Understanding Measurement of length

Length measurement is implemented in practice in many ways. The most commonly used approaches are the transit-time methods and the interferometer methods based
upon the speed of light. For objects such as crystals and diffraction gratings, diffraction is used with X-rays and electron beams. Measurement techniques for three-dimensional structures very small in every dimension use specialized instruments such as ion microscopy coupled with intensive computer modeling.

Standard rulers
The ruler the simplest kind of length measurement tool: lengths are defined by printed marks or engravings on a stick. The meter was initially defined using a ruler before more accurate methods became available.

Gauge blocks are a common method for precise measurement or calibration of measurement tools.

For small or microscopic objects, microphotography where the length is calibrated using a graticule can be used. A graticule is a piece that has lines for precise lengths etched into it. Graticules may be fitted into the eyepiece or they may be used on the measurement plane.
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