Phase-Stepping Interferometry

The present invention has been conceived and developed for extracting phase information in electronic speckle pattern interferometry, or so-called ESPI, by the phase-stepping technique and it is convenient to describe the invention in this context. However, it is to be understood that the invention is applicable to other forms of interferometry for which the phase-stepping technique is appropriate.

This technique involves successively changing the phase of one of the interferometer beams from a datum phase .phi. by 2.pi./k in (k-1) equal steps and recording the respective interference pattern images generated at each different phase. The Fourier series coefficients for the datum phase are then given by the expressions ##EQU1## where N is the image number in the phase-stepping sequence and I.sub.N (x,y) is the image pixel intensity. A solution for .phi.(x,y) is then given, in turn, by the expression

However, the technique is not without practical difficulties.

For a given image resolution, image recording and related equipment needs increase with k and so also, in general terms, will the time taken for computation by virtue of the increased number of pieces of data involved in the expressions to be evaluated.

At the same time, the environmental sensitivity of the optical instrumentation can drift and introduce error in I.sub.N as k increases.

In practice these difficulties are such that the technique is usually deployed with k equal to 3 or 4, when evaluation is respectively given by ##EQU2## When k is 3 the expression to be evaluated is computationally more complex than that for k equal to 4, but environmental sensitivity is higher. Higher values of k are relatively impracticable by virtue of greatly increased equipment cost and/or complexity plus poor sensitivity.

Clearly this situation is such that the case when k is 2 appears particularly attractive. However, in this case it will be seen that .beta..sub.1 is zero.

An object of the present invention is to reduce the difficulties of the above situation and to this end there is provided an interferometric method comprising, and related apparatus for, generating two signals representing the point-by-point variations in intensity of respective patterns of electromagnetic radiation resulting from the interference of first and second beams of such radiation derived from a coherent source, with at least the first beam for each pattern being scattered, before interference with its respective second beam, from a common object surface, and with a corresponding pair of the beams, one for each pattern, having a predetermined relative phase difference of other than a multiple of .pi.; and determining from said two signals values for a datum phase of the radiation at said surface.

The derivation of the invention can be explained by considering, for ESPI, the case for k equal to 3 to produce images at phases .phi.-.delta., .phi. and .phi.+.delta.. These images can be expressed in a simplified manner by the equations ##EQU3## Included in these equations are DC components I.sub.O plus I.sub.R contributed by the object and reference beams producing the images. Elimination of these components, such as by selective frequency filtering or substraction, allows further simplification effectively to represent a situation where k is 2 by forming from only two of the equations an expression for .phi..

Thus from equations (1) and (2), .phi. is given by ##EQU4## and if .delta. is chosen for convenience as .pi./2 or an odd multiple thereof, this expression is further simplified to

A corresponding result is obtainable from equations (2) and (3).

From equations (1) and (3), .phi. is given by ##EQU5## and if .delta. is chosen for convenience as .pi./4, or an odd multiple thereof, cot .delta. equals .+-.1.

For further clarification the accompanying drawing schematically illustrates, by way of example, one embodiment of electronic speckle pattern interferometric apparatus according to the invention.

The illustrated apparatus comprises a coherent light source, laser 1, which projects a beam by way of a beam splitter 2 to an object surface 3 of interest and a reference surface 4, with a television camera 5 or equivalent equipment serving to image light scattered from both surfaces and provide a video signal output representing the relevant image. A phase shifter 6 is located in the beam path from the beam splitter to one of the surfaces 3 and 4, this shifter being of electro-optical or other form selectively operable to effect a predetermined phase shift within the relevant beam portion. The video signal is applied through a filter 7, operable to remove DC components, and then a switch 8, operable in synchronism with the phase shifter to pass alternate ones of a succession of video signals to a frame store 9. A successive pair of video signals are passed, one from the store and one directly by the switch, to a processor 10, with the processor output being passed to an output unit 11 for the purposes of recordal and/or display.

The phase shifter will be operable to effect a relative phase shift of .delta. or 2.delta., video signals representing images formed respectively with and without this shift will be passed as parallel inputs to the processor by different ones of the routes through the frame store or not, and the processor will determine pixel-by-pixel from these inputs, output values for a datum phase .phi. on the basis of the earlier discussion of the invention above. Thus, if the relative phase shift is .delta. the determination follows that for equations (1) and (2) or (2) and (3), while if the shift is 2.delta. the determination follows that for equations (1) and (3).

It will be noted that phase shifting can be effected in respect of the beam portion incident on the object surface or reference surface, but not both. Also the beam can be generated in continuous or pulsed manner with, in the latter case, phase shifting and frame store input switching being suitably synchronised with the pulse timing. Furthermore, to the extent that the datum phase determination is effectively absolute, there need be no change in condition for the object surface, such as caused by deformation or other movement, as is commonly the case for electronic speckle pattern interferometry, although the invention is of course applicable to such situations.

From this clarification it will be seen that the invention provides a basis for improved phase-stepping interferometry or apparatus therefor involving a single phase change step, simplified equipment and computation, and enhanced sensitivity. Moreover, as noted initially above, while conceived and developed for electronic speckle pattern interferometry, the invention is more generally applicable to interferometric procedures and apparatus involving coherent light or other electromagnetic radiation and for which the phase-stepping technique is appropriate, such as holographic interferometry.