**3.1 Artifacts**

Graduated length scales come in many forms, and are made in lengths from a few micrometers to over a several meters. Those longer than a meter or two are usually classified as measuring tapes or rods. Many materials are used including steel, Invar, glass, glassceramics, silicon, and fused silica. Cross sectional shape can be rectangular, "H," modified "U" (flat bottom), or a modified "X" (Tresca). At present, the line scale interferometer is limited to graduation widths ranging from submicrometer to 100 mm, and spacings ranging from less than 1 mm up to 1025 mm. Spacings are generally measured from center to center of the graduations, but can also be measured from edge to edge.

Some devices that are not strictly linear scales are measured in the line scale interferometer. These include end standards in a size range (250 mm to 1000 mm) that can present measurement problems with laser interferometry.

Two dimensional patterns are measured by treating each row and column of graduations as an independent scale and, when possible, an estimate of orthogonality can be made by measuring the diagonals.
