diff --git a/src/osgVolume/Shaders/volume_iso_frag.cpp b/src/osgVolume/Shaders/volume_iso_frag.cpp
index 692ffed17..fe485884e 100644
--- a/src/osgVolume/Shaders/volume_iso_frag.cpp
+++ b/src/osgVolume/Shaders/volume_iso_frag.cpp
@@ -104,9 +104,9 @@ char volume_iso_frag[] = "uniform sampler3D baseTexture;\n"
                          "                vec3 normal = normalize(grad);\n"
                          "                float lightScale = 0.1 +  abs(dot(normal.xyz, eyeDirection))*0.9;\n"
                          "\n"
-                         "                color.x *= lightScale;\n"
-                         "                color.y *= lightScale;\n"
-                         "                color.z *= lightScale;\n"
+                         "                color.x = lightScale;\n"
+                         "                color.y = lightScale;\n"
+                         "                color.z = lightScale;\n"
                          "            }\n"
                          "            \n"
                          "            if (color.w>1.0) color.w = 1.0; \n"