Thermal modeling term. For conductors
between nodes which are circular sections, the equation shown below should
A thermal modeling term. The value of
a radiation conductor is input in units of energy per unit time per degree**4.
It is be computed as:
G = A * e(eff) * F(i-j) * s
G = A * F(i-j) * s
The emittance e, is a measure of how
well a body can radiate energy as compared with a black body. Emittance
is the ratio of total emissive power of a real surface at temperature T
to the total emissive power of a black surface at the same temperature.
The emittance of surfaces is a function of several things including the
material, surface condition, and temperature. The emittance may be altered
by polishing, roughing, painting, etc.
||value of the conductor
||area of the surface i
||Stefan-Boltzmann Constant (energy/length2-time-deg4)
||black body view factor from surface i to j (dimensionless)
||gray body view factor from surface i to j (area)
The view factor F(i-j) is a function of
the geometry of the system only. Many computer programs have been developed
to compute the view factors between complex geometry's; however view factors
between some surfaces with simple geometry's can be hand calculated. The
methods and equations are found in several heat transfer texts.
The gray body view factor F(i-j)
is the product of the geometric shape factor F(i-j) and a factor which
allows for departures from black body conditions (i.e. reflections). For
example, for two parallel flat plates:
F(1-2) = F(2-1) = 1
F(1-2) = [ 1 / ( 1/e1 + 1/e2
-1) ] x F(1-2)
The effective emittance e* between two
surfaces may be used to compute the gray body view factor with the following
F(i-j) = e* x F(i-j)
The error induced by the use of e* is
the result of neglecting secondary reflections from surfaces other than
the two for which the effective emittance was determined.
Reimaging Lens Design
There are two types of lens designs currently in
use with modern FPA systems: Reimaging and Non-Reimaging. A Reimaging lens
is one that has the image in focus at two points within the optical path.
One point is on the detector (as with all lenses) and the second point
is in the middle of the lens at a point called a intermediate focal plane.
This point in the middle of the lens, where the image is refocused, is
used for placing a device in the optical path which will capture energy
from objects outside of the normal field of view (referred to as off-axis
The device that is placed at the intermediate focal
plane is called a Field Stop. The field stop has an opening in it which
corresponds to the field of view of the lens. This is an important feature,
since without this capability imaging and measurement data can be corrupted
by hot or cold objects that reside outside the field of view of a camera's
P/PM users who are using IR FPA cameras for measurement
purposes in industrial environments should be aware of this design factor.
Systems with Reimaging lenses can be used in environments where there are
a variety of hot and cold objects around the object that is being measured.
Systems that do not have this type of lens design can be subject to measurement
errors as a result of off-axis stray energy falling on the FPA detector.
A relationship between a thermistor's
resistance and the temperature.
The resistance to the flow of heat.
Resistance = 1 / Conductance