Photopia Materials Library

Background

Our customers rely on Photopia to produce accurate predictive photometry. Having accurate material models is necessary to have accurate output. Photopia includes a library with over 300 measured materials from most of the major vendors that the lighting industry uses.

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Our Measurement Process

LTI Optics uses a custom-built BRDF/BTDF measurement device to accurately characterize the real reflecting/transmitting properties of various materials such as semi-specular aluminum, hammertone, prismatic lenses, perforated diffusers, and many others. The measurements are collected for a wide range of light incidence angles to capture the true scattering nature of real materials (see image below). Photopia uses the measured data in lookup table format and therefore does not attempt to “curve fit” the data into standardized equations that do not model the wide range of scattering effects observed in real materials. The library of measured data also eliminates the need for the user to “guess” at the scattering properties of materials. Customers can also submit proprietary materials for measurement, ensuring the most accurate data possible for their analyses. To create an accurate material model we perform two types of measurements: integrated reflectance/transmittance, and material scattering.

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Adding a Material to the Library

If you're looking for a material that you don't find in the Library, please let us know and we'll work on adding it. Often, you may use a custom material that isn't generally available. We can add this material to a custom library for your company. For information on getting a material in the Library, please see this order form.

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Modifying a Material in the Library

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 refractive materials. The integrated reflectance and transmittance values can be changed 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 Material Files:

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 that these are 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 Material Files (includes reflective files above, 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 that these are not the straight-through transmittance values, but the fraction of the transmitted light that passes through un-scattered 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 Files:

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 that 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.

To Modify Material Files:

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:

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Differences Between Refractive and Transmissive Materials

Transmissive surfaces are modeled as infinitely thin surfaces in the CAD model and all optical properties of the physical material are assigned to this surface. When a ray strikes a transmissive surface all of the effects of the thick material are accounted for at this single ray/surface interface. The amount of reflected, transmitted and absorbed light is dictated by the material's .RFL and .TRN files which list the reflectance and transmittance as a function of incidence angle, respectively. The scattering properties for the reflected and transmitted light are dictated by the BRDF and BTDF data, respectively. The "front" side of the material always has reflectance and transmittance properties and the "back" side has data on only some of the materials in the library. Examples of transmissive materials are clear glass and plastics, white translucent materials, isotropic* prismatic lenses and isotropic perforated materials.

Refractive surfaces model the entire volume of a lens including the inside, outside and side surfaces. When a ray strikes a refractor surface from the "outside" (from the air, for example), it is partially reflected and refracted according to Fresnel's Equations and Snell's Law of Refraction, respectively. Light is also absorbed within the material according to the path length within the material and the material's extinction coefficient. Once rays have entered into a refractor material such as glass, Photopia searches for intersections with other refractor surfaces on all refractor layers in the model. Once an intersection is found, the ray is either partially internally reflected and partially refracted out or it is Totally Internally Reflected (TIR) back into the material, depending on the incidence angle.

Clear lenses can be modeled with either Transmissive or Refractive surfaces. Transmissive surfaces will result in a faster analysis. If the lens is curved, then only the inside surface of the true lens shape should be modeled if making it a Transmissive surface. If the curvature and thickness of the clear lens are such that there might be some refractive effects, then it should be modeled as a Refractor using its true geometry and thickness.

*Isotropic materials are those for which their orientation within the plane of the material is not important. For example, a flat sheet of translucent white plastic can be rotated within the plane of the material and the scattering of the incident light will be unaffected. If a material such as a plastic lens with extruded linear prisms was rotated there would be a significant difference between the scattered light patterns. The effects of the prisms on the light are different depending on whether the light strikes the prisms from within a plane that is parallel, perpendicular or some other orientation to them. Such materials are referred to as anisotropic. A perforated material is isotropic if it has round holes in a regular pattern, but it would be anisotropic of it contained linear slots.

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When to Use the Solid Model Versions of Transmissive Materials

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.

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