A method of evaluating effective irradiance is discussed for photoresist with the reciprocity law failure property irradiated by fluctuating radiation. The resultant formula for evaluating effective irradiance suggests that both reciprocity law failure and time response for the photochemical reaction should be considered when evaluating effective irradiance. This method is used in an irradiance meter which measures the distribution of near ultraviolet effective irradiance. This irradiance meter is applied to the photoresist exposure process in color TV picture tube fabrication. This meter automatically measures relative irradiance over 51 points on the phosphor screen of a color TV picture tube, in about 1 min. Reproducibility and accuracy for measured values are estimated to be about 0.2% and 1%, respectively. Absolute irradiance is measured at the center of the screen, with an accuracy of about 3%.