• In the design problem it is required to design a greenish-color mirror that maintains the same perceived specular reflected color, under D65 source.
• Polarization of averaged, angular range is from normal incidence to 60 degrees.
• In order to determine if the reflected color at oblique incidence is perceptually similar to the reflected color at normal incidence, each angular color difference is calculated from normal incidence (referred to as the “anchor color”) using a CIE color difference equation, CIEDE2000.
• Layer Thickness ≥ 3nm, Number of Layers ≤ 2000.
• Lightness: L* ≥ 30.
• Chroma: Cab* ≥ 100.
• Hue Angle: 120° ≤ h_ab ≤ 180°

n_sub = 1.52, n_inc = 1.0

Layer Materials:

F = 1.38

L = 1.45

B = 1.65

M = 1.8

T = 2.15

H = 2.45

Design tricks and approaches:

• The breaker condition and the most complicated part – minimization of the design total physical thickness;
• It can be done with standard Gradual Evolution, Needle Optimization techniques
  in OptiLayer; 
• A special additional experimental target for minimization of total thickness was implemented in OptiLayer.
• Additional simplex-type fine tuning based on GPU (Graphics Processor Unit) computations was applied. NVidia GTX 580 was used. GPU code was about 5-8 times faster than code running at two six-core Xeon 3.8 GHz CPUs.

Metal-Dielectric Solutions:

• Main problem with metal layers – if used at the beginning of computations they become too thick and “shield” the rest of the stack.
• To prevent this, special Trapping option of OptiLayer has been applied. 
• When layer thickness appears within trapping limits – it becomes constrained automatically!

A winning result: Design has 121 layers, total physical thickness of 4782.80003 nm, DE2000 = 0; 13 thin Ag layers.

Learn more: https://www.osapublishing.org/ao/abstract.cfm?uri=ao-53-4-A360&origin=search