diff --git a/src/graphics/rendering/settings/default-startup-settings.ts b/src/graphics/rendering/settings/default-startup-settings.ts
index 311249b..42ffaa6 100644
--- a/src/graphics/rendering/settings/default-startup-settings.ts
+++ b/src/graphics/rendering/settings/default-startup-settings.ts
@@ -9,5 +9,6 @@ export const defaultStartupSettings: StartupSettings = {
   paletteSize: 256,
   ignoreWebGL2: false,
   backgroundColor: vec4.fromValues(1, 1, 1, 1),
+  lightPenetrationRatio: 0.75,
   enableStopwatch: false,
 };
diff --git a/src/graphics/rendering/settings/startup-settings.ts b/src/graphics/rendering/settings/startup-settings.ts
index 31f6a4c..b5def2d 100644
--- a/src/graphics/rendering/settings/startup-settings.ts
+++ b/src/graphics/rendering/settings/startup-settings.ts
@@ -24,6 +24,14 @@ export interface StartupSettings {
    */
   shadowTraceCount: number;
 
+  /**
+   * The illumination is multiplied by this constant where
+   * the distance field is negative (i.e. inside objects).
+   *
+   * Should be between 0 and 1, but other values are also permitted.
+   */
+  lightPenetrationRatio: number;
+
   /**
    * Gives the number of possible object colors for the scene.
    *
diff --git a/src/graphics/rendering/shaders/shading-fs-100.glsl b/src/graphics/rendering/shaders/shading-fs-100.glsl
index a277f40..34be6a7 100644
--- a/src/graphics/rendering/shaders/shading-fs-100.glsl
+++ b/src/graphics/rendering/shaders/shading-fs-100.glsl
@@ -2,7 +2,7 @@
 
 precision lowp float;
 
-#define INTENSITY_INSIDE_RATIO 0.5
+#define INTENSITY_INSIDE_RATIO {intensityInsideRatio}
 #define SHADOW_TRACE_COUNT {shadowTraceCount}
 
 {macroDefinitions}
diff --git a/src/graphics/rendering/shaders/shading-fs.glsl b/src/graphics/rendering/shaders/shading-fs.glsl
index 3567e28..27d5439 100644
--- a/src/graphics/rendering/shaders/shading-fs.glsl
+++ b/src/graphics/rendering/shaders/shading-fs.glsl
@@ -2,7 +2,7 @@
 
 precision lowp float;
 
-#define INTENSITY_INSIDE_RATIO 0.5
+#define INTENSITY_INSIDE_RATIO {intensityInsideRatio}
 #define SHADOW_TRACE_COUNT {shadowTraceCount}
 
 {macroDefinitions}