Tiffen filters are manufactured using ColorCore technology, a closely guarded proprietary process that entails permanently laminating the filter material in between two pieces of optical glass that are ground flat to tolerances of a ten-thousandth of an inch, then mounting them in precision metal rings.
The ColorCore process allows Tiffen to control the color and density of Tiffen filters, and the characteristics of special effects filters with much greater accuracy than typical dyed-in-the-mass filters, which usually exhibit color and density variations. When Tiffen filters are ground and polished, the Colorcore is unaffected, so color and density remain uniform. Indeed, when you hold a group of Tiffen filters of the same type against a white sheet of paper, they all look identical -- because they are! Try this with or competitor's filters and you're likely to see slight variations. Maybe that's why most top Hollywood motion picture studios rely on Tiffen filters for their multimillion-dollar productions, or why Tiffen can custom-make an exact replacement for them if one breaks on the job. That's consistency and excellence - two pretty good reasons why, when it comes to filters, you should rely on Tiffen too.
Tiffen 81 Series filters are useful with daylight corrected film in cool light conditions. As letters advance, density and warmth increases.