DIRSIG utilizes a material database file to look-up optical and thermodynamic properties for facets in the scene using the Material ID assigned to each facet. The material database file (denoted with an extension) consists of a series of material description sections. The variables that can be set in each of these MATERIAL_ENTRY sections are described in the following table.
Table 34-2. Tags values and descriptions for section.
| Variable Name | Usage | Units | Description | |
| THERMAL_CONDUCTIVITY | Required | [W/m*K] | The thermal conductivity | |
| MATERIAL_ID | Required | Material ID number | ||
| SPECIFIC_HEAT | Required | [L/C] | Specific heat | |
| MATERIAL_NAME | Required | Name of material | ||
| MASS_DENSITY | Required | [gm/cm3] | Mass density | |
| THICKNESS | Optional | [cm] | Slab thickness | |
| SPECULARITY | Required | (fraction) | Surface specularity | |
| VISIBLE_EMISSIVITY | Optional | (fraction) | Mean solar absorptivity | |
| THERMAL_EMISSIVITY | Optional | (fraction) | Mean thermal emissivity | |
| EXPOSED_AREA | Required | (fraction) | Surface exposed are term | |
| OPTICAL_DESCRIPTION | Optional | (see below) | Controls surface opacity | |
| EMISSIVITY_FILE | Required | Name of spectral emissivity file | ||
| EXTINCTION_FILE | Required * | Name of spectral extinction file | ||
| DOUBLE_SIDED | Optional | Boolean | Control double-sided attribute | |
The units for the specific heat are [L/cm/C] where [L] is a Langley unit and [C] is degrees Celsius. A Langely is equal to 1 calorie/cm^2 or 1 watt/m^2 is equal to 0.086 L/hour. Since several of the material properties are unique to the model, the more uncommon variables are discussed in the following sections.
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 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 Table as a guideline:
The thermal model utilizes solar absorption and thermal emissivity coefficients in 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.
The OPTICAL_DESCRIPTION variable is used to define the opacity of the material. By default, this value is set to OPAQUE, however, the user may wish to give a material transmissive properties. Assigning this variable the value UNIFORM_TRANSMISSION_LAYER will give facets assigned this material a spectral transmission derived from the spectral extinction and the material thickness (from either the per-facet thickness or material thickness). A material may also be designated as the boundary to a NONUNIFORM_TRANSMISSION_LAYER. In this case, the spectral transmission will be derived from the spectral extinction and the path length through the material.
If either of the two transmissive properties are assigned to a material, the EXTINCTION_FILE variable must be set to the file containing the desired spectral extinction data.
The SPECULARITY variable describes the ratio of the reflected radiance from the specular direction over the total reflected radiance. For example, a material that is extremely diffuse will have a SPECULARITY value of 0.0 and a very specular material will have a value of 1.0. In future releases, this term will be replaced by the true bi-directional reflectance distribution function of a material.
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 assigned this material type). 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.
The material database file contains a series of MATERIAL_ENTRY records. In each record, the previously discussed material properties are defined. Optional variables (for example, the THICKNESS, DOUBLE_SIDED variables) can be either included or omitted. Omitted variables that are optional will be assigned default values.
Each material in the file is differentiated based on the ID number assigned to the material. If multiple entries have the same ID, the last one will be the one used at run-time (no warning is issued for duplicate material IDs). The entries do not need to be ordered by ID.
MATERIAL_ENTRY {
NAME = Tree, Maple, Bark
ID = 8031
SPECIFIC_HEAT = 0.57
THERMAL_CONDUCTIVITY = 0.86
MASS_DENSITY = 0.8
SPECULARITY = 0.0
THERMAL_EMISSIVITY = 0.96
EXPOSED_AREA = 0.9
EMISSIVITY_FILE = normaymaple_bark.ems
EDITOR_COLOR = 0.6941, 0.6471, 0.3765
}
MATERIAL_ENTRY {
NAME = Tree, Silver Maple, Leaf
ID = 8194
SPECIFIC_HEAT = 1.0
THERMAL_CONDUCTIVITY = 5.0
MASS_DENSITY = 1.0
SPECULARITY = 0
THERMAL_EMISSIVITY = 0.92
EXPOSED_AREA = 0.13
THICKNESS = 0.05
OPTICAL_DESCRIPTION = UNIFORM_TRANSMISSION
EMISSIVITY_FILE = norwaymaple_leaf.ems
EXTINCTION_FILE = norwaymaple_leaf.ext
EDITOR_COLOR = 0.2000, 0.6000, 0.2000
}