Quick Guide to Precision Measuring Instruments

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PG49Quick Guide to MeasurementGray scale2551270(1)(2)(3)Tool positionTool244241220193977667525353243242220195947366545355244246220195947564565150Dimensional MeasurementAn image consists of pixels. If the number of pixels in a section to be measured is counted and is multiplied by the size of a pixel, then the section can be converted to a numeric value in length. For example, assume that the total number of pixels in the lateral size of a square workpiece is 300 pixels as shown in the gure below. If a pixel size is 10μm under imaging magnication, the total length of the workpiece is given by 10μm x 300 pixels = 3000μm = 3mm.Edge DetectionHow to actually detect a workpiece edge in an image is described using the following monochrome picture as an example. Edge detection is performed within a given domain. A symbol which visually defines this domain is referred to as a tool. Multiple tools are provided to suit various workpiece geometries or measurement data.The edge detection system scans within the tool area as shown in the gure at left and detects the boundary between light and shade.Example of numeric values assigned to pixels on the tool(1) Scan start position(2) Edge detection position(3) Scan end position300 pixels10µm300 pixels10µmHigh-resolution MeasurementTo increase the accuracy in edge detection, sub-pixel image processing is used.An edge is detected by determining interpolation curve from adjacent pixel data as shown below.As a result, it allows measurement with a resolution higher than 1 pixel.Image signal without sub-pixel processingImage signal with sub-pixel processingThe image signal prole approaches an analog waveform like this.Gray scaleTool positionGray scaleTool positionWhen enlarged...A position the system recognizes as an edge may be in error by up to one pixel width using normal image processing. This will prevent the execution of high-resolution measurement.Gray scaleGray scaleTool positionTool position