JPH0546752B

PURPOSE:To prevent the deviation of a gamma control value due to the variance of the halftone density by measuring the halftone density of a diascope and controlling the gamma value of a reference channel on a basis of the measured density. CONSTITUTION:The video signal of the incorporated diascope chart which is picked up by an image pickup tube 1 and passes a video signal processing circuit 2 and is amplified is inputted to a gamma control circuit 3 and is initialized. The gamma control is turned off, and the video signal of a channel G as the reference channel is inputted to a level detecting circuit 4 in this state to detect an actual halftone transmittance of the incorporated diamond scope chart at this time. This detected level is inputted to a CPU6 through an A/D converter 5, and a value corresponding to the video signal level of a halftone part is calculated and is stored in a RAM10. The gamma value of the channel G is controlled on a basis of this value to prevented the deviation of the gamma control value due to the variance of the halftone density.