BRTDfunc for dichroic film? (was mixfunc for dichroic film?)

You can download and view an information sheet describing the characteristics of a specifically engineered dichroic film at:

Basically, the film is spectrally selective, and only transmits light at very specific wavelengths, and reflects all other wavelengths as if it were a mirror. Depending on the incident angle of light in relationship to the surface, a different wavelength (within the band that the film transmits) is transmitted. For instance, imagine a white piece of paper directly below a light source, and you were holding a dichroic film between the light and the paper. As you incline and decline the film, the color transmitted through the film changes (in the specific example at the url above, from purple to blue to purple). Now imagine that you are looking through the film out a window, holding the film perpendicular to the direction of view. As you rotate the film about either the z-axis or the y-axis, the scene shifts from purple to blue to purple, AND the greater the angle of rotation, the more reflective the material appears, until at a certain angle, the film behaves almost like a perfect mirror.

Thus, I was hoping that in Radiance (since it is a ray-tracing program), information regarding the direction and angle of the light, and direction and angle of view is probably already calculated. If we could use this information to determine if-then switches, then a material such as dichroic films which exhibit different behavior under different conditions could be modeled.

I hope I've been clear. Any insight into if this would be possible using BRTDfunc or mixfunc or any other func would truly be appreciated.



ps. Dichroic films are not yet a commonplace architectural material. However, the premise behind trying to model this material is to speculate and experiment with how such a material could be used (whether for aesthetic and/or energy purposes) in an architectural context without the expense and cost (time and money) of a full-scale mock-up. Basically, behavior this material exhibits in a scaled physical model is not indicative of how it will behave in a real environment, and thus Radiance seems to be the perfect tool to explore this material.


Jack de Valpine wrote:

Message: 5
Date: Sun, 29 Feb 2004 23:29:09 -0500
From: Jack de Valpine <>
Subject: Re: [Radiance-general] mixfunc for dichroic film?
To: Radiance general discussion <>
Message-ID: <>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

Hey John,

My apologies, although I seem to remember somebody somewhere talking
about dichoic glazing in radiance (albeit a long time ago), after having
followed my own advice to check in the archives, I have not been able to
find anything. In any event, I think that something like BRTDfunc or
perhaps transfunc will do what you want. Do you have any performance
data that you can share so people can get a better sense of the
performance characteristics of the material.


Hi John,

Judging from what you've said and the spec. sheet for dichroic film, the BRTDfunc seems to be your best bet. You can vary reflectance with angle and wavelength (RGB color) using any function or data you have. Since we're just talking about the mirror and transmitted directions, the BRTDfunc gives you complete control and should deliver accurate and efficient simulations.