Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uflorida!gatech!rutgers!cmcl2!yale!coifman
From: coifman@yale.UUCP (Ronald Coifman)
Newsgroups: comp.graphics
Subject: Re: Ray tracing refraction
Message-ID: <49391@yale-celray.yale.UUCP>
Date: 1 Feb 89 20:33:05 GMT
References: <0XryqWy00Uo1875Ud-@andrew.cmu.edu> <101880003@hpcvlx.HP.COM>
Reply-To: musgrave-forest@yale.edu (F. Kenton Musgrave)
Organization: Yale University Computer Science Dept, New Haven CT
Lines: 45

In article <101880003@hpcvlx.HP.COM> tomg@hpcvlx.HP.COM (Thomas J. Gilg) writes:
>>> I'm trying to add refraction to a ray tracing program I'm writing in C. 
>
>> How do people (or do they) manage refraction when the index of
>> refraction is a function of the wave length of light and in a crystal
>> the direction of light with respect to the orientation of the crystal
>> itself.
>> The reason that this would be valuable is that to ignore it
>> life would be without rainbows and the fire of diamonds.
>
>Question : is the index of refraction linear with repect to the
>wavelength ?  I started writting a ray tracer and took liberty in
>assuming linear response.  Any experts out there ?

  As I had noted some weeks ago in this news group, my Masters thesis was 
on modelling dispersion (refraction as a function of frequency) so I am 
something of an expert on the topic.

  The answer to the above question is "no".  Dispersive power, or amount of 
change of index of refraction with frequency, is modulated by absorption
bands in the transparent material.  The index of refraction curves
asymptotically to infinity at the frequency of the absortion feature.
However, my practical experience has indicated that only an optical lens
designer or some such (overly sensitive!) person notices the difference
between a linear and a nonlinear dispersion curve model.  Besides, it's
not all that easy to get the empirical data for a good nonlinear dispersion
curve.

  A noteable exception to this is the rainbow, where the nonlinearity of the
dispersion curve of water has a very noticeable effect.

  For a thorough discussion of all this, see "Prisms and Rainbows: a Dispersion
Model for Computer Graphics" by F. K. Musgrave in the upcoming Graphics Inter-
face Conference proceedings (June of '89).  Or, as I offered before, email me
a request and I'll send you draft copies of that paper (and another on disper-
sion, if you like).  See also Spencer Thomas, "Dispersive Refraction in Ray
Tracing" in the Jan. '86 issue of Visual Computer (if you can find it!)

					Ken Musgrave
-- 
_____________________________________________________________________
Ken Musgrave			arpanet: musgrave-forest@yale.edu
Yale U. Math Dept.		
Box 2155 Yale Station		Primary Operating Principle:
New Haven, CT 06520				Deus ex machina