centimeter
’
(W*sec/
cm
2
), and since a Watt second is equal to a Joule it would be
in Joules per square centimeter(J/cm
2
). If you are dealing
with small doses, the answer may be expressed in millijoules
per square centimeter(mJ/c m
2
), by multiplying the answer
by 1000, and similarly in microjoules per square centimeter
(µ J/cm
2
) by multiplying by 1,000,000. For example if the
instrument displays
‘1.234
e
-2
J/cm
2
’
, you can multiply by
1000 to get 12.34 mJ/cm
2
as your answer.
3.3.6 Darkroom Integration
When integrating in the dark, you must be able to easily find
two buttons. The first is the
‘INT’ which can easily be found by
placing the palm of your hand on the rig ht side of the instrument
and feeling over for the second button from the right with your
thumb. In a
similar manner, you will locate the ‘HOLD’ button
to stop the integration. This is the easiest to find since it is the
first button in from the right si
de. Once ‘HOLD’ has been
pressed the lights may be turned on to make the readout
without changing the reading. It is also possible to operate
the instrument with a remote cable from another room, and
to print the answers on a small printer, or by sending the data to
a computer over the RS232C or USB interface (see sections 4.
& 6. for more on this).
3.4 Bias Selection
As mentioned in section 2.1 (front panel controls), there
is a bias button labeled
‘5
V
BIAS’
. This raises the input
node of the input amplifier up to +5 volts which, in turn,
reverse biases a silicon detector with 5 volts. If it is a
vacuum photodiode, it adds another 5 volt bias to the 9 volts
that is already applied to the cathode of the phototube,
making the total reverse bias of 14 volts. See the following
sections for more details.
3.4.1 Vacuum Photodiode Bias
Vacuum Photodiodes always require a bias voltage to
operate properly. All of the International Light Vacuum
Photodiodes will operate properly with a bias between 9 and
75 volts. We recommend operating them at the 14 volts
provided with the bias ON, so the readings correlate with
our calibration lab, and with previous instruments that
applied 12 and 15 volts. Another advantage to the larger
voltage is the improvement in peak current capability, for
maximizing the dynamic range of flash measurements. The
difference in D.C. readings between 9 and 15 volts bias, is
only about 1%, but to gain that extra accuracy, use it with
the
‘5
V BIAS
’
light on.
14
3.4.2 Flash Bias
Detector bias has the most significant effect on system
performance in flash measurements. Semiconducto r devices
gain in three ways when placed in a reverse biased
condition, as opposed to the photovoltaic mode (zero bias).
The first improvement is to reduce the junction capacitance,
which makes the instantaneous photo current get to the
photometer faster, rather than being delayed as a junction
charge. The second improvement is the elimination of
junction saturation. This happens because the instantaneous
photocurrent produces an I*R drop across the top
transparent electrode, which in turn allows the junction
voltage to become forward biased, causing junction
saturation. The third advantage has to do with the method
used to measure the charge from a flash. In order to make
the radiometer operate at speeds faster than the clock time of
the computer, it is necessary to temporarily store the flash
charge, so the charge digitizing circuitry can withdraw this
charge and measure it. This temporary storage can handle
2.00e-6 coulombs, so the instrument maintains 6 decades of
dynamic range even down to nanosecond speeds. Below 100
microseconds the limitation is due to the peak current of the
detector, not the instrument. Our SED033 is designed to
handle more than 2.0 milliamps, to still provide 3 decades of
useful range at 1 microsecond. By combining this basic
capability along with a neutral density attenuator, one can
make fast measurements, spanning four or five decades.
Anything below one microsecond is not recommended, since
a multitude of detector speed limitations, including
impedance matching etc., cause inaccuracy.
3.4.3 Low Level Bias
When making low light level measurements, it is
important to minimize the detector leakage current. With a
semiconductor, this can be done by using it in the
photovoltaic mode, with NO BIAS on it at all. This also
minimizes the temperature effects on the detector due to
changes in the internal shunt resistance. Vacuum
Photodiodes must always have a bias on them, so they
should be operated with the
‘
5 V BIAS
’
light ON. As part of
the detector design, we have minimized detector leakage
through the coaxial cable by keeping the current carrying
lead at the same potential as the shield lead, even in the
reverse bias mode. Vacuum Photodiodes have a good low
light level attribute, the absence of 1/f noise. This low
frequency (thermal) noise, can be a serious problem for D.C.
measurement s when using semiconductors. The vacuum
photodiode noise advantage tends to compensate for the
higher semiconductor responsivity, making them continue to
be a very effective transducer, especially for the short wave
length measurement s in the UV, and for their ability to reject
the longer unwanted wavelengths.
代理美国International Light辐照计http://www.testeb.com/yiqi/ilt/zhaoduji.html 深圳市格信达科技 电话18823303057 QQ：2104028976
