ILLUMINATION TECHNOLOGY: Line light uses LEDs for web inspection

In web-based machine-vision systems, high-brightness line lights must maintain uniform illumination across their width. To achieve sufficient brightness and uniformity, many lighting manufacturers use fiber-coupled halogen lamp sources. With the recent advances in ultrahigh-brightness light-emitting diodes (UHB-LEDs), vendors such as Innovations In Optics (Woburn, MA, USA; www.innovationsinoptics.com) are looking to replace these products with modular line lights that use LEDs as the light source.

“Until now,” says Thomas Brukilacchio, president of the company, “the performance of LED line lights has fallen short of industry expectations.” Recently, Brukilacchio and his colleagues have combined both imaging and non-imaging optics with LED chip-on-board (COB) high-density arrays. The resulting intensity surpasses halogen illumination and also has a significantly improved lifetime designed to exceed 30,000 h. Each 4-in. module contains five evenly spaced 0.7-mm-wide by 7-mm-long LED die arrays supplied by either Cree (Durham, NC, USA; www.cree.com) or SemiLEDs (Boise, ID, USA; www.semileds.com).

“To collimate the light from the devices mounted on the T-Clad PCB manufactured by The Bergquist Co. (Chanhassen, MN, USA; www.bergquistcompany.com),” says Brukilacchio, “a specially designed non-imaging plastic collimator was designed with SolidWorks (Concord, MA, USA; www.solidworks.com) and optimized using Zemax CAD-based ray tracing software from Zemax Development Corp. (Bellevue, WA, USA; www.zemax.com). Any number of these modules can then be placed linearly within a custom metal housing for specific web-based inspection length requirements (see figure). To ensure that light from each of the individual modules within the line light is uniform, an aspheric lens is then coupled along the length of the line light.

Replacing fiber or halogen-based illumination sources is the aim of an LED line light from Innovations In Optics. Each line light consists of a number of discrete LEDs mounted on a PCB and coupled to an optical collimator.

“Line lights that use fiberoptics to deliver light to individual regions along the line may suffer from intensity fluctuations or degradation as the halogen lamps used in such systems can vary and degrade over their lifespan,” says Brukilacchio.

To maintain uniform illumination across its LED-based line light, Innovations In Optics’ module can be configured with both a photosensor and a thermistor. By feeding analog measurement from these devices to the device’s power supply controller, these measurements can allow the intensity of light and temperature of the light to be constantly maintained.

“Using LED light sources coupled with novel optics dramatically decreases illumination inefficiencies, while at the same time increasing the lifespan of the light,” says Brukilacchio. Of course, the use of LEDs can provide other advantages, especially for designers of machine-vision lighting systems. While Innovations In Optics’ initial line light will be offered as a 24-in.-long white line light target priced at $2500, future products will incorporate different types of LED modules.

“Instead of mounting five LEDs of the same color in each of the arrays,” says Brukilacchio, “each individual array could consist of a number of red, green, and blue LEDs.” In this way, the system integrator could configure the light as a red, green, blue, or white light source.

By using a calibrated integrating cavity with an aperture of 2.5 mm, Innovations In Optics has measured the light output of its white LED illumination system at greater than 1 Mlux when powering the LED die below their rated 350-mA current specification and cooled by free convection. According to Brukilacchio, light intensities of 5 Mlux are achievable by operating the device at higher current, with auxiliary cooling. The company is marketing the product directly and through distributor and system integrator Visics (Wellesley, MA, USA; www.visics.com) and is looking for both a UK and European distributor.