• Materials
• Lamps

Materials

Q: How do I build custom specular and diffuse reflector materials?

A: If you just own the Photopia Foundation, you can build custom specular materials. If you own the Foundation with the General Materials Library, you can build both custom specular materials and custom diffuse materials.

Perfectly Specular Materials:

1. View the contents in your \Photopia\Lib directory.
2. Locate any of the files with a *.BRD extension that are 2 bytes in size. It may list the file as 1 Kb or you may need to view the properties of the file to ensure it’s 2 bytes. All *.BRD files are either around 70 Kb in size or 2 bytes in size.
3. Make a copy this file AND the associated *.RFL file of the same name in your \Photopia\Lib directory. Rename the files (keeping the same extensions) to whatever you want with 8 characters.
4. Open the *.RFL file in any editor such as Notepad or Wordpad. It should look like one of the following:

5. Edit the reflectance values in your new *.RFL file as you deem appropriate and save the file. (Please note that materials that are not perfectly diffuse can rarely be characterized in this format. You should not attempt to create custom, non-diffuse materials using this format without a thorough understanding of the assumptions that this format imposes.)
6. Open the REFLECT.LIB file in any editor such as Notepad or Wordpad. Go to the end of the file and make a copy of the last line. Here is an example of what this line may look like:

7. Make the appropriate changes to the line(s) you just copied for your custom material(s). Save the file.

1. View the contents in your \Photopia\Lib directory.
2. Locate any of the PERFECT*.BRD and PERFECT*.RFL files. For example, locate the PERFECT1.BRD and PERFECT1.RFL files. These *.BRD files are about 70 Kb in size. You may need to view the properties of the file to ensure it’s the correct size. All *.BRD files are either around 70 Kb in size or 2 bytes in size.
3. Make a copy of these files in your \Photopia\Lib directory. Rename the files (keeping the same extensions) to whatever you want with 8 characters.
4. Open the *.RFL file in any editor such as Notepad or Wordpad. It should look like the following:

5. Edit the reflectance values in your new *.RFL file as you deem appropriate and save the file. The reflectance values should be the same from 0 to 89.999.
6. Open the REFLECT.LIB file in any editor such as Notepad or Wordpad. Go to the end of the file and make a copy of the last line. Here is an example of what this line may look like:

7. Make the appropriate changes to the line(s) you just copied for your custom material(s). Save the file.

Perforated Materials:

Q: Can you provide more details about the perforated materials in the Photopia 1.5 library?

A: The following information should clarify the descriptions in the material list and provide additional information:

23% open wht perf, 0.085" o.c.

This means the holes produce an open area in the material (as viewed straight on) of 23%. The material is painted white and the holes are 0.085" on center.

.020" wht. acrylic / 23% open perf

This is the same perforated steel as above but with a 0.020" thick acrylic inlay on the inside of the material.

.040" wht. acrylic / 23% open perf

This is the same perforated steel as above but with a 0.040" thick acrylic inlay on the inside of the material.

50% open wht perf, 0.09" o.c.

This means the holes produce an open area in the material (as viewed straight on) of 50%. The material is painted white and the holes are 0.09" on center.

.020" wht. acrylic / 50% open perf

This is the same perforated steel as above but with a 0.020" thick acrylic inlay on the inside of the material.

.040" wht. acrylic / 50% open perf

This is the same perforated steel as above but with a 0.040" thick acrylic inlay on the inside of the material.

In addition to these descriptions, the physical characteristics of the perforated material have been measured again recently. The data from these measurements varies somewhat from the descriptions in the current library mainly because the real measurements vary across the material samples as the paint thickness varies and fills the holes in more in some areas than others. Note, the above descriptions about the % open are also in part from manufacturers specifications. Thus, they may not exactly match the measured straight through transmittance values. Despite the variation on what we label these materials, the photometric data for these samples remains the same according to the data in your library.

