**7. Conclusion**

Multiple-wavelength digital holographic interferometry using tunablity of laser diodes for measuring a large step-height with high accuracy was presented. The requirements for performing the phase unwrapping were discussed. We have found that precise knowledge of the recording wavelengths is required for correctly performing the phase unwrapping. The required precision of the knowledge was derived. A simple and fast algorithm for pixel size adjustment was presented. It has been demonstrated that the problem of the phase subtraction in two-wavelength holographic interferometry can be solved by the present method.

Several holograms were recorded through the changes in the wavelength of a laser diode by injection current and operation temperature controls. A pair of holograms with a small wavelength difference less than 0.01 nm was recorded and used for realizing holographic interferometry with a large synthetic wavelength more than 120 mm. Phase differences with synthetic wavelengths from 0.4637 mm to 129.1 mm were extracted by using the holograms. The synthetic wavelengths were calibrated by the comparison between the phase differences and the object heights measured by using a slide caliper. The step-heights of 0.1 mm and 12 mm were correctly detected.
