Smith Hieftje correction method
The Smith-Hieftje background correction is basically another means of "chopping" the signal. The amplifier locks onto the chopper frequency to cancel out the background noise. "Locking on" essentially means that the amplifier will measure the difference between the two signals. 

So one way to chop the "light" from the emission tube would be to insert a "fan blade" to mechanically chop the light on and off.

The Smith-Hieftje background correction is a bit more complicated. Basically what happens is that this emission tube has two parts. A heater to vaporize the source of the atoms in the tube and an electron beam to stimulate an excited state. In this type of emission tube the heater is pulsed high and low. When the heater is low it produces enough metals atoms so that the atomic line emission is created. But when the heater is on high heat then it vaporizes so many atoms that the vapor cloud of metal atoms self-quenches the emitted atomic line emission. The high density cloud of atoms also means that the atoms bump into one another. So instead of the atoms in the cloud all having about the same kinetic energy they have a range of kinetic energies which results in line broadening.