The following are the new measurements of the hole sizes and spacing for the 2 perforated materials in the Photopia library:

23% open

• 0.088" hole spacing (the description in Photopia 1.5 is 0.085")
• 0.041" hole diameter
• Measured perpendicular transmittance: 21%

50% open

• 0.100" hole spacing (the description in Photopia 1.5 is 0.90")
• 0.075" hole diameter
• Measured perpendicular transmittance: 51%

Q: What are the Solid Model versions of the Transmissive materials and when should they be used?

A: Transmissive materials use BRDF & BTDF files to characterize how light reflects and transmits when it is incident upon the material. When a ray strikes a transmissive surface in Photopia, the appropriate reaction is applied to the ray using the BRDF & BTDF data. The important point to note is that the measured BRDF & BTDF data takes into account the effects of the full material thickness. So when a ray in Photopia strikes an infinitely thin polygon with a transmissive material assigned to it, the full reaction of the physical material (with a thickness) is accounted for at this single ray/polygon interface.

When a CAD model is constructed as a solid, every part has a thickness. Thus, if a prismatic lens or white diffuser material is drawn as a solid, it will be constructed with the thickness of the physical part. When this model is exported to Photopia via a STL file it imports as a mesh of polygons that cover the surface of the original solid model. If a transmissive material is assigned to the mesh that models this lens, then a ray will encounter 2 surfaces when passing through the material model. Since the full effect of the material is accounted for at the first ray/surface interaction, it will then be accounted for twice if the ray then strikes a second surface of the lens.

To avoid this problem, we have made special versions of the transmissive materials where the second surface is ignored in such a case. These are the Solid Model versions of the transmissive materials and they should be used when the CAD model was constructed as a solid model. Note however, that the Solid Model versions of the materials only produce the proper effect if all of the surfaces of the lens model are oriented so that their “front” side is facing to the outside of the part. Thus, if the part is on a layer with a color of white, then the part should be rendered as white from all points of view when viewed in the Show Surface Orientation model in Photopia. This is important because the way that the Solid Model materials work is to have the reaction on the “back” side of the material be perfectly transmissive. So when the ray encounters the second surface of a lens model, it strikes the “back” side and is allowed to pass directly through with no losses or scattering.

One significant consequence of this is that it prohibits (or at least limits) the use of materials with different properties on each side. Materials that have a textured surface on one side and a smooth finish on the another, for example, can be properly modeled with a single, infinitely thin surface in Photopia since we can assign unique BRDF & BTDF data to each side of the material. But in the case of Solid Model materials, the “back” side must be set to be perfectly clear, so this flexibility is lost. This is why all of the Solid Model materials in the library are for materials that are the same (or mostly the same) on both sides. If you have a need for another material to be made into a Solid Model material, then contact us about making a new material for you. It is possible to make Solid Model materials for materials that are different on each side as long as light is only incident onto one side of the lens in the luminaire model.

Q: What are the descriptions of all of the material files and which files can I modify to make new materials?

A: You are able to modify the integrated reflectance of reflective materials, the integrated reflectance and transmittance of transmissive materials, and the index of refraction and extinction coefficient of refractor materials. The integrated reflectance and transmittance values can change as a function of incidence angle. You can also change the ratio of the light reflected in a specular manner to that scattered by a material and the ratio of the light transmitted in a straight through manner to that scattered by the material. You cannot change the distribution of the scattered light from materials, however, as this data is measured in our BRDF/BTDF device and stored in binary format. Following are descriptions of the material files:

Reflective Materials:

Filename.Rfl – ASCII file of integrated reflectance values at each incidence angle. Files can contain an arbitrary angle set so long as 0, 90 and 180 degrees are included. Valid reflectance values range from 0.0 to 1.0.

Filename.Brd – Binary file of the BRDF data for a material. This file cannot be modified. A .Brd file only 2 bytes in size indicates the material is specular.

