Objective lens design of subminiature endoscope with image fiber bundles

According to the requirements of small size and large field of view(FOV) for subminiature fiber soft endoscope, the fundamental design criteria was analyzed, the retrofocus objective with a "negative-positive" form was utilized as the initial structure, and the telecentric optical system in image space was determined for this design.Through theoretical calculation and continuous optimization with Zemax optical design software, a designed subminiature endoscope objective lens sample was fabricated finally, with operation wavelength, focal length, FOV and relative aperture of 0.48 μm~0.65 μm, 0.37 mm, 90° and 1:4, respectively.The optical system is composed of 4 pieces of lenses, including two single lenses and one cemented doublet.The result shows that its total length is 3.89 mm and maximum cross sectional diameter is 0.95 mm, which can satisfy the initial design requirements of image telecentric structure.The modulation transferfunction (MTF) value of the lens is approximately 0.7 at Nyquist spatial frequency of 77 lp/mm, near the diffraction limit. Furthermore, the designed lens has the peculiarity of wide FOV, short focal length, fine quality of imaging, reasonable structure and uniform illumination at image plane. It is suitable for the subminiature fiber endoscope demand.