Abstract
An omni-directional optical system can be used as a surveillance camera owing to its wide field angle. In cases in which a system is designed with a central screen obscuring structure to increase the resolution of the off-axis field, however, the conventional methods cannot be used to measure the effective focal length (EFL). We assumed the actual and theoretical distortion values of the fabricated optical system to be the same and determined the system’s EFL by finding the minimum deviation point of the measured and theoretical distortions. The feasibility of the determined EFL was verified through a tolerance analysis of the system. For these precise measurements we also analyzed the sources of error. To verify our proposed measurement method, we measured the focal length of a center-obstructed omni-directional reflective optical system with an 80–135° field of view (FOV). The EFL from the measurement was 0.3739 mm and was only approximately 11 µm different from the EFL calculated using the design software. Thus, the reliability of focal length measurements in omni-directional optical systems was improved.
Highlights
The 9/11 terrorist attack in New York in 2001 has led to tremendous growth in security-related markets
The focal length of a center-obstructed omni-directional reflective optical system with an field of view (FOV) of 80–135◦ was measured by applying the novel measurement method described in this paper
The effective focal length (EFL) calculated using the measurements from our proposed measurement system was 0.3739 mm, which was only ~11 μm from the EFL obtained using the design software
Summary
The 9/11 terrorist attack in New York in 2001 has led to tremendous growth in security-related markets. Most of the key security products that employ optical design technology (e.g., fingerprint identification systems and cameras) are used for building security [1] These devices capture optical images of people approaching a building to help identify possible intruders among numerous visitors [2]. A zoom lens has a limited FOV, and requires mechanical movement if the need-to-be recognized target is beyond its FOV [3,4] In such cases, security may be compromised if the target approaches swiftly while the camera is moving. Security may be compromised if the target approaches swiftly while the camera is moving To overcome this weakness, an optical system with an FOV of 180◦ must be used to monitor a wide FOV range with no mechanical movement
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