Theory Of Operation - Spectro-UV AccuPRO Serie Manual Del Operador

9. THEORY OF OPERATION

LIGHT
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 2TT steradians. The sensor units of the
AccuPRO 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.
SENSOR
OPTICAL STAGE,
A/D CONVERSION,
PROGRAMMABLE
CIRCUITRY
10
SIGNAL PROCESSING
CIRCUIT
MICROPROCESOR
CONTROL
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 (Rd) drops
exponentially as the irradiance increases and the
photo generated voltage is a quasilogarithmic function
of diode irradiance when the external load resistance
is considered. Another major disadvantage is that Rd
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 Rin = Rf/A where A is the open loop gain
and Rf 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.
GRAPHICAL
DISPLAY