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Quick Guide to Precision Vision Measuring Machines
Measuring Instruments
Vision Measurement Difference in Image Quality
Vision measuring machines mainly provide the following pro- Difference between 2-level and 256-level greyscale images
cessing capabilities.
Edge detection
Detecting/measuring edges in the XY plane
Sample image displayed in 2-level greyscale Sample image displayed in 256-level greyscale
Auto focusing Variation in Image Depending on Threshold
Focusing and Z measurement Level
Pattern recognition These three pictures show how the same 2-level greyscale image can
Alignment, positioning, and checking a feature appear vastly different depending on the slice levels (threshold levels)
used. Therefore, 2-level greyscale imaging is not used for high-precision
vision measurement since pixel numeric values will change depending
on the threshold level that is set.
Image Storage Display Dimensional Measurement
screen
CCD Video signal High-speed An image consists of pixels. If the number of pixels in a section to be
camera A/D converter PC measured is counted and is multiplied by the size of a pixel, then the
lens Frame grabber section can be converted to a numeric value in length. For example,
640 pixels assume that the total number of pixels in the lateral size of a square
Amplifier workpiece is 300 pixels as shown in the figure below.
If a pixel size is 10 m under imaging magnification, the total length of
the workpiece is given by 10 m x 300 pixels = 3000 m = 3 mm.
300 pixels 10 µm
K 480 pixels
An image is comprised of a regular array of pixels. This is just like a Edge Detection
picture on fine plotting paper with each square solid-filled differently.
How to actually detect a workpiece edge in an image is described using
Greyscale Images the following monochrome picture as an example. Edge detection is
performed within a given domain. A symbol which visually defines
A PC stores an image after internally converting it to numeric values. A this domain is referred to as a tool. Multiple tools are provided to suit
numeric value is assigned to each pixel of an image. Image quality varies various workpiece geometries or measurement data.
depending on how many levels of greyscale are defined by the numeric
values. The PC provides two types of greyscale: two-level and multi- The edge detection system scans within the tool area
level. The pixels in an image are usually displayed as 256-level greyscale. Tool as shown in the figure at left and detects the boundary
between light and shade.
2-level Multi-level 255 244 241 220 193 97 76 67 52 53 53
greyscale greyscale 243 242 220 195 94 73 66 54 53 55
244 246 220 195 94 75 64 56 51 50
White 1 White
Grey 0
Black Grey 127 Example of numeric values assigned to pixels on the tool
255
Greyscale
Black 0 127
Each pixel is displayed as one of 256 levels
Pixels in an image brighter than a given level between black and white. This allows high- 0 (1) Scan start position
are displayed as white and all other pixels are fidelity images to be displayed. (1) (2) (3) Tool position (2) Edge detection position
displayed as black.
(3) Scan end position
K-17
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