--- ray/doc/ray.1	2011/02/20 06:34:19	1.19
+++ ray/doc/ray.1	2011/10/17 21:53:55	1.24
@@ -898,6 +898,9 @@ If the BSDF has transmission and back-side reflection 
 a parallel BSDF surface may be
 placed slightly less than the given thickness away from the front surface
 to enclose the complex geometry on both sides.
+The sign of the thickness is important, as it indicates whether the
+proxied geometry is behind the BSDF surface (when thickness is positive)
+or in front (when thickness is negative).
 .LP
 The second string argument is the name of the BSDF file, which is
 found in the usual auxiliary locations.
@@ -1548,6 +1551,13 @@ the Ecole Polytechnique Federale de Lausanne (EPFL Uni
 in Lausanne, Switzerland.
 .NH 1
 References
+.LP
+Ward, G., R. Mistrick, E.S. Lee, A. McNeil, J. Jonsson,
+``Simulating the Daylight Performance of Complex Fenestration Systems
+Using Bidirectional Scattering Distribution Functions within Radiance,''
+.I "Leukos",
+7(4),
+April 2011.
 .LP
 Cater, K., A. Chalmers, G. Ward,
 ``Detail to Attention: Exploiting Visual Tasks for Selective Rendering,''