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Monday, November 3, 2014

A Story from the Testing Lab: What is 100% on the ISO Brightness Scale?

Liquid-cooled Zeiss Elrepho
The original diffuse brightness tester, the Zeiss Elrepho, was introduced in 1953 by Carl Zeiss Company of Oberkochen, West Germany. The Elrepho (ELectro-REcording PHOtometer) with its integrating sphere (diffuse) geometry originally used magnesium oxide as the basis for the 100% brightness point just like the GE Brightness Instrument (directional geometry, TAPPI T452). This was chosen since it was the 'whitest' substance available at the time. Since magnesium oxide was very unstable and yellows over a relatively short period of time the basis for setting the 100% brightness point was changed from the reflectance of magnesium oxide to absolute reflectance which is a more theoretical 100% point in the mid-1960's.  This scale change caused all brightness readings to go down i.e. a pulp previously measured at 60% brightness might now read 58.5%.

Measurement on Color Touch
In the early 1970's, a gloss trap was included in the integrating sphere to exclude the specular gloss component.  The insertion of a gloss trap is an attempt to minimize the effect of surface reflectance, leaving only intrinsic brightness to be measured.  Brightness values may be lower by up to 2% due to the inclusion of a gloss trap.

These changes which occurred over a decade resulted in brightness values for the ISO Brightness instrument scale to go down over 2%.  Since that time ISO has developed a more rigorous process for maintaining this absolute reflectance 100% brightness calibration point. This 100% point is considered the perfectly reflecting diffuser.  Standardizing Laboratories transfer this calibration from the perfectly reflecting diffuser to the Authorized Laboratories.  Then Authorized Laboratories transfer this calibration to end-users of the instruments via paper standards which are issued on a monthly or quarterly subscription basis to allow brightness tester users to maintain close agreement with the perfectly reflecting diffuser.

ISO Standard 2469, which was based on the Zeiss Elrepho, was adopted by ISO in 1973.  TAPPI T525 is identical to ISO 2469 and ISO 2470-1 (ISO Brightness). Subsequently, brightness testers conforming to ISO 2469 have been manufactured by Datacolor, AutoElrepho, Minolta and Technidyne Corporation.

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