Find reflections in glass
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 8/1/2004
Optical waveplates used in fiber-optic telecommunication devices. Cut by saws into sizes as small as 1-mm2 and 34-µm thick, the waveplates must be defect free at their center.
THE CHALLENGEAutomate the inspection process so a vision system can inspect the waveplates without human intervention. The system must measure reflected light that hits the glass surface at 45° to detect scratches in the glass. Software must analyze the reflected light to look for common defects.
THE TOOLS- Danaher Motion: x-y stage with 1-µm resolution; z-axis stage with 1-µm resolution; granite slab and column. www.danaherprecision.com
- Edmund Optics: Ronchi ruling glass slide. www.edmundoptics.com
- Hamamatsu: 1300x1024 12-bit digital camera. usa.hamamatsu.com
- Illumination Technology: light source. www.illuminationtech.com
- National Instruments: motion-control card; frame grabber; LabView software with Vision toolkit. www.ni.com
- Schott Fostec: darkfield ring light. www.fostec.com
- Thales Optem: 10X zoom telecentric lens. www.thales-optem.com.
CVI Laser, (www.cvilaser.com), a maker of waveplates used with fiber-optic telecommunication devices, required employees to manually inspect each waveplate after a saw cut it from a substrate. (The saw creates small chips at the edge of the glass, and the manufacturing process can also create scratches and digs.) With manual inspection, one person could spend hours inspecting a batch of waveplates under a microscope. Ammons Engineering (www.ammonsengineering.com) and Graftek Imaging (www.graftek.com) jointly developed an automated inspection system that finds defects without human intervention.
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Defects along a waveplate's edge refract light, making the defects visible to a camera. Courtesy of Ammons Engineering. |
With the new system, an operator places the cut waveplates in a 2x3-in. tray containing a solid, transparent gel that holds them in place and provides a level surface. A granite positioning table moves the tray into position under a camera. The system finds each waveplate and inspects it using dark-field illumination. Light from the ringlight hits the glass at about 45°. In clear, defect-free areas, the light reflects off the surface of the glass without entering the lens. This area shows up dark in the image (see figure). Defects diffuse the light, making it visible to the camera.
To measure the sizes of a waveplate's defects, the system scans the entire tray and creates a large composite image. It then analyzes the image to locate the individual glass pieces. Next, it takes an image of each glass piece and measures its dimensions. Software measures the brightness of each pixel and a defect's size. From the analysis, the software makes a decision as to the severity of a defect.
After that, the telecentric lens zooms in on the individual defects and measures the critical dimensions. From the measurements, software compares the dimensions to limits specified by the operator and determines if the waveplate passes or fails. The operator can review the defects on screen or create a test report.
The system's telecentric zoom lens lets an operator view a calibrated image with resolution from 0.6 µm per pixel to 6 µm per pixel. Telecentric optics provide an undistorted image for the entire field of view, which results in precise measurements anywhere within the image.
To achieve best focus, the system must coordinate its zoom, vision, and motion features. The system knows the position of the x-y table, the z-axis stage, and the zoom lens. As the lens zooms, the z-axis alignment relative to the center of the image may vary, so the system must measure these variations and compensate for them.
RESULTSThe system can inspect a tray of glass waveplates in about 1 hr, in contrast to the manual inspection that could take as long as a day. The system generates reports as Word documents for each waveplate, indicating both the size of any defects and the waveplate's pass/fail status. It also produces images at 1300x1024 pixels. CVI Laser is much more confident in the quality control of its waveplates, and the company has begun using the system to inspect other optical elements for defects.
