With high/ low refractive index materials stacking, the quarterwave stack can then make high pass or low pass filters.

We can also use it to make bandpass filters simply by putting two quarterwave stacks (each a stack of quarterwaves at different wavelengths, say one at 500nm, one at 1000nm) one on top of the other.

The products can be used in exposure machine, inspection instruments, camera lens, and 3C products…, etc.