An optical element has a plano-plano body formed of a first material having a greater refraction index n1 and a second material having a lesser refraction index n2. Both indices are greater than one. The absolute value of the index contrast, log10 (n1/n2), is in the range from about 0.001 to about 0.17, preferably from about 0.01 to about 0.05. The materials have an induced absorbance rate ΔAbs/Dose less than or equal to about 0.4, preferably less than or equal to about 0.2. The materials are arranged such that an interface with at least one cusp is defined therebetween. The cusp has an apex pointed toward the material having the greater index of refraction. The cusp is operative to produce a region of increased light intensity on one surface of the optical element in response to light incident on the other surface of the optical element.