10
6.2Instructionsforreset tozero
Resetthedisplaybeforestartingaseriesof densitymeasurements. If
thepart of the subjectused forzeroingdoesnothavethelowest
density,anegativesign will appearwhilemeasuring lowerdensity.
Reset of transmission reading
§Inphotographicapplicationsyoumaydecideeithertomeasure
thedensityinrelationtotheabsolutelightortobase+fog. Thatis
an unexposed,butadevelopedareaoffilm.
-Ifyouzerotoabsolutelight(nofilminthelight pass), the
base+fog isdisplayedwithapositivevalue andthisvaluehasto
be subtractedfromall densityvalues.
-Ifyouzerotobase+fog, the densityvaluesofanyareawithinthe
negativecouldbereaddirect. The absolutevalueofbase+fog
couldalsobemeasured, whiletakingareadingtoabsolutelight
andignoringthenegativesigninthiscase
§Ifinthick glassplatesorfilterszeroingtothelightsourceis
required,then thesubject tobe readmust beplaced onthe
reading areasothatthelight sourceisnot coveredup,but the
measuring headrestsonthe platewhenthemeasuring headis
depressed.Thefreelightfluxisequivalent toabsolute-zero.
Reset of reflectivereading
§Thereisnototallyreflectiveand diffusingmateriali.e. thereisno
absolutewhite.Bariumsulphateisusedforzeroing,becauseit
reflectsupto99%of diffuseincident light,however, it isalso
sensitivetosoiling anddamage. Normally, the backofafibre
based paperissufficientforzeroing.
§Inphotography, zeroingisdoneonthebasefog, thatisan
unexposedbutdeveloped partof the subject.
§Ifoccasionallymaterialthickerthan2mm istobe measured,then
thematerialandthepaperusedforzeroingshouldbeplaced
underthemeasuringheadtogethertoallowfortheoffset inthe
reading geometry. Thishint can beignored, if you'reworkingwith
theremovedmeasuringhead(Opt.01).
