Patent ID: 7298687

Claim:
An optical recording media objective lens for converging working lights of a first wavelength and a second wavelength at predetermined points onto a first optical recording medium and a second optical recording medium, respectively, the working light of the first wavelength being converged at a first numerical aperture onto the first optical recording medium and the working light of the second wavelength being converged at a second numerical aperture onto the second optical recording medium for recording or reproducing information, said optical recording media objective lens comprising: an aperture adjusting zonal part on at least one of its lens surfaces for apparently eliminating light at the periphery of a light flux having a wavelength λ 1 while maintaining light at the periphery of a light flux having a wavelength λ 2 where λ 1 , is one of the first and second wavelengths and λ 2 is the other wavelength; and the aperture adjusting zonal part is formed between two concentric stepped parts about the optical axis of the objective lens in a manner such that the following Conditions (1) and (2) are satisfied Δ 1 =(2 n+ 1)·λ 1 /2+δ 1 Condition (1) Δ 2 =m ·λ 2 +δ 2 Condition (2) where Δ 1 is the difference in optical path length from the light source to the focus point of the optical recording media objective lens for light having the wavelength λ 1 that would pass through the area where the zonal part would be if the zonal part were to be removed from the optical recording media objective lens versus the light passing through the zonal part; Δ 2 is the difference in optical path length from the light source to the focus point of the optical recording media objective lens for light having the wavelength λ 2 that would pass through the area where the zonal part would be if the zonal part were to be removed from the optical recording media objective lens versus the light passing through the zonal part; m and n are integers; δ 1 is a constant within the range |δ 1 |≦0.25 λ 1 ; and δ 2 is a constant within the range |δ 2 |≦0.25 λ 2 ; wherein the following Condition (3) is also satisfied: 0.95≦( a+c )/ b≦ 1.05 Condition (3) where a is the distance in a direction orthogonal to the optical axis between the boundary of the smaller one of the first and second numerical apertures and the inner one of the two stepped parts, b is the distance in a direction orthogonal to the optical axis between the inner stepped part and the outer stepped part, and c is the distance in a direction orthogonal to the optical axis between the outer stepped part and the boundary of the larger one of the first and second numerical apertures.