Use measured glass IOR for transparent-machines scenes.

The good news? Subtle dispersion effects.
The bad news? Lingering chromatic noise at object edges, even at very high spp.

transparent-machines/frame1266.pbrt

@@ -26,7 +26,7 @@ AttributeBegin
         "string filename" [ "textures/sky.exr" ]
         "float scale" [ 6 ]
 AttributeEnd
-Material "dielectric"
+Material "dielectric"  "spectrum eta" "glass-BK7"
 
 AttributeBegin
     ObjectBegin "single arm 0"

transparent-machines/frame542.pbrt

@@ -27,7 +27,7 @@ AttributeBegin
         "float scale" [6]
         "string filename" "textures/sky.exr"
 AttributeEnd
-Material "dielectric"
+Material "dielectric" "spectrum eta" "glass-BK7"
 
 AttributeBegin
     ObjectBegin "single arm 0"

transparent-machines/frame675.pbrt

@@ -27,7 +27,7 @@ AttributeBegin
         "float scale" [6]
         "string filename" "textures/sky.exr"
 AttributeEnd
-Material "dielectric"
+Material "dielectric"  "spectrum eta" "glass-BK7"
 
 AttributeBegin
     ObjectBegin "single arm 0"

transparent-machines/frame812.pbrt

@@ -27,7 +27,7 @@ AttributeBegin
         "float scale" [6]
         "string filename" "textures/sky.exr"
 AttributeEnd
-Material "dielectric"
+Material "dielectric"  "spectrum eta" "glass-BK7"
 
 AttributeBegin
     ObjectBegin "single arm 0"

transparent-machines/frame888.pbrt

@@ -28,7 +28,7 @@ AttributeBegin
         "float scale" [6]
         "string filename" "textures/sky.exr"
 AttributeEnd
-Material "dielectric"
+Material "dielectric" "spectrum eta" "glass-BK7"
 
 AttributeBegin
     ObjectBegin "single arm 0"