diff --git a/timetrace/README.md b/timetrace/README.md
index 04cc7943..87bc1d19 100644
--- a/timetrace/README.md
+++ b/timetrace/README.md
@@ -7,13 +7,17 @@ Test: sbt test
 Style: sbt scalastyle-generate-config / sbt scalastyle
 
 
+PLAN:
+
+Will start by doing all lighting (even direct) from photon map
+Raytrace out from camera will find first hit, then query PM
+Later extend this to recurse only down the direct (straight) path, in case of participating media
+
 
 TODO:
-* Rendering a sequence of frames
-* Shadowing
+* Participating media
 * Global illumination
 * Specular surfaces
-* Participating media
 * Movable surfaces
 * Movable lights
 * Movable camera
diff --git a/timetrace/src/main/scala/timetrace/RayHit.scala b/timetrace/src/main/scala/timetrace/Hit.scala
similarity index 69%
rename from timetrace/src/main/scala/timetrace/RayHit.scala
rename to timetrace/src/main/scala/timetrace/Hit.scala
index 979f31db..78f25a1e 100644
--- a/timetrace/src/main/scala/timetrace/RayHit.scala
+++ b/timetrace/src/main/scala/timetrace/Hit.scala
@@ -3,9 +3,10 @@ package timetrace
 import timetrace.math.Vector4
 import timetrace.material.Material
 import timetrace.shape.ShapeHit
+import timetrace.math.RayLike
 
-sealed case class RayHit(
-  val ray: Ray,
+sealed class Hit[R <: RayLike](
+  val ray: R,
   val shapeHit: ShapeHit,
   val material: Material) {
 }
diff --git a/timetrace/src/main/scala/timetrace/Ray.scala b/timetrace/src/main/scala/timetrace/Ray.scala
index 1c901a5c..c9360d4d 100644
--- a/timetrace/src/main/scala/timetrace/Ray.scala
+++ b/timetrace/src/main/scala/timetrace/Ray.scala
@@ -2,12 +2,10 @@ package timetrace
 
 import timetrace.math.Vector3
 import timetrace.math.Vector4
+import timetrace.math.RayLike
 
-sealed case class Ray(val start: Vector4, val direction: Vector4.SpatiallyNormalized) {
+class Ray(val location: Vector4, val direction: Vector4.SpatiallyNormalized) extends RayLike {
+  assume(direction.t == -1.0) // Photons only travel backward in time
 
-  override def toString: String = s"Ray($start -> $direction)"
-
-  def march(v: Double): Vector4 = {
-    start + direction * v
-  }
+  override def toString: String = s"Ray($location -> $direction)"
 }
diff --git a/timetrace/src/main/scala/timetrace/Raytrace.scala b/timetrace/src/main/scala/timetrace/Raytrace.scala
index bdd38a33..865b2ea7 100644
--- a/timetrace/src/main/scala/timetrace/Raytrace.scala
+++ b/timetrace/src/main/scala/timetrace/Raytrace.scala
@@ -4,26 +4,28 @@ import timetrace.light.Light
 import timetrace.math.Vector3
 import timetrace.math.MathUtils._
 import timetrace.math.Vector4
+import timetrace.math.RayLike
+import timetrace.photon.Photon
 
 class Raytrace(val scene: Scene) {
 
   def raytrace(ray: Ray): Color = {
     assert(ray.direction.t == -1.0)
 
-    val hit: Option[RayHit] = firstHit(ray)
+    val hit: Option[Hit[Ray]] = firstHit(ray)
     hit.map(calculateDirectLighting _).getOrElse(Color.BLACK)
 
   }
 
-  def firstHit(ray: Ray): Option[RayHit] = {
-    def pickClosest(a: RayHit, b: RayHit) = {
+  def firstHit[R <: RayLike](ray: R): Option[Hit[R]] = {
+    def pickClosest(a: Hit[R], b: Hit[R]): Hit[R] = {
       if (a.shapeHit.t < b.shapeHit.t) a else b
     }
 
     scene.things.flatMap(_.intersect(ray)).reduceOption(pickClosest _)
   }
 
-  def calculateDirectLighting(hit: RayHit): Color = {
+  def calculateDirectLighting(hit: Hit[Ray]): Color = {
 
     def contributionFromLight(light: Light): Color = {
       val hitLocation: Vector4 = hit.ray.march(hit.shapeHit.t)
@@ -41,4 +43,20 @@ class Raytrace(val scene: Scene) {
     scene.lights.map(contributionFromLight _).reduce(_ + _)
   }
 
