This document discusses the details of using the DIRSIG image and data generation model to simulate thermal infrared sensing systems.

Overview

The DIRSIG model includes the ability to model self-emission from surfaces and volumes in the scene. This self-emission arises from blackbody radiation of those surfaces and volumes having temperatures greater than absolute zero.

Heat Transfer Basics

This section will attempt to summarize basic heat transfer, but the reader should be aware that entire books are devoted to the subject. Heat transfer is the energy transferred between an object and its environment due to a temperature difference between the two. There are three basic mechanisms for heat transfer: (1) convection, (2) conduction and (3) radiation. They are described individually for clarity, however in reality a given system will typically exhibit all three processes simultaneously.

As an example, consider a block of wood sitting on a stone slab. The block of wood can be either heated or cooled by air passing over it (convection), by the stone slab on which it is in contact (conduction) and finally by it absorbing photons from the Sun and other sources (radiation).

The following sections give a brief overview of the theory and relevant parameters for these mechanisms.

Convection

Convection is the heat transfer caused by the macroscopic movement of matter. Just as molecules of high temperature possess more kinetic energy, areas of a high temperature (fluid) are less dense than areas of low temperature. The difference in the density (and temperatures) within the fluid create the macroscopic convection currents. As a first example consider a pocket of air. The hot air (less-dense) rises due to buoyant forces and the colder air (more-dense) falls.

To be explicit, convection is really the combined heat transfer due to the macroscopic bulk movement and the random molecular motion. This can be demonstrated considering the example of a fluid in motion and its barrier, both at different temperatures. At the surface boundary, the velocity of the fluid is zero, and thus all of the heat transfer is due solely to the random molecular motion. As you go further from the boundary, the velocity of the fluid increases to some finite value and additionally, the temperature decreases.

The macroscopic heat transfer is due to the fact that the thermal boundary layer grows as the flow progresses in the x direction and heat in this layer is swept downstream and eventually to the fluid outside the boundary layer.

There are essentially two types of convection: forced and natural. The hot air example above is an example of natural convection. Air blowing over the earth as a result of atmospheric wind is an example of forced convection. However, both obey the same rate equation given below.

\begin{equation*} q_\mathrm{conv} = h(T_a - T_b) \end{equation*}

where h is the convection heat transfer coefficient and Ta and Tb represent the temperature gradient.

Conduction

Conduction is the mechanism described by heat transfer through a medium in direct contact with surfaces of different temperatures. The first requirement for conduction is that the medium is not moving. Thus the medium must be a rigid solid, or if fluid, it must have no circulating currents (note that circulation is sometimes referred to as the fourth heat transfer mechanism).

If there is no large-scale movement then the heat transfer must be at the molecular and atomic level. Recall that molecules and atoms at any temperature above absolute zero are in motion. The motion is measured by the kinetic energy of the atoms and molecules. Temperature is by definition the measure of the average kinetic energy. This kinetic energy is due to the motion of the atoms and molecules globally, as well as the internal molecular motion; rotational, vibrational, etc. When atoms or molecules collide, energy is transferred. In terms of temperature, hotter (faster) atoms lose some energy, while the (slower) cooler atoms gain energy. Conduction is defined as the transfer of the energy in this manner.

Consider a group of atoms with high kinetic energy isolated from a group of atoms at a lower temperature via an ideal thermal insulator. At this point nothing interesting happens. If however the thermal insulator is removed, the atoms begin colliding randomly with each other, and at some time t1 > t0, the atoms will reach an equilibrium temperature. Now consider replacing the thermal insulator with a material of some given thermal conductivity. Thermal conductivity is a material property which describes the rate at which the material conducts heat. All materials have some thermal conductivity as a perfect vacuum is the only ideal insulator.

We can now quantify the heat transfer rate in terms of the thermal conductivity, k, temperature difference, dT, and the direction of energy transfer dx.

\begin{equation*} q_\mathrm{cond} = -k\frac{dT}{dx} \end{equation*}

Radiation

Radiation is the mechanism of heat transfer where electromagnetc energy is either absorbed from an external source by the object or emitted by the object into the environment. Remember that any object at a temperature above absolute zero will radiate energy. A perfect radiator, also known as a blackbody, has the characteristic that it is a perfect absorber and in turn a perfect emitter. Planck derived the equation of spectral radiant exitance from a blackbody. It is given by Planck’s equation below.

\begin{equation*} M_{\lambda BB}= 2 \pi h c^2 \lambda^5 (e^\frac{hc}{\lambda k T} - 1)^{-1} \end{equation*}

Integrating Planck’s Equation yields the total exitance from a blackbody. This is knows as the Stefan-Boltzmann equation. Note that the total energy is proportional to the Temperature of the blackbody raised to the fourth power.

\begin{equation*} M = \sigma T^4 \end{equation*}

where sigma is the Stefan-Boltzmann constant.

