PASCO PS-2148 IR Sensor - Manual (Page 3)

Model No. PS-2148 Background
3 ®
Theory of Operation
Consider a blackbody of temperature T
s
, whose shape can be
approximated as an infinite plane, and a flat detector surface parallel to
source with area A
d
.
Because radiation from an infinite plane propagates as a plane wave,
the power flow from the source to the detector (P
sd
) equals the radiation
emitted by a part of the source whose area is equal to the detector’s
area, regardless of the distance between the source and detector.
P
sd
= A
d
σT
s
4
The detector itself also radiates in accordance to the Stefan-Boltzmann
law. If the detector’s temperature is T
d
, then power radiating out of the
detector is
P
d
= A
d
σT
d
4
Therefore, the net power absorbed by the detector is
P = A
d
σ(T
s
4 − T
d
4
)
The net intensity, which is what the sensor measures, is the net power
divided by the detector area.
The net power that flows onto the active detector area by radiation
(P
sd
− P
d
) flows out of the detector by conduction to other parts of the
sensor. A proportion of that power is conducted through the thermopile,
which sets up a temperature different (
∆T) across the thermopile. The
thermopile produces a voltage (V) that is proportional to
∆T.
If the sensor is warmer than the target source, then net power flows out
of the sensor, and
∆T and V are negative.
The net radiated power (P) is proportional the power flow through the
thermopile, which is proportional to
∆T, which, in turn, is proportional
to V; therefore, V is proportional to P:
V = R P
The constant, R , is known as the responsivity of the detector. For the
PS-2148 sensor, R is about 31 V/W.
The sensor amplifies the voltage produced by the thermopile and
converts it into a digital signal. A microprocessor in the sensor
calculates intensity, which is incident power divided by the area of the
detector (2.25 mm
2
). The thermopile voltage and intensity data are sent
digitally to the PASPORT interface or computer.
Pair of
Thermocouples
Thermopile
Cold
Hot
T D
V
Cold
Hot
V
T D
T
s
V Thermopile
T
d
Source
P
sd
P
d
DT
What is a thermopile?
A thermopile is a series
of thermocouples
A thermocouple is the
junction of two different
metals. When two
thermocouples are
connected in series, and
are at different
temperatures, a voltage
proportional to the
temperature difference
occurs between them.
This voltage is usually
very small. In a
thermopile many
thermocouples are
connected in series (as
shown below) to produce
a larger voltage. The
thermopile in the
PS-2148 consists of 120
junctions etched in
silicon.