don't subtract the background from the background image

lib/imaging.py

@@ -193,7 +193,8 @@ class FluorescenceData(ImagingData):
             self.results = dict() if self.results is None else self.results
             self.results.update({
                 label: {
-                    "N": float(N) - float(N_bkgd),
+                    "N": float(N) - float(N_bkgd)
+                        if label != "background" else float(N),
                     "sx": float(sx),
                     "sy": float(sy)
                 }