JP2632307B

PURPOSE:To correctly quantize a picture signal by sampling the average level of the picture signal based on an output of an image sensor corresponding to respective black and white reference plates and making the average levels to the respective black and white reference plates a black reference value and a white reference value. CONSTITUTION:Before an original 1 is read by the image sensor 2, the prescribed area E1 of the white reference plate 3 is read and the prescribed are E2 of the black reference plate 4. Then, the average level of the picture signal based on the output of the image sensor 2 corresponding to the respective reference plates 3, 4 is sampled. The average level to the sampled white reference plate 3 is decided as the white reference value and the average level to the black reference plate 4 is decided as the black reference value. Thereafter, the original 1 is read by the image sensor 2 and the picture signal based on the output of the image sensor 2 during the reading is quantized according to the black reference value and the white reference value decided as mentioned above. Accordingly, the respective black and white reference values are not varied according to the fluctuation in the output of the image sensor 2.