Does a light bulb's brightness depend on voltage or current?

The bulb (assuming incandescent here) brightness depends on the temperature of the filament. That in turn depends on the power being dissipated per length of the filament. read more

The bulb’s brightness depends primarily on the power it dissipates (assuming we are comparing bulbs of the same type, particularly incandescent bulbs with a resistive filament). Power depends on both voltage and current, as you can see from the equation W = E x I . read more

Wattage is what determines the brightness of the light. For example, a 100 watt bulb is brighter than a 60 watt bulb. Lumens is the unit of measurement for that brightness. read more