|Liquid-cooled Zeiss Elrepho|
|Measurement on Color Touch|
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.