add surface model documentation

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diff --git a/doc/source/index.rst b/doc/source/index.rst
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@@ -44,6 +44,7 @@ Subscribe to the mailing list for update announcements!
    install
    tour
    geometry
+   surface
    detector
    render
    simulation
diff --git a/doc/source/surface.rst b/doc/source/surface.rst
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+Surface Models
+==============
+
+Chroma includes a variety of surface models, which control the interaction of photons with objects. The ``Surface`` class contains many (typically wavelength-dependent) surface parameters, some of which apply to each surface model.
+
+To select a particular model for a surface, set ``surface.model = MODEL`` where ``MODEL`` is one of ``{ SURFACE_DEFAULT, SURFACE_SPECULAR, SURFACE_DIFFUSE, SURFACE_COMPLEX, SURFACE_WLS }``.
+
+``SURFACE_DEFAULT``
+-------------------
+
+The default surface model approximates real surfaces by combining a diffuse and specular reflection component. Surfaces may also be absorbing and detecting. The wavelength-dependent probabilities for detection, absorption, diffuse reflection, and specular reflection are provided in uniformly-spaced float arrays of length ``n``, and should sum to unity. The corresponding wavelengths for these arrays are computed from the first (``wavelength0``) and the step size (``step``).
+
+Parameters::
+
+    float *detect
+    float *absorb
+    float *reflect_diffuse
+    float *reflect_specular
+    unsigned int n
+    float step
+    float wavelength0
+    
+``SURFACE_SPECULAR``
+--------------------
+
+A perfect specular reflector. Behavior is identical to ``SURFACE_DEFAULT`` with 100% specular reflection at all wavelengths, but is marginally faster. This model has no parameters.
+
+``SURFACE_DIFFUSE``
+-------------------
+
+A perfect diffuse reflector. Behavior is identical to ``SURFACE_DEFAULT`` with 100% diffuse reflection at all wavelengths, but is marginally faster. This model has no parameters.
+
+``SURFACE_COMPLEX``
+-------------------
+
+This surface model uses a complex index of refraction (``eta``, ``k``) to compute transmission, (specular) reflection, and absorption probabilities. Surfaces may also be photon detectors, but the detection probability is conditional on detection (``detect`` should be normalized to 1). If a surface is not transmissive (``bool transmissive``), transmitted photons are absorbed.
+
+This model accounts for incoming photon polarization in its calculations.
+
+Parameters::
+
+    float *detect
+    float *eta
+    float *k
+    unsigned int n
+    float step
+    float wavelength0
+    float thickness
+    bool transmissive
+
+``SURFACE_WLS``
+---------------
+
+A model of wavelength-shifting surfaces. Surfaces may absorb and reflect (these probabilities should sum to 1). Reflection is diffuse. If a photon is absorbed, it may be reemitted with a probability given in ``float *reemit`` (normalized to 1). The reemission spectrum CDF is defined in ``float* reemission_wavelength`` (x) and ``float* reemission_cdf`` (y), both of length ``reemission_n``. The CDF must start at 0 and end at 1. This model does not enforce conservation of energy, and cannot reemit multiple photons!
+
+Parameters::
+
+    float *absorb
+    float *reflect
+    float *reemit
+    float *reemission_wavelength
+    float *reemission_cdf
+    unsigned int n
+    unsigned int reemission_n
+    float step
+    float wavelength0
+