• General Questions
• Raytrace Settings
• Results
• Converting 2D Part Drawings to 3D Models

General Questions

Q: What are the differences between Transmissive and Refractive surfaces?

A: 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.

Q: Is there a way to run a Photopia analysis in the background without having it slow down the response of my other applications?

A: The priority for Photopia can be set in Windows NT 4.0 and Windows 2000. Note however, that it is less likely to be necessary in Windows 2000 since it does a better job at managing multiple tasks than NT 4.0. To set Photopia's priority, first start Photopia then right click on the task bar at the bottom of your screen and select Task Manager. Then go to the center tab labeled Processes. Then find Photopia.exe in the list that appears and right click on it. Then select Set Priority and change it to Low. When you are not doing anything else that takes processor time Photopia will run at full speed, but when you are trying to type a message or use your CAD software you should see a significant reduction in the delay from these applications. You need to do this each time that Photopia is started, as it will always default to Normal priority. Despite the warning message that Windows issues after you change the task priority, we have yet to see any negative effects from this.

Raytrace Settings

Q: Do you have any suggestions for the Analysis Settings to get the most accurate results?

A: Since Photopia is a probabilistic based raytracing program, the results get more accurate as more rays are traced. The number of rays that are required to obtain accurate results depends on the level of “resolution” you have specified, in other words, the level of detail in the results. You can view the results such as the candela polar plot and the illuminance plane shaded plot and watch as they vary around their final values with each update as the raytrace proceeds. The default update frequency is 10%, so you see results at 10%, 20% and so on until the raytrace is finished. The least detailed result is the luminaire efficiency (LOR). This is a single number that represents how many lumens exit the luminaire compared to how many lumens were generated by the lamps. The exact direction of the exiting lumens is not critical when determining the efficiency. Thus, you can see that the efficiency changes very little after the first update of the results during the raytrace. The candela distribution requires much more detail to be resolved since Photopia needs to determine exactly how many lumens belong in all of the angular zones of the distribution. The more angles that are specified in the distribution, the smaller the angular zones and thus the more rays it takes to determine the correct proportions of lumens in all zones. The most level of detail is generally required for the illuminance planes. Whereas the angular zones in the candela distribution might be separated by 2.5 or 5 degrees, the small patches in a high-resolution illuminance plane might be separated by fractions of a degree in angular spread. With this general understanding, we make the following recommendations for the Raytrace and Photometric Output settings. Keep in mind that these are only general recommendations and you can vary these values as long as you understand the consequences.

Raytrace & Photometric Output Setting Recommendations

*Note: When using CFL lamps, we recommend that you turn ON the Lamp Shadow Check option. This options can be left off for most other lamp types. All other raytrace settings can generally be left at their default values.

Q: Can I do a quick 2D raytrace to see where the light is going in a particular plane?

A: Certainly. We have several 2D lamp models which you can find in the Lamps.xls spreadsheet, including a point source as well as T5 and T8 lamp cross sections. Just import these like any other lamp model, placing them where you want to see the raytrace. General guidelines are as follows: 10,000 rays, 100% update frequency, and turn on 3D ray display with at least 500 rays. Keep in mind that because this process is only meant for seeing ray paths, the candela curves and illuminance planes will not be relevant. Additionally, the Parametric Optical Design Tools (PODT) provide an excellent way to determine how reflector sections are aimed and easily change the aiming without redrawing things in CAD.

Q: What is the best value for the Spawn Limit when using materials that scatter light?

A: Although Photopia has the ability to “spawn” more than a single ray, we recommend that the Spawn Limit be left at its default value of 1 for all material types, including diffuse. Use the number of initial source rays to determine the level of detail of your analysis. For a given number of rays passing through your system, you will get a better sampling of reflection points when using 1 spawned ray, thus getting the best results in the shortest amount of time. Consider an example of sending 10 rays in a general direction toward a surface and spawning 1 ray for each reflection. You end up with 10 different reflection points and 10 reflected rays. However, if you use a spawn limit of 10 and initiate one-tenth the number of source rays, then you end up with 1 reflection point and 10 reflected rays. The latter was a faster analysis (at least if only 1 reflection was traced) however in a system that is attempting to model the real behavior of light with relatively few rays, it is preferred to model as many reflection points as possible. If more than 1 reflection is traced, then spawning generally results in a much slower analysis as well.

