A dichroic mirror includes a substrate, a first film stack, and a second film stack. The first film stack is deposed on the substrate and has a structure represented by a first formula: (2HL)n. The second film stack is deposed on the first film stack and has a structure represented by a second formula: (0.325H0.65L0.325H)m. ‘2HL’ and ‘0.325H0.65L0.325H’ respectively represent a double-layer sub-structure of the first film stack and a sandwiched sub-structure of the second film stack, and ‘n’ and ‘m’, which are integers, respectively represent the number of repetitions of the double-layer sub-structures and sandwiched sub-structures ‘H’ and ‘L’ respectively represent a high refractive index layer and a low refractive index layer, both of which have a height equal to ¼ of the reference wavelength of the dichroic mirror, and ‘2’, ‘0.325’ and ‘0.65’ are height coefficients.