1. Field of the Invention
This invention relates to a dichroic mirror to be suitably used for an optical apparatus or an optical telecommunication apparatus such as a camera, a copying machine or an optical printing machine.
2. Related Background Art
Conventional optical systems of optical apparatuses and optical telecommunication apparatuses typically include a dichroic mirror that reflects light in a specific waveband and transmits light in a different waveband. Such a dichroic mirror comprises a dielectric multilayer film obtained by alternately laying high refractive index films and low refractive index films, each having an optical thickness (refractive index n.times.film thickness d) of .lambda./4, where .lambda. is the designed central wavelength.
Conventional dichroic mirrors normally comprise a relatively large number of component layers for the dielectric multilayer film to increase the reflectivity in the reflection waveband. However, as the number of layers increases, the ripple generated in the transmission waveband (fluctuation of transmission loss generated due to transmittance varying depending on wavelength) increases its influence to reduce the transmittance in the transmission waveband. FIG. 1 shows a graph typically illustrating the spectral transmittance characteristics of a known dichroic mirror. It will be appreciated from FIG. 1 that the transmittance is reduced in the transmission waveband by the ripples generated in that band. In FIG. 1, the horizontal axis represents the wavelength and the vertical axis represents the transmittance (%).