added lux comment

examples/webgl_pmrem_test.html

@@ -122,6 +122,11 @@
                     .setPath( 'textures/equirectangular/' )
                     .load( 'spot1Lux.hdr', function ( texture ) {
 
+                        // This HDR environment map is expressed in nits (lux / sr). The directional light has units of lux,
+                        // so to match a 1 lux light, we set a single pixel with a value equal to 1 divided by the solid 
+                        // angle of the pixel in steradians. This image is 1024 x 512, 
+                        // so the value is 1 / (pi / 512) ^ 2 = 26,560 nits.
+
                         var pmremGenerator = new PMREMGenerator( renderer );
                         radianceMap = pmremGenerator.fromEquirectangular( texture ).texture;
                         pmremGenerator.dispose();