The DIRSIG User's Manual
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5.4. Tutorials

At this time, the GUI does not have a way to create a new file, add entries to an existing file or import spectral data. Therefore, most of these tutorials needs to be performed using a text editor.

5.4.1. Creating a new material database

The format of the material database file is described in the Material Database File section in the Support Application Manuals part of the manual.

The process of creating a new material database file is a simple task. The procedure entails creating an empty text file and adding the appropriate material entries to the file. The following two tutorials discuss the process of creating material entries for different types of materials.

5.4.2. Creating an opaque material

Creating a new material entails adding a new entry to a material database file. The format of the material database file is described in the Material Database File section in the Support Application Manuals part of the manual.

To add an opaque material we will be creating a material entry that has a specific set of optical attributes. To create a material, we will need the thermodynamic properties and a Spectral Emissivity File. The creation of a Spectral Emissivity File from ASCII spectral emissivity or reflectance data is discussed in a tutorial later in this section.

For this tutorial, we will assume that we are making a material for a maple tree trunk. The steps to create the material entail the following general steps:

  1. Identify an material ID that is not in use in the material file. In this case we have arbitrarily identified 8031 as the ID for our new material.

  2. Establish a name for the material. At RIT, we try to construct a name that embeds a basic material taxonomy in the name. In this case, we will will use the name "Tree, Silver Maple, Trunk". If we have multiple types of Silver Maple tree trunks (perhaps reflecting different ages or health) then we might consider making a material for each variant of Silver Maple. Each variant might capture the conceptual difference in the name.

  3. Enter the thermodynamic properties. In addition to the required fields, the optional THERMAL_EMISSIVITY variable is used in this example to correct the broad-band used by the thermal model. Overriding the broad-band thermal emissivity used by the thermal model does not affect the emissivity used in the radiative transfer.

  4. Supply the spectral emissivity file using the EMISSIVITY_FILE variable. In this case, we are treating the tree trunk as a purely diffuse surface by setting the SPECULARITY to 0. Since the default material type is "opaque", we were able to omit the OPTICAL_DESCRIPTION variable. However, we could have included this variable and assigned it the value OPAQUE.

Below is the entry that was produced for this transmissive material: 

MATERIAL_ENTRY {
    NAME = Tree, Silver Maple, Trunk 
    ID = 8031
    SPECIFIC_HEAT = 0.57
    THERMAL_CONDUCTIVITY = 0.86
    MASS_DENSITY = 0.8
    THERMAL_EMISSIVITY = 0.96
    EXPOSED_AREA = 0.5
    SPECULARITY = 0.0
    EMISSIVITY_FILE = tree_silvermaple_bark.ems
    EDITOR_COLOR = 0.70, 0.65, 0.35
}

5.4.3. Creating a transmissive material

Creating a new material entails adding a new entry to a material database file. The format of the material database file is described in the Material Database File section in the Support Application Manuals part of the manual.

To add a transmissive material we will be creating a material entry that has a specific set of optical attributes. To create a material, we will need the thermodynamic properties and a Spectral Emissivity File and a Spectral Extinction File. The creation of a Spectral Emissivity File and Spectral Extinction File from ASCII spectral data is discussed in tutorials later in this section.

For this tutorial, we will assume that we are making a material for a individual maple tree leaves. The steps to create the material entail the following general steps:

  1. Identify an material ID that is not in use in the material file. In this case we have arbitrarily identified 8194 as the ID for our new material.

  2. Establish a name for the material. At RIT, we try to construct a name that embeds a basic material taxonomy in the name. In this case, we will use the name "Tree, Silver Maple, Leaf". If we have multiple types of Silver Maple leaves (perhaps reflecting different health, color, etc.) then we might consider making a material for each variant of Silver Maple. Each variant might capture the conceptual difference in the name.

  3. Override the thickness of all the surfaces assigned this material. For demonstration purposes, we have set the optional THICKNESS variable to the value of 0.05 cm (0.5 mm). This thickness will be used by both the thermal model and the radiation code to compute the transmission from the spectral extinction we will supply later.

  4. Indicate that material uses the plate transmission support by setting the OPTICAL_DESCRIPTION variable to the value UNIFORM_TRANSMISSION.

  5. Supply the spectral emissivity file using the EMISSIVITY_FILE variable. In this case, we are treating the tree leaf as a partially specular surface by setting the SPECULARITY to 0.1.

