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Proceedings Paper

Optical engineering application of modeled photosynthetically active radiation (PAR) for high-speed digital camera dynamic range optimization
Author(s): James Alves; Christian A. Gueymard
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Paper Abstract

As efforts to create accurate yet computationally efficient estimation models for clear-sky photosynthetically active solar radiation (PAR) have succeeded, the range of practical engineering applications where these models can be successfully applied has increased. This paper describes a novel application of the REST2 radiative model (developed by the second author) in optical engineering. The PAR predictions in this application are used to predict the possible range of instantaneous irradiances that could impinge on the image plane of a stationary video camera designed to image license plates on moving vehicles. The overall spectral response of the camera (including lens and optical filters) is similar to the 400-700 nm PAR range, thereby making PAR irradiance (rather than luminance) predictions most suitable for this application. The accuracy of the REST2 irradiance predictions for horizontal surfaces, coupled with another radiative model to obtain irradiances on vertical surfaces, and to standard optical image formation models, enable setting the dynamic range controls of the camera to ensure that the license plate images are legible (unsaturated with adequate contrast) regardless of the time of day, sky condition, or vehicle speed. A brief description of how these radiative models are utilized as part of the camera control algorithm is provided. Several comparisons of the irradiance predictions derived from the radiative model versus actual PAR measurements under varying sky conditions with three Licor sensors (one horizontal and two vertical) have been made and showed good agreement. Various camera-to-plate geometries and compass headings have been considered in these comparisons. Time-lapse sequences of license plate images taken with the camera under various sky conditions over a 30-day period are also analyzed. They demonstrate the success of the approach at creating legible plate images under highly variable lighting, which is the main goal of this application. Graphs of plate contrast over various sky conditions and camera aiming geometries are also presented to quantify the performance of the plate's legibility.

Paper Details

Date Published: 20 August 2009
PDF: 16 pages
Proc. SPIE 7410, Optical Modeling and Measurements for Solar Energy Systems III, 741004 (20 August 2009); doi: 10.1117/12.827483
Show Author Affiliations
James Alves, AR Technologies, LLC (United States)
Christian A. Gueymard, Solar Consulting Services (United States)

Published in SPIE Proceedings Vol. 7410:
Optical Modeling and Measurements for Solar Energy Systems III
Benjamin K. Tsai, Editor(s)

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