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Number of pixels in Irradiance Maps, Plot Points in Candela Distribution Plots, and Smoothing

Question

When viewing an Irradiance Map or a Candela Plot, I find that changing the settings for Number of Pixels, Plot Points, or Smoothing causes the results to vary significantly. How do I know what are the proper settings, and how do I know if I have traced enough rays?

Synopsis

Solution

When analyzing a spatial distribution (Irradiance Map) or an angular distribution (Candela Plot) in TracePro, it is important to understand that the resolution of the plot will significantly influence the appearance of the map or plot. The goal of this Knowledge Base Article is to provide background information about how the maps and plots work, and guidelines for setting the resolution.

Number of pixels in Irradiance Maps

During the ray trace, the position and flux of each ray incident on the surface is stored. The Number of Pixels value on the irradiance/illuminance map determines how many pixels or bins there are across the map. The map is always square, and its dimension is chosen so that it tightly fits the boundary of the surface as projected onto the plane of the map.

The choice for the number of pixels or bin size depends on the problem to be solved. If the results of the Irradiance Map will be compared to a lab measurement, the pixel size of the Irradiance Map should match the pixel size of the detector in the lab. If the Irradiance Map is of a detector surface in the model, the pixel size should be set to match the pixel size of the detector in the model.

In the absence of a known pixel size for comparison, the general rule is that more pixels = higher resolution.

Plot Points in Candela Distribution Plots

The Polar and Rectangular Candela Distribution Plots have an option to set the number of Plot Points, this is analogous to the number of pixels in the Irradiance Map.

The Plot Point value n divides the data into n-1 bins over the angular range. When the angular width is set to 180 deg and the plot points are set to 4096, a resolution of 180/4095 = .04 deg is obtained.

The best choice for the Plot Point value in Candela Options is the one that achieves the same angular resolution as the lab measurement or design requirement. In the absence of a known angular resolution for comparison, the general rule is that more plot points = higher resolution.

Iso Candela Plots

The Polar and Rectangular Iso Candela Plots do not have an option to set the number of Plot Points, the number of bins is fixed as 128 x 128. The resolution of the Iso Candela Plots can be adjusted by changing the angular height and width of the map. For a map spanning +/- 10 deg, the resolution is 20 deg / 128 = .156 deg/bin

Smoothing

When smoothing is ON, the map is smoothed by convolving or spreading the array of flux bin values with a Gaussian shape. The waist of the Gaussian function is equal to the bin size.

Smoothing is useful when setting up a TracePro model and tracing just a few rays to see if the model is working as expected. The Irradiance Map or Candela Plot with Smoothing ON will give an approximation of the final result.

However, when it is time for the final ray trace, the final analysis, Smoothing should be OFF, so the plot is displaying the true distribution of the rays.

Determining when enough rays have been traced

If Smoothing is turned OFF and the plot looks noisy, this is not a sign to turn Smoothing ON again, rather it is a sign that more rays need to be traced.

The following actions will increase the resolution of the plot:

increasing the number of pixels in the Irradiance Map
increasing the number of Plot Points in the Polar or Rectangular Candela Distribution Plot
decreasing the angular height and width of the Polar or Rectangular Iso Candela Plot.

The tradeoff is that the plot will have fewer rays/bin, so it may appear noisy. This is a sign that more rays need to be traced.