In recent years Magneto-optical techniques have been used extensively in memory systems, which allow high density recording and high speed access time. Usually In this method the light beam from a He-Ne laser is linearly polarized by a polarizer and passes or reflected from a magneto–optic recording medium thin layer. The polarized light will modulate and the plane of polarization is rotated. The rotated polarized light passes through an analyzer and focus on a photo-detector. The output of the photo-detector is measure as a photocurrent signal according to Malus law. In all measurement the Cosin Malus law is the base and accepted. But in our present work we found that in practice it is necessary to modify the signal or modify the Malus law. In other word some constant should be added to the Malus law. In this measurement we averaged about 5000 reading for signals in each point and then we have plotted out the Malus law. The measured values are compared between the theoretical and practical measurement

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