+  def generatePhotons(): List[Photon] = {
+    assume(scene.lights.size == 1)
+
+    val light = scene.lights(0)
+
+    val photon: Photon = light.emitPhoton
+
+    val hit: Option[Hit[Photon]] = firstHit(photon)
+
+    hit.map(ph => {
+      val hitLocation = ph.ray.march(ph.shapeHit.t)
+
+      new Photon(hitLocation, ph.ray.direction, ph.ray.color)
+    }).toList
+  }
+
 }
diff --git a/timetrace/src/main/scala/timetrace/Renderer.scala b/timetrace/src/main/scala/timetrace/Renderer.scala
index 18e0168b..01a5380c 100644
--- a/timetrace/src/main/scala/timetrace/Renderer.scala
+++ b/timetrace/src/main/scala/timetrace/Renderer.scala
@@ -22,11 +22,13 @@ import timetrace.material.WhiteDiffuseMaterial
 
 object Renderer {
 
+  private val PHOTON_SCATTERING_PARTITIONS = 1000
+
   def main(args: Array[String]): Unit = {
     val camera: Camera = DefaultStillCamera
     val scene = new Scene( //
       List(Thing(new Plane(Vector3(0.0, 1.0, 0.0).normalize(), -1.0), WhiteDiffuseMaterial)), //
-      List(new SinglePulsePointLight(Vector3(0.0, 1.0, 0.0), Color.WHITE, 1.0)))
+      List(new SinglePulsePointLight(Vector3(0.0, 1.0, 0.0), Color.WHITE, 0.0, 1.0)))
 
     val downscale = 4
 
@@ -40,7 +42,9 @@ object Renderer {
     val sparkContext = new SparkContext(sparkConf)
 
     try {
-      val photons: RDD[Photon] = sparkContext.parallelize(1 to 1000, 1000).flatMap(generatePhotonBatch(job))
+      val photons: RDD[Photon] = sparkContext //
+        .parallelize(1 to PHOTON_SCATTERING_PARTITIONS, PHOTON_SCATTERING_PARTITIONS) //
+        .flatMap(generatePhotonBatch(job))
 
       val photonMap: Broadcast[PhotonMap] = sparkContext.broadcast(buildPhotonMap(photons.collect))
 
@@ -95,11 +99,11 @@ object Renderer {
   }
 
   def generatePhotonBatch(job: RenderJob)(n: Int): Seq[Photon] = {
-    Iterator.continually(generatePhotonsFromSingleEmission(job)).flatten.take(job.photonCount / 1000).toSeq
-  }
+    val raytracer: Raytrace = new Raytrace(job.scene)
+
+    val photonsToGenerate = job.photonCount / PHOTON_SCATTERING_PARTITIONS
 
-  def generatePhotonsFromSingleEmission(job: RenderJob): Seq[Photon] = {
-    List(Photon(null, null, null))
+    Iterator.continually(raytracer.generatePhotons()).flatten.take(photonsToGenerate).toSeq
   }
 
   def convertToImageFile(job: RenderJob)(frame: Frame): (Int, Array[Byte]) = {
diff --git a/timetrace/src/main/scala/timetrace/Thing.scala b/timetrace/src/main/scala/timetrace/Thing.scala
index 57f48f27..6f061429 100644
--- a/timetrace/src/main/scala/timetrace/Thing.scala
+++ b/timetrace/src/main/scala/timetrace/Thing.scala
@@ -2,10 +2,11 @@ package timetrace
 
 import timetrace.shape.Shape
 import timetrace.material.Material
+import timetrace.math.RayLike
 
 sealed case class Thing(val shape: Shape, val material: Material) {
 
-  def intersect(ray: Ray): Option[RayHit] = {
-    shape.intersect(ray).map(sh => RayHit(ray, sh, material))
+  def intersect[R <: RayLike](ray: R): Option[Hit[R]] = {
+    shape.intersect(ray).map(sh => new Hit[R](ray, sh, material))
   }
-}
\ No newline at end of file
+}
diff --git a/timetrace/src/main/scala/timetrace/light/Light.scala b/timetrace/src/main/scala/timetrace/light/Light.scala
index c3948933..a5910ec5 100644
--- a/timetrace/src/main/scala/timetrace/light/Light.scala
+++ b/timetrace/src/main/scala/timetrace/light/Light.scala
@@ -2,8 +2,11 @@ package timetrace.light
 
 import timetrace.math.Vector3
 import timetrace.Color
+import timetrace.photon.Photon
 
 trait Light extends Serializable {
   val location: Vector3
   def colorAtTime(t: Double): Color
+
+  def emitPhoton: Photon
 }
diff --git a/timetrace/src/main/scala/timetrace/light/SinglePulsePointLight.scala b/timetrace/src/main/scala/timetrace/light/SinglePulsePointLight.scala
index b97b6864..f9d26c1d 100644
--- a/timetrace/src/main/scala/timetrace/light/SinglePulsePointLight.scala
+++ b/timetrace/src/main/scala/timetrace/light/SinglePulsePointLight.scala
@@ -2,13 +2,17 @@ package timetrace.light
 
 import timetrace.Color
 import timetrace.math.Vector3
+import scala.collection.immutable.NumericRange
 
