We describe a laser wavelentgh calibration system based on a grating spectroscope used with the method of overlapping orders. Illuminated by a neon spectral lamp it gives about 1500 reference lines. A small mirror, put on the grating axis, is lightened by a He–Ne laser and gives an angular measurement which helps in the identification of the lines. Calibration of the laser wavelength is obtained by linear interpolation between two neighboring reference lines.