To further understand how a single ray spawn can model scattered light, think of light acting as a particle (a photon). When a single photon strikes a diffuse reflector material there are 2 possibilities for its fate. First, it may never reflect from the material. The probability of this happening is equal to the absorption of the material. If the photon is reflected, it retains its original magnitude and reflects in a single direction. The overall material reflectance is dictated by the ratio of reflected photons to the total number incident onto the surface. The reason why the material scatters light in all directions is simply because a large number of photons are directed onto it, each reflecting in unique directions. But if you directed only a single photon onto it, then it would not reflect light in all directions.

Photopia uses almost the same logic with its rays. The only change is that it reflects all rays (instead of absorbing some) and adjusts the magnitude of the ray lumen values upon reflection to account for material absorption. But it scatters in the same way, choosing a single direction probabilistically. When many rays are traced it produces the complete scattering effects of the material.

Results

Q: Can illuminance planes be placed inside of luminaires? If so, how is the light onto them calculated?

A: Illuminance planes can be placed within the geometry of the luminaire. Keep in mind that illuminance planes only accumulate lighting falling onto their "front" side. The "front" side is the side you are viewing as you think of the corner points as lower left, lower right and upper left. Illuminance planes will not block light if they are placed within the bounds of the optical geometry. In version 1.5, illuminance planes only accumulate light that is on its way out of the luminaire. In other words, Photopia only checks to see if rays intersect illuminance planes after it has determined that the rays will not intersect any other surfaces in the model. In version 2.0, intersections with the illuminance planes is checked while the light is still interreflecting within the luminaire, so they accumulate all light that falls onto them, not just the light that is exiting the luminaire.

Q: How are the luminaire luminance values calculated in the Photopia photometric report? Why are the average luminaire luminances in the Photopia photometric report lower than those shown on a report I received from a laboratory report?

A: The average luminaire luminance values are calculated by dividing the candela value at the view angle by the apparent area of the luminaire as viewed from that angle. Differences between predicted and measured average luminance values may be attributed to differences in the candela value and/or differences in the luminous area seen from the view angle. The most common source of major differences between predicted and measured values is due to differences in the luminous area of the luminaire. Check the width, length and height dimensions in the Photopia photometric report and see if the values match those in your physical test report. The dimensions shown in the Photopia report come from the IES file for the luminaire and are defined to be the "luminous" dimensions. By default, Photopia uses the maximum X, Y and Z dimensions of the luminaire CAD model to determine the luminous width, length and height, respectively. Photopia 1.5 allows you to override these values by entering luminous dimensions in the Photometric Report tab of the Photometric Output Specification screen. Since the physical dimensions of the luminaire do not always match the luminous dimensions, it is important to check these values and enter proper values when they differ. The most common difference is in the height of the luminaire. Many luminaires are recessed or have opaque sides, yet Photopia will use the physical Z dimension as the luminous height if you do not intervene. In version 1.1, you must wait until the analysis is finished, close Photopia and then edit the .IES file for the luminaire and change the dimensions as needed. Once the .IES file is changed, you need to delete the .RPT file for the luminaire. Once you open the project in Photopia again and view the report, the new dimensions will be used to calculate the average luminaire luminances. The .IES and .RPT files are located where you saved the project. See Appendix B of the User's Guide for details on the IES format.

Q: My measured photometry shows a much sharper cutoff than that measured by Photopia. Where does the discrepancy come from?

A: This discrepancy may come from the angular resolution of the photometry you have specified in Photopia. Real photometers measure the luminous intensity at specific points while Photopia counts the number of lumens that fall into a particular zone and calculates an average luminous intensity for that zone. These methods produce similar results for small angular zones. As a test, try decreasing your specified angular increments and increasing the number of rays. The angular settings may be changed in the Photometric Settings window. The number of rays may be increased in the Raytrace Settings window. If you continue to see a discrepancy, please contact Lighting Technologies.

Q: The photometric results for my luminaire do not make sense even though all of the surfaces in my model seem to be oriented properly. Sometimes rays go through reflector surfaces and other surfaces that are reflective absorb much more light than they should.

