Optical element, mold, and optical device

An optical element having an antireflection effect for a predetermined range of use wavelength and formed of a transparent material having a predetermined range of refractive index corresponding to the use wavelength range, comprising:

• an incident plane and an emission plane;
• and
• a conical microstructure formed on at least one plane of the incident plane and the emission plane and comprising a first structure formed of a plurality of conical microprojections with an average height H1 and a second structure formed of a plurality of conical microrecesses with an average height H2,
• wherein the average height H1≦1/3*λL/nL and the average height H2≦1/3*λL/nL, wherein an average pitch P between the first structure and the second structure is H1/2≦P≦1/3*λS/nS and H2/2≦P≦1/3*λS/nS,
• wherein a lower limit of the use wavelength is defined as λS, an upper limit of the use wavelength is defined as λL, a lower limit of the refractive index corresponding to the use wavelength is defined as nS, and an upper limit of the refractive index corresponding to the use wavelength is defined as nL.