Patent ID: 6949891

Claim:
A light quantity adjusting method, which is used for driving a light emitting device with a drive current at a first level which is generated in accordance with a modulation signal based on an image signal, and driving said light emitting device with a drive current obtained by adding a drive current at a second level to the drive current at the first level for at least one or both of a predetermined time period right after rise of the modulation signal and a predetermined time period right before fall of the modulation signal, said light quantity adjusting method comprising the steps of: storing a resolution ratio between a drive current setting resolution of first set data used to determine the drive current at the first level and a drive current setting resolution of second set data used to determine the drive current at the second level; measuring light emission quantities of said light emitting device based on predetermined two set data which are set in advance to become higher in level than an oscillation threshold current of said light emitting device, but not to exceed in level a rated current of said light emitting device in adjustment of the light quantity, respectively, to calculate and store rates of change in light emission quantity of said light emitting device for the predetermined two set data; and calculating a value in the first set data as a first set value so that the light emission quantity of said light emitting device corresponding to the drive current at the first level agrees with a target value using the rate of change of the light emission quantity on a straight line connecting two points corresponding to the two set data obtained through the measurement, and calculating a value in the second set data as a second set value so that the light emission quantity of said light emitting device corresponding to the drive current at the second level agrees with a target value using the resolution ratio and the rate of change in the light emission quantity on the straight line, thereby driving said light emitting device with the drive current at the first level and the drive current at the second level which are determined based on the first set value and the second set value obtained through the calculation to carry out the light quantity adjustment by which an error due to degradation or time-dependent change of a light emission efficiency is suppressed.