  6. Supply the spectral extinction file using the EXTINCTION_FILE variable.

Below is the entry that was produced for this transmissive material: 

MATERIAL_ENTRY {
    NAME = Tree, Silver Maple, Leaf
    ID = 8194
    SPECIFIC_HEAT = 1.0
    THERMAL_CONDUCTIVITY = 5.0
    MASS_DENSITY = 1.0
    EXPOSED_AREA = 0.13
    THICKNESS = 0.05
    SPECULARITY = 0.1
    OPTICAL_DESCRIPTION = UNIFORM_TRANSMISSION
    EMISSIVITY_FILE = tree_silvermaple_leaf.ems
    EXTINCTION_FILE = tree_silvermaple_leaf.ext
    EDITOR_COLOR = 0.2000, 0.6000, 0.2000
}

5.4.4. Creating an emissivity file from reflectance data

Importing spectral reflectance data entails creating a new spectral emissivity database file. The format of this file is described in the Spectral Emissivity File section in the Support Application Manuals part of the manual. This common task usually arises because the user has extracted a spectral reflectance curve or set of curves from another source and they wish to incorporate them into their simulation. For the purpose of this tutorial, we will assume that is the case and walk through the process of importing this data.

A brief synopsis of the steps involved in this process is included below. The DIRSIG Spectral Emissivity File is an ASCII file, so the user must insure that their data is also in ASCII. Working with ASCII data allows the user to use a simple spreadsheet (like Excel) to perform any necessary conversions or sorting that is described in the outlined process.

  1. Obtain the spectral reflectance curve or curves to be imported.

  2. Sort the data by wavelength or frequency . The Spectral Emissivity File expects to see the spectral values in increasing wavelength (microns) or decreasing frequency (wavenumbers, cm-1) order. The model does not assume that the data appears at any specific spectral resolution or that it is sampled on constant spectral centers.

  3. Convert the reflectances to emissivities. Since the model assumes thermodynamic equilibrium, we can convert the reflectance values to emissivities as 1 - reflectance.

  4. Creating a new emissivity database file. Using Excel or a simple text editor the user needs to start constructing the emissivity file. The first line of the file is the total number of curves in the file. Set this number based on the number of curves you wish to import.

  5. Create the angular weighting values. The next 91 lines in the emissivity file is the angular weighting factors. It is assumed that the emissivity curves in the files are normal or nadir emissivity values. Generally, the emissivity changes as a function of view angle. These angular weighting factors were introduced when the model was used exclusively in the thermal infrared region of the spectrum. These weighting coefficients would be used to weight the normal emissivity curves for other view angles.

    Since the model is now used in both the reflective and emissive regions of the spectrum these single, wavelength independent weighting coefficients have limited use. Most modern emissivity files have all of these weighting values equal to 1.0. Therefore, the next 91 lines in the file should be all 1's.

  6. Import the computed emissivity curve(s). The next line in the file is the curve delimiter tag, which is the string CURVE_BEGIN. Following this line, the emissivity values you converted from the reflectance data can be inserted. At the end of this curve, the user can start another curve using the CURVE_BEGIN tag followed by the next emissivity curve.

    There is not a special tag to denote the end of the spectral data, therefore after the final curve in the file simply terminate the input. Avoid blank lines anywhere in this file including at the end of the file.

5.4.5. Creating an extinction file from transmission data

Importing spectral transmission data entails creating a new spectral extinction database file. The format of this file is described in the Spectral Extinction File section in the Support Application Manuals part of the manual. This common task usually arises because the user has extracted a spectral transmission curve from another source and they wish to incorporate them into their simulation. For the purpose of this tutorial, we will assume that is the case and walk through the process of importing this data.

A brief synopsis of the steps involved in this process is included below. The DIRSIG Spectral Extinction File is an ASCII file, so the user must insure that their data is also in ASCII. Working with ASCII data allows the user to use a simple spreadsheet (like Excel) to perform any necessary conversions or sorting that is described in the outlined process.

  1. Obtain the spectral transmission curve to be imported.

  2. Sort the data by wavelength or frequency. The Spectral Extinction File expects to see the spectral values in increasing wavelength (microns) or decreasing frequency (wavenumbers, cm-1) order. The model does not assume that the data appears at any specific resolution or that it is sampled on constant spectral centers.

  3. Convert the transmission values to extinction coefficients. Since the model currently only allows the user to input extinction data, the transmission values will need to be converted to extinction coefficients. In order to do this, the user will need to know the thickness of the material this transmission data will be applied to. This thickness is either a facet-level attribute (see the Geometric Database File description) or it is included in the material entry in the material database file using the THICKNESS variable. In either case, this thickness is in centimeters, but the extinction file is extinction per kilometer.

    To convert the transmission values to extinctions the following equation can be used:

    where "d" is the thickness of the material in kilometers (make sure to convert the thickness). If your transmission curve has values of zero at some wavelengths then computing the logarithm of this value will result in an error. To create an optically thick (zero transmission) extinction value, simple substitute in a very large number (e.g. 1e16).

  4. Creating a new extinction database file. Using Excel or a simple text editor the user needs to start constructing the extinction file. The first line of the file is the total number of curves in the file, which is limited to 1 at this time.

  5. Importing the computed extinction curve. The next line in the file is the curve delimiter tag, which is the string CURVE_BEGIN. Following this line, the extinction coefficients you converted from the transmission data can be inserted. There is not a special tag to denote the end of the spectral data, therefore after the final curve in the file simply terminate the input. Avoid blank lines anywhere in this file including at the end of the file.

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