depends on the particular optical geometry used. For the arrangement shown in Figure 7.5, the

sensitivity direction lies along the blue vector “k” that bisects the directions of the illumination

and object beams. An ESPI setup with similar geometry to that used for the Moiré interferom-

eter shown in Figure 7.3 would have in-plane sensitivity. When used for ESPI measurements,

a diffraction grating would not be needed on the specimen surface. Both the out-of-plane ge-

ometry shown in Figure 7.5 and the in-plane geometry shown in Figure 7.3 are suitable for

hole-drilling applications.

A significant feature of ESPI is that it can work with a plain specimen surface, without

attachment of the diffraction grating needed for Moiré measurements. is makes it possible to

do ESPI measurements rapidly, thus making it suitable as an industrial quality control tool. As

determined. e algorithms used for doing this have become quite sophisticated, and with a

well-calibrated optical system, displacements of C= 0.02 pixel can be resolved.

e 3-D technique, schematically illustrated in Figure 7.8b, involves imaging the region

of interest with two cameras and using stereoscopic imaging to determine deformations in three

dimensions. e equipment is more complex than for the 2-D technique, and careful setup

and calibration are required. For hole-drilling measurements the out-of-plane displacements

are small compared with the in-plane displacements, so in this case the extra data available from

3-D measurements provide only modest advantage.

e DIC method has both advantages and disadvantages compared with the interferomet-

ric methods. Its major disadvantage is its relatively low sensitivity, about an order of magnitude

less when measuring a typical 2 mm diameter hole. It also requires the painting of a speckle

pattern on the specimen surface. However, once the pattern has been applied, the surface is

quite durable and can be cleaned if it becomes dirty. By comparison, a surface for ESPI mea-

surements cannot be touched at all; even very minor abrasions will damage measurement quality.

In addition, DIC can directly evaluate displacements in two in-plane directions, also the out-

of-plane direction if the 3-D technique is used. e large amount of available data improves the

quality of the resulting residual stress evaluation. By comparison, typical ESPI systems provide

displacement data in only one direction. Multi-axis ESPI systems do exist, but are uncommon.