-class SinglePulsePointLight(val location: Vector3, val color: Color, val pulseDuration: Double) extends Light {
+class SinglePulsePointLight(val location: Vector3, val color: Color, val minT: Double, val maxT: Double) extends Light {
   def colorAtTime(t: Double): Color = {
-    if (0.0 <= t && t < pulseDuration) {
+
+    if (minT <= t && t < maxT) {
       color
     } else {
       Color.BLACK
     }
   }
+
+  def emitPhoton = ???
 }
diff --git a/timetrace/src/main/scala/timetrace/light/StaticPointLight.scala b/timetrace/src/main/scala/timetrace/light/StaticPointLight.scala
index 0d7fead3..db7e7c98 100644
--- a/timetrace/src/main/scala/timetrace/light/StaticPointLight.scala
+++ b/timetrace/src/main/scala/timetrace/light/StaticPointLight.scala
@@ -5,4 +5,6 @@ import timetrace.math.Vector3
 
 class StaticPointLight(val location: Vector3, val color: Color) extends Light {
   def colorAtTime(t: Double) = color
+
+  def emitPhoton = ???
 }
diff --git a/timetrace/src/main/scala/timetrace/math/RayLike.scala b/timetrace/src/main/scala/timetrace/math/RayLike.scala
new file mode 100644
index 00000000..bff68433
--- /dev/null
+++ b/timetrace/src/main/scala/timetrace/math/RayLike.scala
@@ -0,0 +1,11 @@
+package timetrace.math
+
+trait RayLike {
+  def location(): Vector4
+
+  def direction(): Vector4.SpatiallyNormalized
+
+  def march(v: Double): Vector4 = {
+    location + direction * v
+  }
+}
diff --git a/timetrace/src/main/scala/timetrace/photon/Photon.scala b/timetrace/src/main/scala/timetrace/photon/Photon.scala
index 82c7b815..ed5b77af 100644
--- a/timetrace/src/main/scala/timetrace/photon/Photon.scala
+++ b/timetrace/src/main/scala/timetrace/photon/Photon.scala
@@ -3,7 +3,9 @@ package timetrace.photon
 import timetrace.Color
 import timetrace.math.Vector3
 import timetrace.math.Vector4
+import timetrace.math.RayLike
 
-case class Photon(val location: Vector4, val direction: Vector3.Normalized, val color: Color) {
-
-}
\ No newline at end of file
+/// Represents light from a light source as it is inbound on an interacting thing
+case class Photon(val location: Vector4, val direction: Vector4.SpatiallyNormalized, val color: Color) extends RayLike {
+  assume(direction.t == 1.0) // Photons only travel forward in time
+}
diff --git a/timetrace/src/main/scala/timetrace/shape/Plane.scala b/timetrace/src/main/scala/timetrace/shape/Plane.scala
index 70620b25..18c4baaa 100644
--- a/timetrace/src/main/scala/timetrace/shape/Plane.scala
+++ b/timetrace/src/main/scala/timetrace/shape/Plane.scala
@@ -2,8 +2,8 @@ package timetrace.shape
 
 import timetrace.math.Vector4
 import timetrace.Ray
-import timetrace.RayHit
 import timetrace.math.Vector3
+import timetrace.math.RayLike
 
 /**
  * Set of points x, satisfying x dot normal == offset
@@ -12,8 +12,8 @@ class Plane(val normal: Vector3.Normalized, val offset: Double) extends Shape {
 
   override def toString = s"Plane($normal, $offset)"
 
