diff --git a/doc/images/medium_heterogeneous_density_1000.jpg b/doc/images/medium_heterogeneous_density_1000.jpg
new file mode 100644
index 00000000..dc0badcf
Binary files /dev/null and b/doc/images/medium_heterogeneous_density_1000.jpg differ
diff --git a/doc/images/medium_heterogeneous_density_200.jpg b/doc/images/medium_heterogeneous_density_200.jpg
new file mode 100644
index 00000000..f177b326
Binary files /dev/null and b/doc/images/medium_heterogeneous_density_200.jpg differ
diff --git a/doc/images/medium_heterogeneous_density_40.jpg b/doc/images/medium_heterogeneous_density_40.jpg
new file mode 100644
index 00000000..044c482e
Binary files /dev/null and b/doc/images/medium_heterogeneous_density_40.jpg differ
diff --git a/src/integrators/photonmapper/photonmapper.cpp b/src/integrators/photonmapper/photonmapper.cpp
index 98ba423a..d7c01d5d 100644
--- a/src/integrators/photonmapper/photonmapper.cpp
+++ b/src/integrators/photonmapper/photonmapper.cpp
@@ -184,10 +184,13 @@ public:
 			Log(EDebug, "Global photon map full. Shot " SIZE_T_FMT " particles, excess photons due to parallelism: " 
 				SIZE_T_FMT, proc->getShotParticles(), proc->getExcessPhotons());
 
-			m_globalPhotonMap = proc->getPhotonMap();
-			m_globalPhotonMap->setScaleFactor(1 / (Float) proc->getShotParticles());
-			m_globalPhotonMap->build();
-			m_globalPhotonMapID = sched->registerResource(m_globalPhotonMap);
+			ref<PhotonMap> globalPhotonMap = proc->getPhotonMap();
+			if (globalPhotonMap->isFull()) {
+				m_globalPhotonMap = globalPhotonMap;
+				m_globalPhotonMap->setScaleFactor(1 / (Float) proc->getShotParticles());
+				m_globalPhotonMap->build();
+				m_globalPhotonMapID = sched->registerResource(m_globalPhotonMap);
+			}
 		}
 
 		if (m_causticPhotonMap.get() == NULL && m_causticPhotons > 0) {
@@ -212,10 +215,13 @@ public:
 			Log(EDebug, "Caustic photon map full. Shot " SIZE_T_FMT " particles, excess photons due to parallelism: " 
 				SIZE_T_FMT, proc->getShotParticles(), proc->getExcessPhotons());
 
-			m_causticPhotonMap = proc->getPhotonMap();
-			m_causticPhotonMap->setScaleFactor(1 / (Float) proc->getShotParticles());
-			m_causticPhotonMap->build();
-			m_causticPhotonMapID = sched->registerResource(m_causticPhotonMap);
+			ref<PhotonMap> causticPhotonMap = proc->getPhotonMap();
+			if (causticPhotonMap->isFull()) {
+				m_causticPhotonMap = causticPhotonMap;
+				m_causticPhotonMap->setScaleFactor(1 / (Float) proc->getShotParticles());
+				m_causticPhotonMap->build();
+				m_causticPhotonMapID = sched->registerResource(m_causticPhotonMap);
+			}
 		}
 
 		if (m_volumePhotonMap.get() == NULL && m_volumePhotons > 0) {
@@ -241,11 +247,12 @@ public:
 				SIZE_T_FMT, proc->getShotParticles(), proc->getExcessPhotons());
 
 			ref<PhotonMap> volumePhotonMap = proc->getPhotonMap();
-			volumePhotonMap->setScaleFactor(1 / (Float) proc->getShotParticles());
-			volumePhotonMap->build();
-	
-			m_bre = new BeamRadianceEstimator(volumePhotonMap, m_volumeLookupSize);
-			m_breID = sched->registerResource(m_bre);
+			if (volumePhotonMap->isFull()) {
+				volumePhotonMap->setScaleFactor(1 / (Float) proc->getShotParticles());
+				volumePhotonMap->build();
+				m_bre = new BeamRadianceEstimator(volumePhotonMap, m_volumeLookupSize);
+				m_breID = sched->registerResource(m_bre);
+			}
 		}
 
 		/* Adapt to scene extents */
@@ -311,7 +318,7 @@ public:
 			transmittance = rRec.medium->getTransmittance(mediumRaySegment);
 			mediumRaySegment.mint = ray.mint;
 			if (rRec.type & RadianceQueryRecord::EVolumeRadiance &&
-					(rRec.depth < m_maxDepth || m_maxDepth < 0))
+					(rRec.depth < m_maxDepth || m_maxDepth < 0) && m_bre.get() != NULL)
 				LiMedium = m_bre->query(mediumRaySegment, rRec.medium);
 		}
 
