Today, most people in the graphic arts are aware that the light under which color originals is viewed has been standardized and specified (5000˚K (a.k.a D50) lighting - ISO 3664 - "Viewing and lighting standards"). However, this was not always so.
The "chromocritic" was developed by the Macbeth company of Philadelphia to solve this problem in an ingeniously practical way by assuring accurate interpretation of transparency colors.It contained two manually controlled light sources by which the client could select a certain temperature light at which the transparency pleased him.
Once "pleasing color" had been achieved, an indicating meter would identify the light settings that the chromocritic had been set to. The meter readings, along with the transparency, could then be sent to another ad agency or to the color etcher or lithographer. By setting up their chromocritic to the same readings they could be assured they were seeing the transparency the same way that the client had seen and approved it.
The etchers would then use their skills at modifying halftone dots through exposure and etching, combined with their knowledge of the color capability of the final print process to recreate on press, as closely as possible, the approved color that the client had seen on their chromocritic.