Source Ellipse
The Source Ellipse is a flat elliptical (and optionally annular) surface which emits rays. Although the origin of each ray launched lies on the surface of the ellipse with a uniform distribution, the distribution of the ray directions may be any one of these:
a) All rays appear to emit from a point placed anywhere relative to the source. The location of this point is defined in the parameter list below. When used in this mode, the source acts like an imaging point source.
b) Rays emit in a cosine distribution of the form
where the exponent Cn may be any value between zero and 100, including non-integer values, and θ is the angle measured from the local surface normal. The larger Cn, the narrower the distribution becomes. If Cn = 1.0, the distribution is Lambertian.
Note this distribution is rotationally symmetric about the local Z axis.
c) Rays emit in a Gaussian distribution of the form
where l and m are the direction cosines of the ray in the X and Y axis directions and Gx and Gy are constants. This form may be used to define a far field pattern that is different in the X or Y directions. The larger Gx and Gy are, the narrower the distribution becomes in the respective directions.
The nature of the ray distribution is defined by the parameter values. If Cn, Gx, and Gy, are all zero, then all rays will appear to emit from a virtual point source. If Cn is 1.0 or greater, then the cosine distribution will result (no matter how the source distance or Gx or Gy are set). If Cn is zero, but either Gx and Gy is non-zero, then a Gaussian distribution will result.
The parameters used to define this source are:
| Parameter # | Definition |
| 1-5 | See " Parameters common to all source objects " . |
| 6 | X Half Width: The x half width in lens units. |
| 7 | Y Half Width: The y half width in lens units. |
| 8 | Source Distance: The distance along the local z axis from the apparent source point to the location of the source object. This value may be positive or negative. If zero, the rays are collimated. If positive, the apparent source point is behind the object. Considered only if Cn, Gx, and Gy are all zero. |
| 9 | Cosine Exponent: The power on the cosine term. This is Cn in the cosine distribution expression above. The ray cosines do not support Sobol sampling when this parameter is not zero. |
| 10 | Gaussian Gx: The X term in the Gaussian distribution. Ignored if Cn is not zero. The ray cosines do not support Sobol sampling when this parameter is not zero. |
| 11 | Gaussian Gy: The Y term in the Gaussian distribution. Ignored if Cn is not zero. The ray cosines do not support Sobol sampling when this parameter is not zero. |
| 12 | Source X: The X coordinate of the point that emits the rays. If Source Distance is zero, then the Source X parameter is the X direction cosine of the collimated ray bundle. Considered only if Cn, Gx, and Gy are all zero. |
| 13 | Source Y: The Y coordinate of the point that emits the rays. If Source Distance is zero, then the Source Y parameter is the Y direction cosine of the collimated ray bundle. Considered only if Cn, Gx, and Gy are all zero. |
| 14 | Min X Half Width: The minimum x half width in lens units. Use a value greater than zero and less than the X Half Width to define an annular region. The ray coordinates do not support Sobol sampling when this parameter is not zero. |
| 15 | Min Y Half Width: The minimum y half width in lens units. Use a value greater than zero and less than the Y Half Width to define an annular region. The ray coordinates do not support Sobol sampling when this parameter is not zero. |
Use with LightningTrace
If the source is modeled as an imaging point source, then the X and Y Half Widths (parameters 6, 7) and the Source X and Source Y values (parameters 12, 13) contribute to the calculation of the angular distribution for the source; otherwise these parameters have no impact on the LightningTrace results. The size and spatial distribution of the source is neglected, and thus the Min Half Widths in X and Y (parameters 14, 15) are treated as zero by LightningTrace. Note that collimated input beams cannot be modeled by LightningTrace; Lightning- Trace will return zero flux for the beam if the Source Distance (parameter 8), Cosine Exponent (parameter 9), and Gaussian terms in X and Y (parameters 10, 11) are all zero.
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