Index: /trunk/ppMerge/src/ppMergeScaleZero.c
===================================================================
--- /trunk/ppMerge/src/ppMergeScaleZero.c	(revision 13956)
+++ /trunk/ppMerge/src/ppMergeScaleZero.c	(revision 13957)
@@ -199,4 +199,22 @@
                         view->chip = j;
                         view->cell = k;
+
+			// This step applies the dark exptime / background relationship: F =
+			// polymomial(exptime).  Below, the input images are scaled by the
+			// output of this relationship.
+
+			// For devices with linear dark and no residual bias, this relationship
+			// is not defined in the configuration system, and the transformation
+			// is defined (when loaded by the concepts) to be F = exptime.  Thus,
+			// in this case, the resulting dark image consists of counts/sec.
+
+			// For devices with a different relationship, the transformation yields
+			// the expected number of counts in the given exposure, based on the
+			// recorded exptime.  Thus, in this case, the resulting dark image
+			// should have a median value of 1.0.
+
+			// note that these corrections must be applied differently when used to
+			// correct a science image.  
+
                         darktime->data.F32[i][cellNum] = pmFPADarkNorm(fpa, view,
                                                                        darktime->data.F32[i][cellNum]);