Neutral Density of 1.0 log units that. is used in Brightness Sense Testing with the Amblyomewter or the Equalizer.  Testing is performed with the filter over one eye to create a 1.0 log unit defect fot the brain to neutralkize and then repeated witht the filter over the other eye.  When a patient has a defect, the filter adds to that defect which makes the defect appear larger and more obvious.  It is most useful to amphify small defects.