A diffractive optical element has been synthesized with laser phototechnology. It can transform a point source into a bright caustic (a light line) stretched along the optical axis. The element transforms a spherical wave into a conic and a point source in the object space to a ring structure in the image space. We investigated how the parameters of the light line (the length and the effective diameter) depend on the wavelength of the light source and the movement of the source along the optical axis. Also the interference field outside the caustic (Fresnel and Poisson fringes) was investigated. For an explanation of experimental data the geometrical theory of interferometers was used. The conditions for the appearance of fringes in white light are given. A device for rectilinear control of large tool beds and for centering machine components was manufactured. Experiments on determining the macroshape and the microshape of objects were performed.

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