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Glossary R through Z and #

Select the first letter of the word from the list to jump to appropriate section of the glossary. If the term you are looking for starts with a digit or symbol, choose the '#' link.
Volume 1 ABCD EFGH	Volume 2 IJKLM NOPQ	Volume 3 R S T U V W X Y Z #

- R -

Radial Conductors
Thermal modeling term. For conductors between nodes which are circular sections, the equation shown below should be used.

Radiation Conductors
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
or
G = A * F(i-j) * s
where:

G value of the conductor

A area of the surface i

e(eff) emittance (dimensionless)

s Stefan-Boltzmann Constant (energy/length2-time-deg4)

F(i-j) black body view factor from surface i to j (dimensionless)

F(i-j) gray body view factor from surface i to j (area)

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.

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 equation:
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 stray radiation).

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 lens.

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.

Resistance Temperature Characteristic:
A relationship between a thermistor's resistance and the temperature.

Resistance (thermal):
The resistance to the flow of heat.
Resistance = 1 / Conductance

- S -

Series Conductors
Thermal modeling term. Series conduction paths between nodes may be combined to create one conductor value by the following equation:

G(tot) = 1 / (1/G1 + 1/G2 +...+1/Gn) = R1 + R2 + ... + Rn

Second Surface Mirror:

The metal deposit provides the absorptance property. Silver and aluminum are the most popular metals and are often used on 2.0 and 5.0 (50 and 125 microns) FEP, also referred to as FOSR, (Flexible Optical Surface Reflector). This combination of materials obtains low absorptance over emittance ratios for low operating temperatures.

An interesting combination of materials such as 5 mil (125 microns) FEP and chromium can produce a "black" mirror.


Metal Deposits	Solar Absorptance
Silver	.06 - .09
Aluminum	.10 - .14
Copper	.20 - .30
Germanium	.50 - .70
Inconel	.60 - .70
Chromium	.70 - .80


FEP Thickness Inches	Emittance
0.0005	0.4
0.001	0.5
0.002	0.6
0.005	0.77
0.010	0.85

Set Point:
The temperature at which a controller is set to control a system.

SI:
System Internationale. The name given to the standard metric system of units.

Solar constant:
In space, normal to the sun line.


Perihelion	Aphelion	Mean
1414 W/m²	1323 W/m²	1367 W/m² 433 Btu/ft²-hr

Specific Gravity:
The ratio of mass of any material to the mass of the same volume of pure water at 4°C.

Specific Heat:
The ratio of thermal energy required to raise the temperature of a body 1° to the thermal energy required to raise an equal mass of water 1°.

Stefan-Boltzman constant:
5.6697E-08 W/m²-K⁴
5.6697E-12 W/cm²-K⁴
1.355E-12 cal/cm²-K⁴-sec
1.714E-09 Btu/ft²-hr-R⁴Super Cooling:
The cooling of a liquid below its freezing temperature without the formation of the solid phase.

Super Heating:
1. The heating of a liquid above its boiling temperature e without the formation of the gaseous phase.
2. The heating of the gaseous phase considerably above the boiling point temperature to improve the thermodynamic efficiency of a system.

- T -

Thermal Coefficient of Resistance:
The change in resistance of a semiconductor per unit change in temperature over a specific range of temperature.

Thermoelectric Cooler:
A solid state refrigerator based on the Peltier effect. Typically very small; on the order of 1x1 inch. A single stage devices can create a -37C temperature differential between hot and cold sides. A three stage devices can create a -60C temperature differential. Use ranges from beer coolers to spacecraft.

Thermal Conductivity:
The ability of a substance to conduct heat. Mathematically, the ratio of heat flow to the rate of temperature change in the particular substance.

Thermal Control:
Thermal control is the engineered approach to control the thermophysical aspects of a system. Typically, temperature is controlled but sometimes heat flow is the controlled parameter.
Reasons for thermal control include:

Preclude catastrophic thermal failure
Increase some performance characteristic
Increase reliability

Reliability concerns have taken on new importance as a result of various studies that show a strong correlation between electronic equipment failures and:

High temperatures
Thermal cycling

Thermal Gradient:
The distribution of a differential temperature through a body or across a surface.

Thermal model
First and fore most a thermal model is tool. It is used to build the knowledge base of the thermal engineer. This knowledge enable the engineer to create a design that meet the requirements.

This could be accomplished (and sometimes is) with physical models and prototypes. The time and expense of this approach is often prohibitive.

Another approach is to construct a computer based, mathematical model of a thermal system. Such a model can be run through multiple conditions or configurations in seconds or minutes. Parametric (sensitivity) studies can also be quickly performed. Expense is incurred chiefly during the initial construction of the model not the running of the model.

Thermistors
Negative temperature coefficient thermistors are used to measure temperatures below 150C. They have sensitivities of several hundred ohm per 1C. Their cost range from $1 to $20. Their various configurations range from glass beads to stainless steel probes. Drawback is the non-linear response.

Transpiration cooling

Transpiration cooling requires a liquid or gas coolant that flows through the surface of a severely heated component and exits the component from the heated surface through small pores in the surface. The coolant will both reduce the connective part of any heating and also removes heat from the surface in a very efficient way. Transpiration cooling is presently used in local regions of commercial turbine and rocket engines.

- U -

Ultraviolet:

That portion of the electromagnetic spectrum below blue light (380 nanometers).

- V -

Variable Integration Time
Variable Integration Time (VIT) refers to a characteristic of the acquisition control of a FPA. The Integration Time is the period of time that the FPA is allowed to collect IR photons. Typically, an FPA runs at a maximum integration time of 16 milliseconds, which is one complete frame.

Arrays that have Variable Integration Time have the capability of capturing photons over shorter periods of time. This reduces the amount of energy that the detector captures at any given temperature. A common use for FPAs with VIT is to have high temperature imaging and measurement capabilities without needing filters. Some modern FPAs will operate up to 450°C simply by using VIT.

For the P/PM user, having a FPA with Variable Integration Timing is a time saver since one can view higher temperatures by changing the electrical characteristics of the detector rather than installing an optical filter. Systems without adequate VIT typically require several filters to cover a span of -10°C to 1500°C.

- W -

- X -

- Y -

- Z -

- # -