A: Check to see if you checked the Raytrace option to "Optimize for open luminaire". If you did and if your luminaire has surfaces that shadow each other as light goes from the lamp to the reflector surfaces or as light interreflects from within the reflector, then you cannot use this raytrace option. When Photopia traces rays in your reflector, it must check to see if a given ray intersects with each of the polygons that comprise your luminaire. Normally, Photopia will check all polygons for an intersection point because if a ray were extended to infinity, it may intersect more than one polygon. In such a case, Photopia uses the intersection that is closest to the ray emanation point as the proper intersection point. Since checking for all of these intersections takes time, the raytrace can be speeded up if, after Photopia finds the first intersection of the ray with a polygon, it stops and does not check for more polygon intersections with this ray. In this case the ray will simply react with the first polygon with which Photopia finds an intersection. While this is clearly not acceptable for all luminaires, it is fine in luminaires that are open (no shadowed surfaces), like if the luminaire were an open bowl type design.

Q: What is the definition of the luminaire Spacing Criterion?

A: Luminaire Spacing Criterion is a term defined by the Illuminating Engineering Society of North America (IESNA) that is intended to indicate the largest ratio of the luminaire spacing to the mounting height (Spacing / Mounting Height) that can be achieved and still have even illumination underneath an array of luminaires. This value is generally reported in directions along and across the lamp axes. Thus, values are generally reported for horizontal angles of 0 and 90 degrees, and sometimes beyond depending on the symmetry of the luminaire. The following steps are followed to calculate the spacing criterion (S.C.) in a given direction:

• First set an arbitrary mounting height such as 10ft.
• Calculate the illuminance directly underneath the luminaire at this mounting height using the inverse square cosine law.
• Then calculate the illuminance along a horizontal line that is below the luminaire at the defined mounting height and in the direction of interest (along or across the lamp axis). These values are also calculated using the inverse square cosine law. The distance between these points should be small, such as every 3 inches.
• Find the distance along the line at which the illuminance is equal to ½ the value directly under the luminaire. Then multiply that distance by 2 and divide it by the mounting height to get the S.C.. The point at which ½ of the center illuminance is achieved will receive the same amount of light from the next luminaire in the array, so that will result in the illuminance directly between 2 luminaires being equal. Note that this does not guaranty that the illuminance will be equal everywhere, just at the point directly under the luminaires and the points ½ way between.
• This calculation is repeated for the other directions.

Q: Can you explain the Lumen Summary in the results?

A: See the descriptions below for detailed explanations for the lumen summary results:

LUMENS EXITING SYSTEM

Lumens (and % of the total) that exit the luminaire with each Reflection of the analysis. The 0th reflection is light that exits the luminaire directly. The Reflection # would be more accurately listed as a “Reaction” #, since this really refers to each time a ray interacts with a surface, and the surface can be reflective, transmissive or refractive. This data is useful to indicate how quickly the light exits the luminaire. If the reflector is designed to get all of the light out in a single reflection, then this information will confirm or disprove that. The % exiting with the 0th reflection is interesting since that shows the direct component from the bare lamp. The rest of the light is therefore being controlled by the reflector. Note that no light will exit with the 0th reflection when a cover lens is in place. If the cover lens is a single transmissive surface, then the direct light from the lamp will be shown in the 1st reflection. If the cover lens is made from a solid model with an inner and outer surface, then the direct light won’t be seen until the 2nd reflection.

LUMEN INTERACTION WITH SYSTEM

Absorbed : The amount of light that is absorbed on each layer and by each material in the model. This data is useful to see which parts of your model are responsible for the bulk of the losses in the system. This therefore helps you prioritize which parts to improve with a more reflective material to increase the efficiency of the design. It is also useful when troubleshooting problems in the analysis. If your efficiency (LOR) is much lower than expected, then review this data and see where the light was lost. If an extraordinary amount of light was absorbed a one particular layer, then that’s likely the source of the problem. The surfaces on this layer could be oriented backwards, for example, resulting in the black side of the surface facing the lamp.

Incident Lumens : The 'Incident' light is the total amount of light that is incident upon all of the surfaces on the layer (or material). But note that the same lumens can come into contact with a surface multiple times on various reflections so these values can get greater than 100%. For example, a ray can leave the lamp and hit the reflector, then get reflected to another portion of the reflector, and so on, and each time that ray hits the surface, its lumen value is added to the total amount of incident light. This data is helpful when you want to know how much light goes through the bulb of a lamp or through its arc when the materials assigned to those components doesn't absorb much or anything. Also, in general these values tell you just how much each component in your model interacts with the light.

