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.
* 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.