diff --git a/tech/light_transport.wiki b/tech/light_transport.wiki
index 1451999..f7ea7ff 100644
--- a/tech/light_transport.wiki
+++ b/tech/light_transport.wiki
@@ -84,3 +84,36 @@ This method will output the _probability of a given outward direction_
 Something like `probability of happening = f(incoming direction, point, outgoing direction)`.
 
 This is known as the _Bidirectional reflectance distribution function (BRDF)_.
+
+
+What about materials that reflect some light allow some right through
+(transmitted).
+
+Properties of a transparent material
+
+* Helmholtz-reciprocity
+  * direction of the ray of light can be reveresd
+  * This means the odds of the light bouncing from A to B are the same as the
+    odds of boucing B to A
+* Positivity
+  * It is impossible for an exit direction to have a negative probability
+* Energy conservation
+  * An object may reflect OR absorb incoming light, but no more light can come
+    out than the incoming amount
+
+== Rendering equation ==
+
+Sum of all light reflections and absorbtions at a point
+
+Note that an object can emit light itself. It also receives light from
+different directions, which it will either reflect or absorb. Therefore
+
+`Light exiting = Emitted light + reflected incoming light`
+
+AKA
+
+{{{
+Light output(x, vector w) = Light emitted(x, vector w)
+  + Integral of (Light incoming to x from vector w * BRFD(vector w, x vector w
+  prime) cos theta dw
+  }}}