IRGenâ is an IR sensor simulation program that generates thermal IR databases for real-time simulation. IRGen creates the 3-D IR data base as viewed by a thermal IR sensor, using first-principles models of heat transport, radiation, and sensor processing. IRGen is designed to provide good modeling accuracy along with ease of use and great flexibility in accommodating user-specific sensor and target signature models.

An overall diagram of the data inputs and outputs to IRGen is shown in Figure 1. IRGen accepts a visual data base with specification of the material code for each surface in the data base. It outputs an IR data base, in which the visual colors are replaced by the IR gray shades that would be displayed by a specified IR sensor for the given thermal and atmospheric environment.

IRGen’s input visual and output IR data bases are in the DWB^TM and OpenFlightÔ format, which are the most widely used simulation data base formats.

IRGen can operate in either a static or dynamic mode. In the static mode, it generates a database, which represents the appearance of the scene at a user-specified time of day. This mode is suitable for short time simulations, such as tactical missile flyouts. In the dynamic mode, the IR radiance image changes as a function of time of day or other scenario variation. (The dynamic mode is currently available only on SGI platforms, and requires PerformerÔ 2.0.)

IRGen will run on any Silicon Graphics (SGI) workstation, under IRIX version 5.3 or higher. It works with DWB and flight format data bases created by the standard versions of Designer’s Workbench and MultiGen.

Since IRGen data bases are standard flight files, they can be imported into any simulation program that works with visual flight files, such as those based on Performer, GVSÔ, EasySceneÔ, or VegaÔ. DWB or OpenFlight data bases can also be used for simulation on Windows NT^®, Evans & Sutherland, and other image generation platforms.

The data base input to IRGen is a material-coded visual data base created by the Designer’s Workbench and MultiGen modeling system. Using Designer’s Workbench and MultiGen, each polygon in the data base is labeled with a material code, which specifies the properties of that surface. (Designer’s Workbench and MultiGen provide an IR material code attribute to support the IRGen material codes.) The material code is then used by the thermal model to compute surface temperature and radiance. Vertex material code labeling is also possible, allowing the modeling of surfaces with thermal gradients, which are then rendered as smooth-shaded polygons.

The process of material code insertion is performed only one time. The global attribute modification features of Designer’s Workbench and MultiGen can be used to facilitate the process of assigning material codes to large data bases. IRGen supports most of the modeling features of Designer’s Workbench and MultiGen, including levels of detail, subfaces, external references, and instances.

Any number of IR data bases can be produced from a single visual data base. For example, it may be desirable to have several IR data bases for the same scene, corresponding to different sensors, different times of day or night, or different seasons of the year.

The thermal model is the key element of IRGen. Given a user-specified thermal environment, it computes the surface temperature of every polygon in the data base. This is a first-principles calculation using the time-dependent heat transport equation, integrated with a finite-difference time-dependent solution method. IRGen has accurate models for exterior and interior heat sources for each surface, as shown graphically in Figure 2. The heat transport properties and finite-element structure of each surface are contained in the material data base, and are referenced using the material code for the surface.

The external heat sources include direct sun, diffuse sun, sky thermal radiation, ground thermal radiation, and convection. The internal heat sources include free or forced convection with air, or heat transfer with a medium other than air. The user can specify the internal heat sources as part of the material description.

The thermal model computes the surface radiance from the surface temperature, emissivity, and reflected radiance terms. The radiance is computed in the sensor spectral band, and is the input to the sensor model, which computes the gray shade that would be seen by the specified sensor display.

The output of IRGen is a second flight file, in which the surface colors represent the gray shades displayed by the specified IR sensor. It is this IR flight file that is used as the data base for a thermal IR imaging simulation. The surface radiance computed by IRGen are stored as comments in the IR DWB or flight file and can be viewed with Designer’s Workbench and MultiGen. Also, the IRView feature of Designer’s Workbench and MultiGen can be used to switch instantly between the visual view and the most recent IR view of a data base.

The material data base contains the thermal and radiative properties of the materials, which are used in the data base. The material code is used as an index to retrieve the properties

IRGen is delivered with a material data base, which contains the most common type of man-made and natural background materials. The material editor option allows the user to add new materials to the data base and modify existing material properties.

The list of properties contained in the material data base is given in Table 1.

Identification

material code
descriptive label

Thermal properties

thickness
specific heat
density
conductivity

Surface properties

mid-IR emissivity
long-IR emissivity
spectral emissivity (optional)
solar reflectivity

Interior heat transport

interior convection
interior temperature
interior flow velocity
fixed temperature
two-sided

Finite-element integration data

integration time increment
integration settling time
number of nodes
node heat capacity array
node conductive transport array
node radiative transport array

Special Thermal Properties

shadow
wet surface
aerodynamic heating

Texture

IRGen allows users to integrate proprietary or restricted IR signature models into IRGen using the user-specified model feature. This feature provides for the designation of certain material codes as references to external programs. When IRGen encounters a data

base surface with a user-specified model code, it invokes the user's external program, sending it the required data for signature computation, and receiving the surface gray shade.

This feature can be used for special user models such as jet engine plumes, vehicle engines, and complex vegetation models.

The environment model contains the parameters, which specify the thermal environment of the data base. Sets of environment parameters can be stored and saved in a library. The environment parameters include time and location (used by the solar ephemeris in IRGen) air temperature and wind speed

profiles, and sky radiance.

The sensor model contains the parameters, which specify the properties of the simulated sensor. Sets of sensor parameters can be stored and saved in a library. The user-specified sensor parameters include spectral band, spectral response, display dynamic range, and white/black hot modes. Display dynamic range, can be specified either in terms of temperature or in terms of radiance.

The Material Editor adds and modifies materials in the IRGen material data base. The editor facilitates new material creation by providing a catalog of standard materials such as steel and aluminum. For materials in the catalog, the user does not have to enter detailed material data. The Material Editor automatically computes the finite-difference integration data from the thermal properties of the material. Non-homogeneous (layered) materials can also be created with the Material Editor by specifying the heat transfer coefficients.

The IRGen Texture Option automatically generates thermally correct IR textures in an SGI image file format. The texture option will work either with existing visual textures, or with the IR texture creation system that is provided with the option. Visual textures are converted to thermally correct IR image textures, with texture modulation depending on the surface temperature and the sensor dynamic range.

The IR textures creation system forms calibrated IR texture maps from either IR camera images, or from an algorithm. The algorithm process generates synthetic texture maps as 2-D random processes with user-defined parameters. Once a calibrated IR texture is generated, it is automatically adjusted to provide the correct tray shade modulation for any thermal environment and sensor.

The Digital Feature Analysis Data (DFAD) option works with the Designer’s Workbench and MultiGen DFAD option. It automates the material code assignment process by reading the DFAD surface material code (SMC) and feature ID (FID) provided by the Designer’s Workbench and MultiGen DFAD option. Thus, for a data base that is completely specified by DFAD data, no material code assignment is necessary.

The DFAD option is delivered with a default set of material assignment and properties for each combination of SMC and FID. The assignments and properties can be modified by the user.

The atmospheric option integrates a full user-friendly implementation of LOWTRAN 7 into the IRGen system. With this option, LOWTRAN is used to compute solar transmittance and sky radiance, which can then be input into the real-time simulation to set the graphics attenuation distance and fog color parameters. The computed sky radiance is also an input to the thermal model.

IRGen 3.0 uses the MotifÔ graphical user interface. User inputs and data base generation are handled through a system of pull-down menus and dialog boxes. All user inputs are checked for validity.