Answer:
If a bulb is rated at 60 W it uses 60 W (60 J/s) for its operation.
If only 18% of this power input provides useful light then the light produced each second is
P = .18 * 60 J / s = 10.8 J / sec
In one sec W = P * t = 10.8 J/s * 1 sec = 10.8 joules