UNACCOUNTED LUMENS

Reached interreflection limit : This is the amount of light trapped in the luminaire because there were too few reflections specified to allow it to either become absorbed or exit the luminaire. This value therefore indicates if you have specified enough reflections in your analysis. You should specify enough reflections so that this value is 1% or less. You will then get a more accurate efficiency (LOR) prediction. Note that if you see 10% of your light lost in this category, then you can’t assume that with more reflections all of this light will exit. Allowed to interreflect more, some of this light will become absorbed. In fact, a good estimate of the amount that will eventually exit is to multiply this value by the current predicted efficiency of the luminaire.

Fell below continuation minimum : This is the amount of light lost in the analysis because the ray magnitudes were diminished to a point below the Magnitude Threshold in the Raytrace Settings. The default value for the Magnitude Threshold is 1%, so this means that when a ray interreflects enough times so that its magnitude is less than 1% of its initial value, then the ray will be terminated and its lumens added to the total in this category. If a significant amount of light (more than 1%) is lost in this category in your analysis, then you should specify a lower Magnitude Threshold and run the analysis again to obtain a more accurate efficiency (LOR) prediction.

Could not find in/out refractor facet : Light lost in this category is from rays that are incident onto the wrong side of refractive surfaces. If a refractor’s surfaces are oriented backwards for example (the “back” sides facing to the outside of the material), then all of the light incident onto the refractor will be lost in this category. Light will also be lost in this category if it enters a refractor through an opening in the mesh describing the refractor. For example, if a flat glass part is modeled with just a top and bottom surface, but no side surfaces, then any light entering the sides will be lost in this category. The same is true for light that enters through the top surface and exits out the sides. For light to interact with a refractor properly, it must enter though the “front” side of a refractive surface and exit out the “back” side of a refractive surface.

Reached try limit for scatter bounce : Light is lost in this category if there is a problem with the BRDF/BTDF data and Photopia cannot find a valid angle into which the ray can be scattered. This is generally associated with high incidence angle light. If you have an analysis with more than 1% of your light lost in this category, then the project should be sent to LTI for review.

Lost elsewhere (i.e. outside distribution) : This totals the amount of light that is directed toward angles that were not a part of the candela distribution. For example, if you specify a vertical angle set of 0 to 90, thus covering the lower hemisphere, yet some light exits your luminaire through a hole in the top that is not covered, then that light exiting out the top will be included in this category. This will not add to the efficiency (LOR) of the distribution. Note that in Type B photometry, no symmetry is assumed in the vertical angles, so you will see a significant amount of light lost in this category if you start your vertical angles at 0 degrees instead of –90. Vertical angle sets for Type B photometry should go from –90 to +90.

Converting 2D Part Drawings to 3D Models

Q: What is the procedure to convert my 2D part to a 3D model?

A: Follow these steps:

1) Import the 2D drawing containing the reflector profile by selecting File / Import CAD File from the main menu. The following steps assume that the 2D geometry is constructed in the XY plane and therefore imports into the XY plane of Photopia’s World Coordinate System (WCS). Photopia uses local coordinate systems called Construction Planes (CPlanes). The current CPlane is indicated in Photopia’s CAD system using a red line for the X-axis and a blue line for the Y-axis. The orientation of the WCS is always shown in the lower left corner of the CAD view with the same color conventions, while adding a green Z-axis. The origin (0,0,0) of the CPlane is indicated with a circle and cross hair. See Chapter 4 of the User’s Guide for more information about CPlanes. The image below shows the Photopia CAD screen in the Isometric View and illustrates the WCS icon in the lower left corner and the CPlane axes.

2) Define new layers in Photopia for 2D profiles and 3D surfaces for the various luminaire components, i.e. Profile-Main, Refl-Main, Profile-Louver, Refl-Louver, Profile-Lens, Tran-Lens, etc.

3) Delete extraneous geometry so that only the essential geometry remains that is required to make the 3D surfaces of the various luminaire parts, i.e. lines or polylines describing the reflector profile, lens profile and lamp holder locations. To delete entities in Photopia’s CAD system, you can select them individually by clicking on them or select by window or crossing by dragging a window from left to right or right to left, respectively. If all items are linked together in a block, then you can use the EXPLODE command to separate them.

