Measuring a camera system's F-number using retroreflections

Derek J. Burrell; Allen J. Parker; David P. Haefner; Stephen D. Burks

doi:10.1117/12.3012784

7 June 2024 Measuring a camera system's F-number using retroreflections

Derek J. Burrell, Allen J. Parker, David P. Haefner, Stephen D. Burks

Author Affiliations +

Proceedings Volume 13045, Infrared Imaging Systems: Design, Analysis, Modeling, and Testing XXXV; 130450T (2024) https://doi.org/10.1117/12.3012784
Event: SPIE Defense + Commercial Sensing, 2024, National Harbor, Maryland, United States

Abstract

Direct measurement of F-number presents a known challenge in characterizing electro-optical and infrared imaging systems. Conventional methods typically require the sensor to be evaluated separately from the lens, indirectly calculating F-number from measurements of effective focal length and entrance-pupil diameter. When a focal plane array is positioned behind the optics and cannot be removed, some potential options could be to quantify signal-to-noise ratio or depth of field using incoherent light. In either of these cases, the result is subject to extraneous camera parameters and sensitive to noise, aberrations, etc. To address these issues, we propose an alternative measurement routine that utilizes a coherent point source at the focus of an off-axis Newtonian collimator to generate collimated light. This allows us to place the system under test at optical infinity, where retroreflections from its focal plane depend solely on angle of incidence, wavelength of illumination and F-number. Thus by measuring retroreflected power as a function of incidence angle, we can back out the system’s F-number with a high degree of confidence. We demonstrate this concept through numerical simulation and laboratory testing, along with an unconventional knife-edge technique for gauging the entrance-pupil diameter in situ. Together these two measurements enable us to calculate effective focal length (and in turn pixel pitch by measuring instantaneous field of view) for a comprehensive system description. We further show that a working F-number and effective image distance are attainable through this method for finite-conjugate systems. These tools improve our ability to update existing system models with objective measurements.

Conference Presentation

(2024) Published by SPIE. Downloading of the abstract is permitted for personal use only.

Citation Download Citation

Derek J. Burrell, Allen J. Parker, David P. Haefner, and Stephen D. Burks "Measuring a camera system's F-number using retroreflections", Proc. SPIE 13045, Infrared Imaging Systems: Design, Analysis, Modeling, and Testing XXXV, 130450T (7 June 2024); https://doi.org/10.1117/12.3012784

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