into electrical signals the signals are then supplied to a processing circuit 6 and an encoder 7 which outputs video signals the r g and b color sensors each have the same structure as that of the image pickup device described with fig1 to 4 however in this embodiment the thickness of a gate conductor made of polysilicon of each of the r g and b color sensors is set to a particular value proper to each color r g and b respectively fig6 is a graph showing spectral sensitivities of image pickup devices each having a respective gate conductor thickness proper to one color from among r g and b as shown in fig6 the sensitivity of each color can be improved by setting the thicknesses of gate conductors polysilicon of blue green and red color image pickup devices to 500 650 and 800 u00e5 respectively as compared with the thinnest thickness of polysilicon of 100 u00e5which is more difficult to control during a manufacturing process than thicknesses such as 500 650 and 800 u00e5 a method of calculating the spectral distribution of an image pickup device will be described below with the calculated spectral distribution a proper gate conductor thickness can be obtained refraction indices of polysilicon and bulk silicon are shown in fig7 and 9 a refraction index is represented as a complex refraction index n ik and an absorption coefficient u03b1 is represented by u03b1 2 u03c0 u03bb u00b7 k a difference of refraction indices between polysilicon of about 100 u00e5 fig7 and polysilicon of about 3000 u00e5 fig8 results from a change of the crystal property of polysilicon the value of a complex refraction changes with the thickness.