4) Change the properties of the 2D entities so that they reside on the appropriate layers just created i.e. put the main reflector profile on layer Profile-Main. To change an entity’s layer, select it in the CAD view and change its layer in the property control on the right side of the screen. To de-select an entity, press the Escape key.

5) Move the entities (lines, arcs, etc.) that comprise the 2D profiles of the luminaire components so that the center of the luminaire opening is located at (0,0,0) in Photopia’s WCS. Type Move or “M” at the command line or choose Modify / Move from the main menu to start the Move command.

6) Use the ROTATE command to rotate the 2D profiles of the luminaire components so that the reflector opening is oriented toward the –Y axis, assuming the luminaire produces downward directed light. Rotate it toward the +Y axis if the luminaire produces upward directed light.

7) Change the view to an isometric view that allows the geometry to be seen in 3D. This can be done by right clicking your mouse in the CAD view and choosing Standard Views / Isometric View from the pop-up menu.

8) The 2D profiles need to be rotated into the Front View (World XZ plane) so that the 3D luminaire is constructed in the right plane. Photopia requires that the beam center for direct luminaires be toward the world –Z axis and the beam center for indirect luminaires be toward the +Z axis. The final step references more information about the required luminaire orientation. Since Photopia does not yet include a 3D rotate command, you will use the standard ROTATE command, which always rotates about the Z-axis of the current CPlane. Therefore, in order to rotate the reflector profile(s) about the world X-axis, the CPlane needs to be set to one of the side views. You will use the Right Side View, which is the world YZ plane. Select Settings / Construction Plane from the main menu, enter “O” for Ortho and “R” for Right. You will see the CPlane axes rotate so that its X-axis aligns with the world Y and its Y-axis aligns with the world Z.

9) Select all of the reflector profiles and start the ROTATE command. Enter 0,0 for the base point and 90 degrees for the rotation angle. The reflector should now be rotated into the world XZ plane as shown in the following image.

10) If the reflector is comprised of linear extrusions, then you will use the EXTRUDE command. This command extrudes the reflector along the Z-axis of the current CPlane, centering the extrusion about the location of the profile. If the reflector needs to be revolved, then you will use the REVOLVE command. This command revolves the reflector about either the X or Y axis or an arbitrary axis you select. Whether you are extruding or revolving your reflector, you need to change the CPlane to the Front View. So start the CPLANE command, then entering “O” for Ortho and “F” for Front.

11) If your reflector profile includes arc entities, then you need to set the resolution of the 3D mesh that will represent the arc. A higher resolution models the shape of the arc more smoothly. For extrusions, enter the SURFTAB1 command and specify the number of segments that will be used to represent it in the mesh. For revolved surfaces, the SURFTAB1 parameter represents the number of segments around the circumference of the mesh, i.e. the smoothness of the circle. The resolution of arc for revolved surfaces is set by the SURFTAB2 parameter.

12) If the reflector is a linear extrusion, then construct the end plates using either the POLYGON command or by drawing a horizontal line across the end of the reflector and then using the EXTRUDE command to create a flat plane. Note that POLYGONS in Photopia are triangles, so you will need to draw 2 to make a rectangular end plate. You can use the osnap features to help draw these entities.

13) Once the 3D reflectors have been constructed, then you need to ensure they are properly oriented. You can check a surface’s orientation by putting the CAD view in the Show Surface Orientation mode. This is an option under View Style on the pop-up menu you get by right clicking in the CAD view. Photopia renders the “front” side of surfaces in the color of its layer and the back side in the opposite color. The reflector materials are applied to the front side of materials and the back side is generally black, so it is important that they are properly oriented. View your model in this mode and orbit around it to confirm the surface orientations. If any of the meshes are oriented incorrectly, then use the Modify / Orient tool to fix them. Select the mesh, then run this tool and enter “R” for reverse.

14) Import the appropriate lamp model for the design by selecting File / Import Lamp on the main menu. Enter the location(s) for the lamp. You will be prompted for multiple locations, so just press Enter again at prompt when you are finished entering the locations you require. Use the rotate command in the appropriate CPlane if the lamp needs to be rotated into different orientation than its default.

15) See the FAQ topic on Photometric Conventions for more information about the proper orientation of the luminaire, which depends on the type of distribution it creates and your region of the world. Different regions use different standard conventions for photometry.