Compound Parabolic Concentrator (CPC)
A CPC is defined by the following parameters:
| Parameter # | Description | Face Name | Face # |
| 1 | The radial aperture at z = 0, a. | Side Faces | 0 |
| 2 | The maximum acceptance angle in degrees, θ . | Side Faces | 0 |
| 3 | The length along the local Z axis, L. | Side Faces | 0 |
| 4-5 | Unused. | NA | NA |
| 6 | The "Is Volume?" flag. | Side Faces | 0 |
| 7 | The volume index of refraction. | Side Faces | 0 |
A CPC is used to concentrate light entering one end of the CPC to the other end. Only rays that make an angle less than the acceptance angle with respect to the local Z axis will pass through the CPC; other rays will reflect back out. This type of CPC is the "Basic CPC" as described in detail in "High Collection Nonimaging Optics" by W. T. Welford and R Winston, Academic Press (1989).
If the "Is Volume?" flag is zero, then the object is a hollow shell. Otherwise, the object is a closed solid volume. If the CPC is a closed volume, it may be made of a refractive material. The reference index of the material can be entered for parameter 7.
The index called the "Volume Index" will be used to rescale the acceptance angle (via Snell's law) to take into account the material of the CPC.
The refracted acceptance angle, 
, is given by: 
Where 
is the acceptance angle, 
is the volume index.
This refracted acceptance angle will be used to determine the CPC shape and maximum length. This means that the volume index controls the shape of the CPC.
The volume index does not automatically pick up from the index of the material used for the CPC. It's the material index that determines the index that the rays see. In most cases, the user will want to select a wavelength and determine the index of the CPC material at that wavelength. That index can be entered as the "Volume Index" for the CPC. The shape and acceptance angle of the CPC will then be correctly matched to the material used to model the CPC.
The maximum value for the CPC length is given by:
Longer lengths will be truncated to this value. The radial coordinate of points on the CPC as a function of the z coordinate along the axis is given by the positive real root of this quadratic equation:
Where
The acceptance angle must be between 0.0 and 89.0 degrees, inclusive. Face Numbers: Front face Face 1, rear face Face 2, outer faces Face 0.
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