PURPOSE: To position a photosensor photodetection part in the center of a light sport speedily with high precision by providing a light spot center position sensor which operates by coupling with a photosensor.

CONSTITUTION: This is a device which measures the profile of light for the exposure of the fluorescent screen of a color cathode-ray tube. An XY driving jig 9 is fixed to a panel 8 and a light profile sensor 4 is fitted to an XY table. A light spot center position sensor which is not shown in the figure is pvovided between a shadow mask 8b equipped on the panel and the light intake window of the sensor 4 to operate by coupling with the sensor 4. The XY table is driven until the position sensor is aligned to the spot center of light to be measured which is passed through an aperture 8c in the mask, and after the sensor 4 and spot are positioned, the sensor 4 is scanned in directions X and Y to measure the quantity of light in each direction.