A new technique is proposed to provide a profile measurement, especially for computer discs, wafers or glass substrates that are highly accurate in flatness. A reflection moire technique using phase shifting method is tried to this flatness measurement. To apply this method to large size samples such as LCD flat panel displays, two trials extension of measurement size and elimination of the reflected light from the back plane of the glass substrate, are developed. One is to combine small areas one after another the flatness of which is measured in advance. The other is using UV wavelength. A wavelength of 313 nm, which is absorbed into the LCD glass substrate, adopted as the light source, and synthetic fused silica lenses are used in optics. It is possible to measure the flatness of the front plane. The technique, which produces much higher accuracy than conventional techniques, can be available in various of optical measurements.