Abstract
Illumination for line-scan machine vision systems is required to produce a highly asymmetric elliptical beam pattern, to maximize system speed and accuracy. The use of LED emitters with symmetric Lambertian emission patterns is challenging in this context, requiring significant beam reshaping. A design for a collimated line-light, with long working distance, utilizing LEDs with symmetric Lambertian emission patterns, is presented. Using a combination of Fresnel lenses and total internal reflection (TIR) optics, an elliptical beam with a high degree of collimation is achieved. TIR elements are designed based on an adaptation of a freeform lens design method published by Chen et al . [Opt. Express 20, 10 (2012)]. Practical performance of the design is verified experimentally using a prototype unit. In addition, the design is compared, using ray tracing software, to line-lights constructed using commercially available symmetric and elliptical TIR lenses, and its superior performance is confirmed. The optical design described is fully manufacturable and suitable for both small- and large-scale production.
Highlights
The use of LED emitters with symmetric Lambertian emission patterns is challenging in this context, requiring significant beam reshaping
Line based inspection, using linear image sensors, is a common tool used in machine vision technology, where non-destructive in-line inspection is required [1], [2], with machine vision inspection methods providing improvements over manual inspection generally [3]
The design utilizes total internal reflection (TIR) designed using an adaptation of the free-form method described by Chen et al [26], coupled to a Fresnel lens which can be user selected to comply with mechanical tolerances of a specific system, while maximizing collimation
Summary
Line based inspection, using linear image sensors, is a common tool used in machine vision technology, where non-destructive in-line inspection is required [1], [2], with machine vision inspection methods providing improvements over manual inspection generally [3]. Using standard line-scan technology, web inspection can be performed at a rate on the order of meters per second, significantly outperforming manual inspection methods in speed, accuracy and resolution [11] Lighting requirements for these systems are challenging, and in particular the irradiance of the lighting can be a rate limiting factor, due to the short exposure times. The limited power output from a single LED package, combined with the relatively high cost of microlens array optics, makes this strategy prohibitively expensive for many line-scan machine vision applications, where the web inspection area can be many metres in width [21]. A freeform secondary optics design based on the use of two cylindrical spherical lenses, producing a rectangular beam for a street-lighting application [13]. A method is proposed for LED line-light collimation and beam shaping, involving control of the LED radiation pattern in orthogonal axes, parallel and perpendicular to a linear LED array
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