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Q&A: Variation in Max Candela value (Bin Size)

Question
I have found that the value of Max Candelas reported at the bottom of the Polar Iso-Candela Plot and the Rectangular CandelaDistribution are different. However, when smoothing is ON, the values are much closer. Why is there such a variation in the Max Candela value that is reported?

Synopsis
Variation in Max Candela value (Bin Size)

Solution
“Since a Candela is a unit of Power / Solid Angle, then the calculation of a Candela value is made by summing the flux (power) of all rays that fall within a specific Solid Angle and then dividing by that Solid Angle. The Max Candela value is then selected from the table of Candela values. If the distribution of rays is not even, then the size of the Solid Angle selected will make a significant difference in the Max Candela value obtained.

To compare this to making actual measurements on an optical bench, it is like measuring the power using a large detector or a small detector. The large detector will tend to average out the maximum and minimum values, whereas the small detector will show a larger variation between max and min.

For the TracePro Candela Plots, each plot is constructed of an array of “bins” or cells like a grid overlaying the plot. TracePro makes a candela calculation for each bin, and these are the values displayed on the plot. The max value is taken from these values and reported separately at the bottom of the plot. The factors that influence the Max Candela value reported are the size of these ” bins”, as well as the degree of smoothing that is applied to the plot.

Effect of Bin Size:
When comparing the Max Candela value from one TracePro Candela Plot to another, the Bin Size must be the same. The Bin Size for each Candela Plot can be determined from the Candela Options selected as follows:

Distribution Plots: Number of Bins = Plot Points value entered in Candela Options, Bin Size = Angular Width (deg) / # Plot Points
Iso Plots: Number of Bins is fixed at 128×128, Bin Size = 2 x Angular Width (deg) / 128

Effect of Smoothing:
Smoothing is a graphical tool that attempts to predict how a plot would change if more rays had been traced. With Smoothing OFF, the flux of each ray is spread over an infinitesimal area of the plot. Therefore, when viewing Max Candela values, a Smoothing OFF plot will tend to have a higher Max Candela value than the same plot with Smoothing ON. As more and more rays are traced, this difference should diminish.

One more consideration when comparing Max Candela Values is that the Candela Distribution Plots only represent a “slice” of the total data, where the Iso Candela Plots include the full angular field. If the angular distribution is not symmetrical, the peak intensity may not occur along the “slice” represented by the Candela Distribution Plot.”

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