diff --git a/src/medium/heterogeneous.cpp b/src/medium/heterogeneous.cpp
index de9db8d5..5ed1291e 100644
--- a/src/medium/heterogeneous.cpp
+++ b/src/medium/heterogeneous.cpp
@@ -86,14 +86,21 @@ static StatsCounter earlyExits("Heterogeneous volume",
  *     }
  * }
  * 
+ * \renderings{
+ *     \medrendering{40}{medium_heterogeneous_density_40}
+ *     \medrendering{200}{medium_heterogeneous_density_200}
+ *     \medrendering{1000}{medium_heterogeneous_density_1000}
+ *     \caption{Renderings of an index-matched isotropic heterogeneous medium using different density multipliers (\lstref{hetvolume})}
+ * }
+ * 
  * This plugin provides a flexible heterogeneous medium implementation, which 
  * acquires its data from nested \code{volume} instances. These can be 
  * constant, use a procedural function, or fetch data from disk, e.g. using a 
  * memory-mapped density grid. See \secref{volumes} for details.
  *
  * Instead of allowing separate volumes to be provided for the scattering
- * absorption parameters \code{sigmaS} and \code{sigmaA} (as is done in
- * \pluginref{homogeneous}, this class instead takes the approach of 
+ * and absorption parameters \code{sigmaS} and \code{sigmaA} (as is done in
+ * \pluginref{homogeneous}), this class instead takes the approach of 
  * enforcing a spectrally uniform value of \code{sigmaT}, which must be
  * provided using a nested scalar-valued volume named \code{density}.
  *
@@ -106,6 +113,48 @@ static StatsCounter earlyExits("Heterogeneous volume",
  * which contains local particle orientation that will be passed to
  * scattering models that support this, such as a the Micro-flake or 
  * Kajiya-Kay phase functions.
+ *
+ * \vspace{4mm}
+ *
+ * \begin{xml}[label=lst:hetvolume,caption=A simple heterogeneous medium backed by a grid volume]
+ * <!-- Declare a heterogeneous participating medium named 'smoke' -->
+ * <medium type="heterogeneous" id="smoke">
+ *     <string name="method" value="simpson"/>
+ *
+ *     <!-- Acquire density values from an external data file -->
+ *     <volume name="density" type="gridvolume">
+ *         <string name="filename" value="frame_0150.vol"/>
+ *     </volume>
+ *
+ *     <!-- The albedo is constant and set to 0.9 -->
+ *     <volume name="albedo" type="constvolume">
+ *         <spectrum name="value" value="0.9"/>
+ *     </volume>
+ *
+ *     <!-- Use an isotropic phase function -->
+ *     <phase type="isotropic"/>
+ *
+ *     <!-- Scale the density values as desired -->
+ *     <float name="densityMultiplier" value="200"/>
+ *  </medium>
+ *
+ * <!-- Attach the index-matched medium to a shape in the scene -->
+ * <shape type="obj">
+ *     <!-- Load an OBJ file, which contains a mesh version
+ *          of the axis-aligned box of the volume data file -->
+ *     <string name="filename" value="bounds.obj"/>
+ * 
+ *     <!-- Reference the medium by ID -->
+ *     <ref name="interior" id="smoke"/>
+ *
+ *     <!-- If desired, this shape could also declare
+ *          a BSDF to create an index-mismatched 
+ *          transition, e.g.
+ *
+ *     <bsdf type="dielectric"/>
+ *     -->
+ * </shape>
+ * \end{xml}
  */
 class HeterogeneousMedium : public Medium {
 public:
diff --git a/src/medium/materials.h b/src/medium/materials.h
index 75d5c8d9..97a1acbe 100644
--- a/src/medium/materials.h
+++ b/src/medium/materials.h
@@ -110,8 +110,6 @@ static void lookupMaterial(const Properties &props, Spectrum &sigmaS, Spectrum &
 				sigmaA *= densityMultiplier;
 				if (eta)
 					*eta = matEntry->eta;
-				SLog(EInfo, "Setting sigmaS = %s, sigmaA = %s", 
-					sigmaS.toString().c_str(), sigmaA.toString().c_str());
 				return;
 			}
 			++matEntry;
diff --git a/src/volume/constvolume.cpp b/src/volume/constvolume.cpp
index bc54ded5..3263bdc2 100644
--- a/src/volume/constvolume.cpp
+++ b/src/volume/constvolume.cpp
@@ -128,6 +128,21 @@ public:
 		return m_float;
 	}
 
+	std::string toString() const {
+		std::ostringstream oss;
+		oss << "ConstantDataSource[value=";
+		if (m_type == Properties::EFloat)
+			oss << m_float;
+		else if (m_type == Properties::EPoint)
+			oss << m_vector.toString();
+		else if (m_type == Properties::ESpectrum)
+			oss << m_spectrum.toString();
+		else
+			Log(EError, "Invalid volume data type!");
+		oss << "]";
+		return oss.str();
+	}
+
 	MTS_DECLARE_CLASS()
 protected:
 	int m_type;
diff --git a/src/volume/gridvolume.cpp b/src/volume/gridvolume.cpp
index 18e7c55a..81d47432 100644
--- a/src/volume/gridvolume.cpp
+++ b/src/volume/gridvolume.cpp
@@ -263,7 +263,7 @@ public:
 							m_channels);
 				break;
 			default:
-				Log(EError, "Encountered a volume data file of unknown type!");
+				Log(EError, "Encountered a volume data file of unknown type (type=%i, channels=%i)!", type, m_channels);
 		}
 
 		m_volumeType = (EVolumeType) type;
@@ -578,6 +578,16 @@ public:
 		return 1.0f;
 	}
 
+	std::string toString() const {
+		std::ostringstream oss;
+		oss << "GridVolume[" << endl
+			<< "  res = " << m_res.toString() << "," << endl
+			<< "  channels = " << m_channels << "," << endl
+			<< "  aabb = " << m_dataAABB.toString() << endl
+			<< "]";
+		return oss.str();
+	}
+
 	MTS_DECLARE_CLASS()
 protected: 
 	FINLINE Vector lookupQuantizedDirection(size_t index) const {