Method and apparatus for optically inspecting a mold for manufacturing ophthalmic lenses for possible mold defects

A method for optically inspecting a mold (10) for manufacturing ophthalmic lenses for possible mold defects, the method comprising the steps of:

• a. taking an image (41) of the mold (10) along a center axis (Z) of the mold (10) that extends through an apex (15) of the mold and normal to a mold surface (16) at the apex (15) by
• illuminating the mold (10) with light (26) incident at an azimuthal illumination angle (ϕ1) and a polar illumination angle (Θ) with regard to the center axis (Z), and
• collecting and detecting light (28) reflected or scattered, respectively, from the mold (10) using an imaging system (30) arranged along and aligned with the center axis (Z), wherein a focal plane (31) of the imaging system (30) has an axial position (z1) along the center axis (Z) in the range of the mold (10);
• b. repeating step a. for one or a plurality of different azimuthal illumination angles (ϕ2, ϕ3, ϕ4, ϕ5) around the center axis (Z) at the same polar illumination angle (Θ) and the same axial position (z1) of the focal plane (31) to generate a plurality of images (41, 42, 43, 44, 45) of the mold;
• c. generating a masked dark field image (54) for each image of the plurality of images (41, 42, 43, 44, 45) taken at the different azimuthal illumination angles (ϕ1, ϕ2, ϕ3, ϕ4, ϕ5) at the same axial position (z1) of the focal plane (31) by applying a respective specular reflection mask (40) to each image of the plurality of images (41, 42, 43, 44, 45) of the mold such as to mask out image regions (44a, 44b) that include direct specular reflections (23, 24) from the mold (10), so as to obtain a plurality of masked dark field images;
• d. generating a focal plane image (61) for the axial position (z1) of the focal plane (31) by averaging pixelwise over the masked dark field images (54) generated at the different azimuthal illumination angles (ϕ1, ϕ2, ϕ3, ϕ4, ϕ5) at the same axial position (z1) of the focal plane (31);
• e. repeating steps a. to d. for one or a plurality of different axial positions (z2, z3) of the focal plane (31) to generate a plurality of different focal plane images (61, 62, 63);
• f. identifying one or more image features (71, 72, 73, 74) in the plurality of different focal plane images (61, 62, 63), each image feature (71, 72, 73, 74) showing a possible mold defect;
• g. determining for each identified image feature (71, 72, 73, 74) a respective sharpest focal plane image (61, 62, 63) of the plurality of different focal plane images (61, 62, 63) by determining for which focal plane image (61, 62, 63) of the plurality of different focal plane images (61, 62, 63) a pre-defined sharpness measure of the identified image feature (71, 72, 73, 74) is at a maximum;
• h. generating for each identified image feature (71, 72, 73, 74) a respective image section (81, 82, 83, 84) out of the respective sharpest focal plane image (61, 62, 63), wherein the identified image section (81, 82, 83, 84) includes the identified image feature (71, 72, 73, 74);
• i. generating a composed dark field image (90) of the mold (10) by composing the respective image sections (81, 82, 83, 84) for each identified image feature (71, 72, 73, 74) such as to enable determination as to whether the possible defects of the mold (10) allow the mold (10) to be used for manufacturing ophthalmic lenses.