Design method of a modular side-view objective compatible with existing photographic objectives

Mobile devices have become the most widely used camera platforms. However, acquiring real-time images of the paper surface during planar operations such as writing and drawing remains challenging. Positioning a camera above the paper surface enables straightforward imaging but blocks the user's line of sight and restricts head and arm motion. Capturing images from the side of documents avoids these issues but introduces trapezoidal distortion and reduces resolution in the distal region of the paper. This study presents a modular side-view objective that is integrated with a tablet camera and omits the sensor and autofocus module, enabling cost-effective imaging in large-scale production. Based on a theoretical analysis of pupil aberrations, we introduce field angle constraints and controlled entrance pupil size errors to minimize trapezoidal distortion and enhance the resolution in the distal region. The proposed side-view objective employs a freeform mirror to generate a relay pupil conjugate to the entrance pupil with reduced pupil aberrations, thereby suppressing stray light. To match the exit pupil to the tablet camera, an iterative design method evaluates and corrects exit pupil aberrations using light-footprint geometry, yielding an accurate exit pupil. Performance and tolerance analyses demonstrate that the design achieves a modulation transfer function above 0.25 at 100 cycles/mm, effectively controls trapezoidal distortion, suppresses stray light, remains compatible with tablet cameras, and maintains good manufacturability.