|
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.
|
