Image signal correcting circuit

Claim 2

An image signal correcting circuit characterized by an image signal correcting means comprising: a light guide; a light source means for providing an illumination light through the light guide for an observed object; a logarithmic compressing means for logarithmically compressing an image signal; a two-dimensional filter means for relatively suppressing a low band component to eliminate an illumination unevenness caused by the illumination light and enhancing a high band component of said logarithmically compressed signal; and an exponential converting means for converting the image signal, having passed through said two-dimensional filter means, to be of an exponential characteristic.

Claim 3

An image signal correcting circuit according to claim 1 wherein said two-dimensional filter means has a filter characteristic variable means which can variably set a signal gain for the high band component and a signal gain for the low band component respectively independently.

Claim 4

An image signal correcting circuit according to claim 1 wherein said two-dimensional filter means comprises a delaying means consisting of two 1H-delay parts each delaying by 1H a picture signal from said logarithmic compressing means, a coefficient multiplying means for multiplying through a coefficient multiplier respective non-delayed, 1H-delayed and 2H-delayed signals from said delaying means by a coefficient, an adding means consisting of three adders for adding the non-delayed, 1H-delayed and 2H-delayed signals having passed through said coefficient multiplier, a current converting means for converting signals, having passed through said adders, to current signals, a current adding means consisting of a delay part for two picture elements for current-adding signals from said current converting means and an impedance converting means for putting out said current-added signals.

Claim 5

An image signal correcting circuit according to claim 1 wherein said two-dimensional filter means comprises a first delaying means consisting of two 1H-delay lines each delaying by 1H the image signal from said logarithmic compressing means, an adding means for adding through adders respective non-delayed, 1H-delayed and 2H-delayed signals from said first delaying means, a second delaying means for delaying by one picture element the 1H-delayed signal from said first delaying means, a first coefficient multiplying means for multiplying by a coefficient the signal from said second delaying means, a current converting means for converting to current signals the signals having passed through said adders, a current adding means consisting of a delay line for two picture elements for current-adding the current signals from the current converting means, a second coefficient multiplying means for multiplying said current-added signals by a coefficient and a subtracting means for subtracting from the signal, from said first coefficient multiplying means, the signals from said second multiplying means.

Claim 6

An image signal correcting circuit according to claim 2 wherein said filter characteristic variable means comprises a first gain variable means for making variable the signal gain for the high band component, a γ-correcting means for γ-correcting the signal from said first variable means, a second gain variable means for making variable the signal gain for the low band component said second gain variable means including a noise suppressing means for suppressing noises in said logarithmically compressed signal and an adding means for obtaining an added output of the signal from said γ-correcting means and a signal provided from the noise suppressing means.

Claim 7

An image signal correcting circuit according to claim 1 wherein said image signal is a brightness signal in an electronic endoscope.

Claim 8

An image signal correcting circuit according to claim 1 wherein said image signal is provided by a solid state image pickup element in an electronic endoscope.