The present invention discloses a dual spiral fluorescent lamp that is smaller in volume than a conventional one. The charge storage time is changed over responsive to the turning-on-and-off period of the light source or the integral multiple thereof to set up a stored light amount for each pixel stepwise and difference in the stored light amount between steps is interpolated by continuous gain control of read-out signals of pixels. The opening can extend from the first surface towards the second surface and can have an inner wall extending away from the first surface.