THE INPUT OPTICS
The lambertian (cosine) response of the sensor head
is desirable for many measurement applications,
especially those where the angle from the source to
the detector is variable or those situations where the
angle from the source is ''extended,'' such as in the
measurement of a fluorescent lamp at distances
comparable to or shorter than its length. In the latter
case, the extended source provides radiation from
many angles, all of which must be properly
''weighted'' as to their effectiveness on the plane
represented by the sensor surface.
In actual practice, it is difficult to make a sensor
conform to the ideal response over the entire solid
angle of 2
TT steradians. The sensor units of the
AccuMAX meter minimize this problem by being
outfitted with optimal transmission diffusing materials
for various spectral regions. These diffusion
materials are mounted close to the surface of the
sensors so that the oblique rays are not obstructed.
The spectral range is selected by adding an
appropriate UV interference filter within the optical
stack before the sensor cell assembly.
THE SENSOR CELL
Photovoltaic Operation
When a p-n junction is operated with no externally
applied voltage, it is considered to be operating in
the photovoltaic mode. Under this zero applied
voltage condition and low levels of incident light, the
p-n junction will generate a current proportional to
the light power incident on the active surface. This
photon-induced current, or photocurrent, will divide
between the diode parallel dynamic resistance and
the parallel load resistance. The dynamic resistance
is normally a high value and is an inverse exponential
function of forward voltage. The direction of
current flow will produce a voltage across the diode
that opposes the band-gap potential of the
photodiode junction, thus forward biasing it. As a
result, the value of the diode dynamic resistance (R
d
)
drops exponentially as the irradiance increases and
the photogenerated voltage is a quasilogarithmic
function of diode irradiance when the external load
resistance is considered. Another major
disadvantage is that R
d
typically has a wide spread
of values over different production batches.
One way of achieving sufficiently low load resistance
and eliminating the effect of the diode parallel
resistance is to feed the photocurrent into the virtual
ground of an operational amplifier.
The output voltage is the result of the photocurrent
being driven by the amplifier through the feedback
resistor and the input impedance R
in
= R
f
/A where A
is the open loop gain and R
f
the feedback resistor.
This circuit has a linear response and is low noise
due to the almost complete elimination of leakage
current with the zero bias. This results in a
proportional voltage being presented to the signal
conditioning section of the electronics.
LIGHT
OPTICAL STAGE,
A/D CONVERSION,
PROGRAMMABLE
CIRCUITRY
MICROPROCESOR
CONTROL
GRAPHICAL
DISPLAY
SIGNAL PROCESSING
SENSOR CIRCUIT
14
9. SPECTRAL RESPONSE (Theory of Operation)
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