The source of error of image measuring instrument

In the technical parameters of image measuring instruments, we often see (3+L/200)um data, which is the measurement accuracy of the image measuring instrument. How did it get it? This is every new contact of us. A doubt in the minds of the customers of the image measuring instrument.

If you want to understand the measurement accuracy of the image measuring instrument, you must know the source of the error of the image measuring instrument. The measurement on the image measuring instrument is the measurement of single-axis or two-dimensional plane coordinates. When measuring, focus first, then align, read (count), and then calculate and process. The reading comes from the ruler or grating system, the focus alignment relies on the optical image system, and there is also an illumination source that directly affects the measurement effect and accuracy. Because, based on the image method measurement instrument, if the test piece cannot be effectively and correctly illuminated, The measurement result obviously deviates from its true size. In addition to the aforementioned factors, environmental conditions are also factors that restrict measurement accuracy and cannot be ignored. Based on the above analysis, we can summarize the sources of error in several aspects.

  1) Error of grating ruler counting;

  2) Error caused by straightness and angular pendulum when the workbench moves;

  3) Work The error of the verticality band of the two measuring axes of the table;

  4) The error of the non-vertical band between the optical axis of the lens and the worktable;

  5) Measuring the error caused by changes in ambient temperature;< /p>

  6) Focus and alignment errors caused by changes in light source lighting conditions.

   Among these factors, the first four errors are hardware errors, which have been formed and fixed during the manufacturing process of the image measuring instrument. Generally, they cannot be changed. The errors caused by temperature influence must be controlled Measure the temperature of the chamber and the isothermal process to reduce its influence.

  The latter item is often overlooked. In actual measurement, when the lighting conditions of the light source change, it directly affects the lighting effect and image quality of the measured workpiece, mainly because the image of the image measuring instrument is Through the CCD reception, although the CCD has the function of automatically adjusting the gain, when the brightness is too high, the adjustment function will be lost, causing the image of the measured workpiece to shrink. When the brightness is too low, the image of the workpiece becomes larger.

  This effect can be ignored when measuring the distance between repeated pattern structures, as long as the lighting conditions remain the same throughout the measurement process, because each repeated pattern structure is increasing at the same time Or become smaller, the distance measurement calculation directly eliminates the influence of image distortion, such as measuring glass ruler, grid plate engraved line spacing; except for this special case, such as measuring the diameter of the circle, the length and width of the workpiece, will be Significant errors come.