Ray Ancestry

This article introduces the concept of ray ancestry as it applies to both specular and scattering events. The conventions associated with ancestry are illustrated graphically.

Ray ancestry is associated with splitting of an incident ray into transmitted, reflected and/or scattered rays. Genealogical terms such as parent, child, grand-child, etc., or generation [0,1,2,..] are commonly used to describe this property. Control of this parameter is exercised in the settings of the Raytrace Controls.

When a ray is incident upon an interface between two materials of differing refractive index, it undergoes specular splitting into a reflected and transmitted ray if the angle of incidence is less than the critical angle. The parentage of these rays is determined by the Parent Ray Specifier option on the assigned Raytrace Control dialog. The default setting for this option is Largest incoherent power which is determined by the Coating specification. If the Coating is of type Uncoated, Thin Film, Quarterwave Layer, or General Sampled then angle of incidence will also play a part in determining which ray is the parent. The user has three other choices for Parent Ray Specifier: Transmitted [transmitted ray always parent], Reflected [reflected ray always parent] or Monte-Carlo (1 ray only) [probabilistic determination].

The picture below illustrates graphically the parentage of specular split rays incident on two interfaces under four distinct conditions when Largest incoherent power is selected. When both interfaces have R ≤ T as in Figure a, the parent ray (0) is transmitted along with even generations while the reflected rays are of odd generation. With R ≤ T on the first and R > T on the second as in Figure b, the parent ray is ultimately reflected and increasing generations both reflect and transmit. With R > T on the first and R ≤ T on the second as in Figure c, the parent ray is immediately reflected and increasing generations both reflect and transmit. Finally, with R > T on both interfaces as in Figure d, the parent is immediately reflect with all subsequent reflections and transmissions of generation two.

The following picture illustrates the ancestry of scattered rays as an incident ray reflects between two rough mirror surfaces. The blue rays are generation 0, specular reflection at both surfaces; the red rays are generation 1, scatter at M1 and specular reflection at M2; the green ray is generation 2, scatter at M1 then scatter at M2.

Limits on ray ancestry are controlled by the Ancestry Level Cutoff option on the Ray Control dialog. For all Raytrace Controls, the default specular ancestry is 2, default scatter ancestry is 1. This setting allows specular rays to split twice and one generation of scatter.

As a practical example of a specular ancestry cutoff setting, consider the fact that any lens system can give rise to reflections among various surfaces when illuminated by an external source such as the sun. This process is commonly referred to as ghosting. In order for these reflections to reach the image plane, an even number of reflections must be allowed (2,4,6,..) on the Raytrace Controls assigned to the lens surfaces. Two reflections is termed first order ghosting, four reflections is termed second order ghosting, etc.

Another practical example of specular ancestry cutoff arises in modeling the Fabry-Perot effect. While expressions for the transmittance and reflectance of a Fabry-Perot arise from an infinite summation of terms, limits on ray splitting necessarily lead to truncation of this sum. In situations corresponding to Figures a-c, the ancestry level directly determines how many terms are retained. On the other hand, situations corresponding to those in Figure d require only that the specular ancestry cutoff remain at its default value of 2. Summation is then truncated either when the Intersection Count Cutoff is exceeded or when ray fluxes fall below the Ray Power Cutoff Threshold.

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