Apparatus and method for monitoring print quality produced by a digital printing mechanism in real-time. The power source unit includes a secondary battery, a power switch, a power switch control portion, and a use-status sensing portion for sensing the status of use by a user of the optical device. The current switch has a plurality of switches respectively connected to the light source such that the control unit connected to the current switch can control an amplitude of current flowing through the light source by turning the switches on and off, thereby controlling a brightness of the light source.