Filename.Rsc – ASCII file specifying the ratio of the reflected light that is reflected in a specular manner. Note, this is not the specular reflectance values, but the fraction of the reflected light that is specular at each angle. To derive specular reflectance from these values, multiply these values by the reflectance at each incidence angle. Valid values range from 0.0 to 1.0. This file may or may not be associated with a material. A data byte indicating whether or not the material has a specular reflectance component is contained in the .Brd file.

Transmissive Materials (all reflector files plus the following):

Filename.Trn – ASCII file of integrated transmittance values at each incidence angle. Files can contain an arbitrary angle set so long as 0, 90 and 180 degrees are included. Valid transmittance values range from 0.0 to 1.0.

Filename.Btd – Binary file of the BTDF data for a material. This file cannot be modified. A .Btd file only 2 bytes in size indicates the material is clear or image preserving.

Filename.Tsc – ASCII file specifying the ratio of the transmitted light that is transmitted in a straight through manner. Note, this is not the straight through transmittance values, but the fraction of the transmitted light that passes through unscattered at each angle. To derive straight through transmittance from these values, multiply these values by the transmittance at each incidence angle. Valid values range from 0.0 to 1.0. This file may or may not be associated with a material. A data byte indicating whether or not the material has a straight through transmittance component is contained in the .Btd file.

Refractor Materials:

Filename.Rfc – ASCII file containing 3 values: the index of refraction of the outside medium (usually assumed to be air at 1.0), the index of refraction of the material averaged over the visible spectrum and the extinction coefficient in units of inches. Note, the extinction coefficient is applied in the equation: Trans = e^(k*L), where "e" is the exponential (2.71…), "k" is the extinction coefficient, and "L" is the distance the ray travels in the material in inches. Thus, "k" is a value per inch.

You can modify any of the ASCII files described above with a text editor such as Notepad. If you want to create a new version of a file without changing the original copy, then follow the specific instructions below:

1. Go to the \Photopia\Lib subdirectory.
2. Find a material that has all of the same general characteristics as the new material you wish to create. For example, pick PAINT001 if you want a reflective material that has a specular component.
3. Copy all of the files for the original material to files with a new filename of your choice. Keep the filename prefix to 8 characters or less.
4. Modify the data in the files as you require.
5. Add a reference to your new material in either the REFLEC.LIB, TRANSMIT.LIB, or REFRACT.LIB file, depending on if your file is reflective, transmissive, or refractive, respectively. These files are ASCII files. Open the proper file in Notepad or another ASCII file editor and copy the last line in the file to a new line. Then modify the data as you require. There are 5 entries on each line, with each piece of data separated by a TAB character. In order, the data is: manufacturer, designation, description, value (either reflectance, transmittance, or index of refraction), and material filename prefix.
6. Save and close the .LIB file and then your new material should be listed inside Photopia.

Q: How can I model a luminaire with multiple lamps of different types?

A: Photopia 2.0 allows you to use as many different lamp types and instances of those lamps as you require. Earlier releases do not allow multiple lamp types, but do allow multiple instances of a given lamp type.

Q: Why don't all of the lamp models in the standard lamp library have .IES files?

A: There are 2 ways to define how light emanates from Photopia lamp models, using .IES files is one and using the tube luminances is another. We call the second method a "Theory" driven lamp model since its light distribution is derived from the lamp geometry and luminance data only. This is required for lamps that we consider "open." That is, the lamp tubes are spaced far enough apart so that another optical element can be placed between them. The circular fluorescent lamps for example, are often used with a circular reflector in the center. This changes the way that the lamp tubes interact with each other and thus changes the bare lamp candela distribution. In this case, using an .IES file that was measured when the lamp tubes interacted without any obstructions is not appropriate. So most U-lamps, 2D lamps and circular lamps are "Theory" lamps. If you want the bare lamp .IES files for of a "Theory" lamp, then you can easily generate one by running a bare lamp analysis through Photopia. Be sure to run at least 1,000,000 rays and at least 5 interreflections in order to get a relatively smooth distribution and the proper lumen output. If you require additional details on how to create "Theory" lamp models then please contact Photopia product support.