a structure for reducing internal reflections in an optical system includes a stack of layers including a first layer having a first refractive index , a second layer having a second refractive index , a third layer having a third refractive index , a fourth layer having a fourth refractive index , and a fifth layer having a fifth refractive index . the second layer is arranged between the first layer and the third layer , and the fourth layer is arranged between the third layer and fifth layer . further , the third refractive index is greater than the second and fourth refractive indexes , the second refractive index is greater than the first refractive index , and the fourth refractive index is greater than the fifth refractive index .