In reality real objects are not perfect absorbers or emitters. We introduce emissivity as the ratio of the object’s spectral exitance to that of an ideal blackbody at the same temperature. From the equation it is obvious that the emissivity is a number between 0 and 1 and it can vary spectrally (as a function of wavelength).

An object is simultaneously absorbing and emitting with surrounding surfaces. Determination of the overall rate of heat transfer becomes complicated very quickly. A common example used in heat transfer is a small object completely surrounded by a larger surface (picture a ball hovering within a larger sphere). Whereas convection and conduction rely on a medium for heat transfer, radiation can occur in a vacuum. The heat transfer of the surface and surround is given in the following equation.

\begin{equation*} q_\mathrm{rad} = \varepsilon \sigma {(T_o^4 - T_s^4)} \end{equation*}

where e is the spectral emissivity, To is the tempeature of the object and Ts is the temperature of the surround.

Thermodynamic Properties

This section will discuss the common parameters describing heat transfer including thermal conductivity, heat capacity, absorption coefficients, emission coefficients, etc.

  • Discuss the conductivity of air and the dependence on humidity.

Modeling Approaches

  • Discuss how steady state temperature is the result of equilibrium of the three heat transfer mechanisms. Hence, numerical modeling involves solution of differential equations.

  • Discuss

Temperature Model Options

Data-Driven Temperatures (data-driven)

Relevant section in the Temp Solvers Manual.

Tempeature Map (data-driven)

Relevant section in the Temp Solvers Manual.

Balfour (predictive, emperical fit)

Relevant section in the Temp Solvers Manual.

THERM (predictive, physics-driven)

Discuss THERM as a 1D model (what that means exactly), the lack of lateral conduction (since its 1D), etc.

The units for the specific heat are [L/cm/C] where [L] is a Langley unit and [C] is degrees Celcius. A Langely is equal to 1calorie/cm2 or 1watt/m2 is equal to 0.086L/hour.

Exposed Area

The exposed area term is used to communicate the amount of exposure the facet has to the ambient air. For example, a facet may be a slab of parking lot asphalt that is exposed to the air only on the top side. In contrast, a facet may be a vertical panel or fence that is exposed to the ambient air on both the front and back sides. The differences between these two cases is very important since the convective load on each surface differs drastically. The sign of this exposure term is used to indicate one of these two possibilities. In addition to controlling to the convective processes, this term plays a roll in the radiational loading of a surface (for example, describing how much of a surface is exposed to the cold sky). Using the original documentation from the DCS Corporation, we provide the following as a guideline:

Common Value Range Description -0.35 through -0.65 Surfaces exposed on both sides +0.35 through +0.65 Surfaces exposed on the top side only

Broad-Band Optical Properties

The thermal model utilizes solar absorption and thermal emissivity coefficients the radiational loading computations. These two coefficients are derived from the spectral emissivity curves assigned to each material. If the emissivity file assigned to a given material contains multiple curves, then each coefficients will be computed for each curve and used in conjunction with the texture mapping routines. The VISIBLE EMISSIVITY and THERMAL EMISSIVITY variables allow the user to shift the mean value of these respective coefficients. The most common reason values are set is to account for biased values that are derived from incomplete spectral emissivity curves. For example, in cases where the user has only thermal spectral emissivity values, the VISIBLE EMISSIVITY variable will have to be set so that the thermal model knows how much solar insolation is absorbed by the surface.

Thickness

The thickness property utilized by the thermal model and radiometry model can be assigned on a facet- by-facet basis during the scene construction process. If the user wishes to change this value at run-time, all the per-facet thicknesses can be overridden by the THICKNESS variable (this will only affect facets as- signed this material). By default, facets are modeled as single-sided entities in all radiometric and thermodynamic calculations. In some instances, the user may wish to make facet have both a front and a back side (commonly for shadowing purposes) which can be accomplished by setting the DOUBLE SIDED variable to TRUE.

MuSES (predictive, phyics driven)

This is out of scope for use to completely document a code nearly as complex as DIRSIG itself. But, we should say that MuSES understands conduction, self radiation (one part of an object radiating onto another), etc.