-  def intersect(ray: Ray): Option[ShapeHit] = {
-    val t = (offset - (ray.start.truncateTo3 dot normal)) / (ray.direction.truncateTo3() dot normal)
+  def intersect(ray: RayLike): Option[ShapeHit] = {
+    val t = (offset - (ray.location.truncateTo3 dot normal)) / (ray.direction.truncateTo3() dot normal)
 
     if (t > 0.0 && t < Double.PositiveInfinity)
       Some(new ShapeHit(t, normal))
diff --git a/timetrace/src/main/scala/timetrace/shape/Shape.scala b/timetrace/src/main/scala/timetrace/shape/Shape.scala
index 1c1e8825..507d3b0a 100644
--- a/timetrace/src/main/scala/timetrace/shape/Shape.scala
+++ b/timetrace/src/main/scala/timetrace/shape/Shape.scala
@@ -1,10 +1,10 @@
 package timetrace.shape
 
 import timetrace.Ray
-import timetrace.RayHit
 import timetrace.math.Vector4
+import timetrace.math.RayLike
 
 trait Shape extends Serializable {
 
-  def intersect(ray: Ray): Option[ShapeHit]
+  def intersect(ray: RayLike): Option[ShapeHit]
 }
diff --git a/timetrace/src/test/scala/timetrace/RayTest.scala b/timetrace/src/test/scala/timetrace/RayTest.scala
index 62a90e81..5629d433 100644
--- a/timetrace/src/test/scala/timetrace/RayTest.scala
+++ b/timetrace/src/test/scala/timetrace/RayTest.scala
@@ -22,14 +22,14 @@ class RayTest extends UnitSpec {
       {
         val v = new Ray(start, direction)
 
-        v.start should equal(start)
+        v.location should equal(start)
         v.direction should equal(direction)
       }
     }
   }
 
   it should "have a nice toString" in {
-    val ray = Ray(
+    val ray = new Ray(
       Vector4(1.0, 2.0, 3.0, 4.0),
       Vector4(0.0, 1.0, 0.0, -1.0).spatiallyNormalize())
     ray.toString() should equal("Ray([1.0, 2.0, 3.0, 4.0] -> [0.0, 1.0, 0.0, -1.0])")
diff --git a/timetrace/src/test/scala/timetrace/RaytraceTest.scala b/timetrace/src/test/scala/timetrace/RaytraceTest.scala
index 5499d658..6559220b 100644
--- a/timetrace/src/test/scala/timetrace/RaytraceTest.scala
+++ b/timetrace/src/test/scala/timetrace/RaytraceTest.scala
@@ -24,7 +24,7 @@ class RaytraceTest extends UnitSpec with ColorMatchers {
       List(Thing(new Plane(Vector3(0.0, 0.0, 1.0).normalize, 0.0), WhiteDiffuseMaterial)),
       List.empty)
     val ray = new Ray(new Vector4(0.0, 0.0, 1.0, 0.0), new Vector4(0.0, 0.0, -1.0, -1.0).spatiallyNormalize())
-    val hit: Option[RayHit] = new Raytrace(scene).firstHit(ray)
+    val hit: Option[Hit[Ray]] = new Raytrace(scene).firstHit(ray)
 
     hit should be('defined)
   }
diff --git a/timetrace/src/test/scala/timetrace/shape/PlaneTest.scala b/timetrace/src/test/scala/timetrace/shape/PlaneTest.scala
index 2e5afd29..97538f27 100644
--- a/timetrace/src/test/scala/timetrace/shape/PlaneTest.scala
+++ b/timetrace/src/test/scala/timetrace/shape/PlaneTest.scala
@@ -8,7 +8,6 @@ import org.scalacheck.Gen
 import timetrace.math.Vector4Test
 import timetrace.RayTest
 import timetrace.Ray
-import timetrace.RayHit
 import timetrace.Generators
 import timetrace.RayTest
 import timetrace.math.Vector3
@@ -28,7 +27,7 @@ class PlaneTest extends UnitSpec {
     forAll(PlaneTest.planes, RayTest.rays) {
       (plane: Plane, ray: Ray) =>
         {
-          val currentSide = Math.signum((ray.start.truncateTo3 dot plane.normal) - plane.offset)
+          val currentSide = Math.signum((ray.location.truncateTo3 dot plane.normal) - plane.offset)
           val eventualSide = Math.signum(ray.direction.truncateTo3() dot plane.normal)
 
           val rayHit: Option[ShapeHit] = plane.intersect(ray)
@@ -45,7 +44,7 @@ class PlaneTest extends UnitSpec {
 
   it should "fail to intersect when rays are parallel" in {
     val plane = new Plane(Vector3(1.0, 0.0, 0.0).normalize, 0.0)
-    val ray = Ray(Vector4(1.0, 0.0, 0.0, 0.0), Vector4(0.0, 1.0, 0.0, 1.0).spatiallyNormalize())
+    val ray = new Ray(Vector4(1.0, 0.0, 0.0, 0.0), Vector4(0.0, 1.0, 0.0, 1.0).spatiallyNormalize())
 
     val rayHit = plane.intersect(ray)
   }