The working principle of linear scale

Moire fringe

Transmission grating, for example, when the indicator on the grating lines and scale on the grating lines formed between a small Angle theta, and two relative of parallel grating ruler facets, in under the irradiation of light source, is located in a nearly vertical grid lines, the formation of light and dark stripes. The stripes, called "moire fringe" (right). Strictly speaking, the direction of moire fringe arrangement is with two pieces of grating lines perpendicular bisector phase Angle. Moire fringe in the two bright lines or the distance between two dark grain as the width of the moire fringe, expressed as a W.

W = omega / 2 * sin (theta / 2) = / omega theta.

Moire pattern has the following features:

(1) the change of moire fringe patterns

Two pieces of grating relative move through a lattice spacing, moire fringe moving over a stripe distance. By light diffraction and the interference effect, the changing rule of the moire fringe approximation (more than) string function, change cycles and the relative displacement of the grating lattice spacing for synchronization.

(2) the amplification effect

In the case of two raster grid line Angle smaller, moire fringe width W and raster grid, the grid line omega has the following relationship between Angle of theta. Type, theta unit for rad, W unit is mm. Due to the Angle is small, the sine theta is small, then

W = omega/theta.

If omega = 0.01 mm, theta = 0.01 rad, is the type available W = 1, the grating magnified 100 times.

(3) the homogenizing error function

Moire fringe is formed by several grating stripe share, 100 lines per mm grating, for example, 10 mm width of moire fringe has 1000 lines, and such lattice spacing between adjacent error is averageness, eliminated due to uneven blade spacing error caused by, fracture, etc.