Light Dark Sky

Selecting Light Fixtures for Dark Sky Usage Using 3dop Pro

The following example illustrates some of the issues presented to specifiers when selecting fixtures suitable for dark-sky usage and also demonstrates some of 3dOP’s features.

Figures 1 and 4 illustrate a typical comparison between a non-shielded refractor type decorative area fixture and a full cutoff shoebox fixture. Notice how you can easily see the difference in light at and above 80 degrees and 90 degrees.

The cutoff classification is shown in the lower right hand corner of the display in case there is any doubt. The fixture’s lamp efficacy and maximum candlepower value with location is also displayed. As a side note, be careful when using the lamp efficacy value to select fixtures, as it can be misleading if a fixture was tested several years ago and the lamp lumens at the time of testing were less than the currently available initial lumens.

When looking at the relative intensity comparison of these two fixtures, the difference becomes even more apparent. You can see from the “thermal” type image that a large portion of the refractor type fixture’s output is not focused downward (see Figure 2). The cutoff fixture’s output is nicely distributed below 90 degrees. The fixture efficiency data along the bottom of the display further supports these conclusions (see Figure 5).

The absolute intensity module displays the actual footcandles output by each fixture. The non-cutoff fixture output is displayed in Figure 3 and the full cutoff fixture output is displayed in Figure 6. The horizontal output from the refractor type fixture and associated high glare can be easily seen. In comparison, the full cutoff fixture outputs more light on the ground plane and much less on the walls of the cube. Each fixture is placed at the cube center 10 feet from each wall.

After the Dark Sky tool is used to evaluate the fixture selection, a typical design can be laid out as shown in Figure 7. This design was done using the area display grid, with two fixtures 80 feet apart and 27 feet above the ground and the statistics mode turned on. A statistical area of 180’ x 110’ was created and the results displayed at the bottom of the display were used to quickly determine the necessary spacing to meet the design criteria. This screen illustrates how easily and quickly design criteria can be calculated.

Figures 1 and 4 illustrate a typical comparison between a non-shielded refractor type decorative area fixture and a full cutoff shoebox fixture. Notice how you can easily see the difference in light at and above 80 degrees and 90 degrees.

The cutoff classification is shown in the lower right hand corner of the display in case there is any doubt. The fixture’s lamp efficacy and maximum candlepower value with location is also displayed. As a side note, be careful when using the lamp efficacy value to select fixtures, as it can be misleading if a fixture was tested several years ago and the lamp lumens at the time of testing were less than the currently available initial lumens.

When looking at the relative intensity comparison of these two fixtures, the difference becomes even more apparent. You can see from the “thermal” type image that a large portion of the refractor type fixture’s output is not focused downward (see Figure 2). The cutoff fixture’s output is nicely distributed below 90 degrees. The fixture efficiency data along the bottom of the display further supports these conclusions (see Figure 5).

The absolute intensity module displays the actual footcandles output by each fixture. The non-cutoff fixture output is displayed in Figure 3 and the full cutoff fixture output is displayed in Figure 6. The horizontal output from the refractor type fixture and associated high glare can be easily seen. In comparison, the full cutoff fixture outputs more light on the ground plane and much less on the walls of the cube. Each fixture is placed at the cube center 10 feet from each wall.

After the Dark Sky tool is used to evaluate the fixture selection, a typical design can be laid out as shown in Figure 7. This design was done using the area display grid, with two fixtures 80 feet apart and 27 feet above the ground and the statistics mode turned on. A statistical area of 180’ x 110’ was created and the results displayed at the bottom of the display were used to quickly determine the ne

cessary spacing to meet the design criteria. This screen illustrates how easily and quickly design criteria can be calculated.

 

Sometimes, footcandles do not tell the whole story or are meaningless to a customer. As shown in Figure 8, the photorealistic display instantly gives the user a view of what the design will actually look like to determine if any changes need to be made.

 

About the Author

Ian R. Ibbitson, Vice President and General Manager of ALLSCAPE, has over twenty years experience in the lighting, entertainment and electronics industries. Dr. Ibbitson joined ALLSCAPE a Philips group brand in 1995 and is currently in charge of several commercial outdoor lighting divisions, including Quality Lighting, and Metrolux. Dr. Ibbitson has a BSc. in electrical/electronic engineering and a PhD in computer aided design from the University of Sunderland in England.

In Search of Pure Dark Skies