at any point in the image is the sum of the light from the object (object beam) and the second beam (reference beam). If the object moves in the direction of viewing, the distance travelled by the object beam changes, its phase changes, and therefore the amplitude of the combined beams changes. When the second speckle pattern is subtracted from the first, fringes are obtained which represent contours of displacement along the viewing direction (out-of-plane displacement). These are not interference fringes, and are sometimes referred to as 'correlation' fringes since they map out areas of the speckle pattern which are more or less correlated. Strictly speaking, the fringes represent purely out-of-plane displacement only if the surface is illuminated normally (this requires a beam splitter to be used to illuminate the object), but the dependence on in-plane movement is relatively small unless the object illumination is well away from the normal direction. The fringes in the image above are out-of-plane fringes. The plate has been rotated about a vertical axis and the fringes represent contours of constant displacement. The contour interval is about 0.3μm since a He-Ne laser was used in the system. As with many interferometric techniques, it is not possible to identify the zero-order fringe without additional information from the system. That means a rigid body motion of a half wavelength (0.3μm) towards the camera does not change the fringe pattern. Holographic interferometry provides the same information as out-of-plane ESPI fringes. Out-of-plane vibration measurement The optical arrangement is the same as for out-of-plane displacement above. The object is vibrated at a specific frequency. Those parts of the object which do not move will continue to be speckled. It can be shown that parts of the object which vibrate with amplitudes of nλ/4 have higher speckle contrast than those parts which vibrate at (n+½)λ/4. This system is simpler to operate than either of the displacement measuring systems, as the fringes are obtained without any recording being required. The vibration mode can be observed in the image from the camera as a variation in speckle contrast rather than as a variation in intensity but it is quite difficult to discern. When the image is high-pass filtered, the variation in contrast is converted to a variation in intensity, and a fringe pattern of the form shown in the diagram is observed where the fringes are clearly visible. Holographic interferometry can be used