Electrochromic

Electrochromic materials undergo pigment changes and color shifts by redox reactions in an electrochemical cell. By applying a small voltage and current, these cells can switch from transparent to pigmented, for example white or black. There is a wide range of materials and behaviors.

Mass produced examples include car mirrors and window light shutters for thermal control and privacy. The 787 Dreamliner passenger windows are electrochromically adjustable from clear to black.

Adjusting reflectivity changes light pressure. Changing from transparent to a perfect Lambertian white reflector changes the on-axis light pressure from 0 to ( 1 + 1/π ) ( P/c ). Off axis sideways thrust is proportional to sin( 2 θ ) , with a contribution from incident light but none from the reflective light.

Transition metal electrochromics can shift all the way from transparent to a mirror. The light pressure is 50% higher, and the off-axis sideways thrust from the mirror doubles.