Patent ID: 7122975

Claim:
A filament lamp light quantity control method for controlling a light quantity of a filament lamp, comprising: providing a current detection sensor connected in series to the filament lamp having a characteristic in which a change in a light quantity of a lamp is substantially proportionate to the square of a power consumption; providing a multiplication circuit for generating a voltage V w proportionate to the product of a terminal voltage V of the filament lamp and an output voltage V i of said current detection sensor; providing a multiplication circuit for generating a voltage V WW proportionate to the square of said voltage V w ; providing an error amplifier circuit for controlling an error to the minimum by comparing the output voltage V WW of said multiplication circuit and the control voltage V c ; providing a constant voltage source unit for supplying an electric power to the filament lamp, the unit being connected to an output terminal of the error amplifier circuit; and controlling the light quantity of the filament lamp by controlling said control voltage.