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the light is picked up from a small spot in the image plane.
This is very nice for measuring the brightness of an
illuminated segment of an alpha numeric display, or for
measuring the dot brightness on the face of a CRT.
Unfortunatel y these systems are very expensive. Another
alternative is to use simple lenses to image a small portion
of a test field, onto an aperture which has a detector behind
it. This is very effective especially if the source is a
repetitive configuratio n in a production situation. A custom
set up can be made to specifically measure that one small
emitting surface. Our Radiance accessory has a 1.5 degree
field of view, with the objective lens being about one inch in
diameter. This is effective as long as the target is larger than
one inch. If you must back up from it, the target requirement
gets bigger by the distance times 2*(tan 1.25), plus some
margin for error. If you were a foot away, the target should
be at least 2 inches in diameter.
8.9.4 Uniform Receiver Sensitivity
As mentioned in section 8.3, the best receiver for
uniformity is the input port of an integrating sphere. The
uniformity we now are talking about is that which is
measured perpendicular to the optical axis, over the input
surface. For Flux measurements it is necessary to have this
uniformity, so that small errors in centering the beam do not
contribute to much of a change in the measured reading.
Our narrow beam adapter (HNK15) attachment, is designed
to accept a few millimeters of axial misalignment without
appreciable changes in the reading. This is necessary to
allow for non critical positioning of laser beam
measurements.
8.10 Temporal Response
This refers to the light time response. There are many
factors that should be considered when measuring fast light
pulses, including the need for instantaneous information, or
whether dose information is adequate. To obtain
instantaneous data, the entire electrical system, including the
final oscilloscope, must be properly designed and matched.
This includes the connecting cable, characteristic
impedances, and matching amplifiers. In most cases it is
necessary to put a preamplifier in the detector head to match
the high impedance of the detector to the coaxial cable.
When you do this, the dynamic range is limited to a few
decades at best, and auto ranging is not practical. For these
reasons we have chosen to concentrate on dose measuring,
which generally matches the ultimate goal of the light pulse,
and is compatible with the same detectors used for c.w.
measurements. When we say
‘ultimat
e
goal’
, we mean that
the energy in a pulse is generally the factor that determines
its effectiveness to perform work, and therefore is the best
figure of merit when making a measurement. Capacitance is
one of the properties of a detector that is very detrimental to
fast instantaneous measurements, but does not produce an
error when integrating, since it just tends to store the charge
for subsequent removal. By designing charge measuring
electronics into the instrument, we can rely on charge
storage in both the detector and in a capacitor on the front
end of the system. By measuring with a 5 volt reverse bias
on the detector, we now eliminate junction saturation due to
internal series resistance voltage drop in the detector, plus
we store the charge in the capacitance of the detector
(approx 4 nanofarads for SED033), the coaxial cable
(approx 0.23 nanofarads), and the 390 nanofarad capacitor
located on the front end of the amplifier. At 5 volts bias, the
system can temporarily store more than 1 microcoulomb of
charge, which is quantized in one measurement cycle (half
of a second). The lowest charge of 10 picocoulomb, will
still have a 5% resolution, providing a pulse dynamic range
of five decades. There are other limitations, such as the
maximum current output from the detector, and the intrinsic
speed of the detector itself which will be covered separately
below.
8.10.1 Low Duty Cycle (fast pulse)
The ILT1700 can measure a single fast pulse as long as
the pulse rise time is generally greater than 5 microsecond,
and the peak detector current is less than 2.0 milliamps.
These limitations refer to the SED033 detector, which
generally gives the best performance for flash
measurements. The vacuum photodiodes are very fast
devices, but the peak output current is about 1.0
microamperes, which is a limitation to high speed,
high magnitude light sources. (attenuators can be used to
decrease sensor sensitivity) If one wishes to measure a
continuous stream of pulses, the ILT1700 has a unique
capability of averaging the reading, even if the duty cycle is
extremely low. This of course is also limited by the
boundaries of the peak current, and intrinsic detector speed.
Another method for measuring low duty cycle signals, is to
integrate while counting the number of flashes. Then divide
the total integral by the number of flashes to get the average
charge in one flash. Still another technique is to integrate
for a known period of time, and then divide by the number
of seconds to get the average reading/sec. Data is
updated exactly every half second.
8.10.2 High Peak Amplitude
If you have a very bright C.W. source that would produce
more than 2 milliamps from the detector, the instrument will
read “HI” on the display. If the peak current is the result of
a
high amplitude flash, the limit usually is due to the detector
saturation. The most often used silicon detector (SED033) can
operate up to 2.0 peak milliamps before saturation, while the
vacuum photodiodes (SED240 or SED220), have a peak less
than 1.0 microamperes. In flash mode, there system may not
read “HI”
because the measurement integral does not detect the
peak. ILT offers attenuation filters (QNDS1, QNDS2, QNDS3)
for attenuation by a factor of 10, 100, and 100 respectively. To
test, simply add the QNDS fil ter, without changing the
calibration factor and the output should drop by the same
attenuation rate as that of the filter.(ie QNDS1 x 10)
You can easily determine the current levels:.
1. enter a calibration factor of 1, and select that factor, or
2. multiply the readings taken by the calibration factor being
used. Both provide the detector current in amperes.
For further applications assistance, check out our
complimentary handbook on our website.
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
