Breakthrough Photography Circular Polarizing Filters (CPOL)
The Breakthrough Photography X4 circular polarizer filters (CPOL) have an exceptionally neutral spectral weighting as can be seen in the three comparisons below (toggle). [The filters were previously designated X3, the X4 moniker does not indicate a change; it’s a trademark headache issue].
In particular, some polarizers transmit more green / yellow / red than blue. The Breakthrough Photography X4 circular polarizer filters maintain a nearly flat spectral transmission profile, meaning they generate a more neutral result that avoids color bias. The important range for visible-light photography is 420nm (violet)* to 650nm (extreme dark red, near infrared).
I’ve just received some CPOL filters for testing, and I’ll put them to use soon in some testing.
See also Breakthrough Photography X3 Neutral Density Filters and Breakthrough Photography Night Sky Filter.
* Most digital sensors utilize a bandpass filter around 420 nm, the 400-420nm range is of little importance with most cameras.
Graphs courtesy of Breakthrough Photography.