It takes a couple of seconds for the low-power, low-voltage fluorescent light fixture to supply enough energy to the tube to establish an arc across, or through, the gas used in the light fixture.
This is because the small quantity of mercury that is used to create the arc needs to heat up just enough to start ionizing.
The mercury arc, which produces light at a wavelength of 337.1 nanometers just outside the visible spectrum in the ultraviolet range, excites the atoms in the coating inside the tube and they then emit photons in the visible range.
The delay happens as the arc is being established.
Unlike the electricity supply that powers modern fluorescent light fixtures in the home and office, the 24-volt electrical systems in aircraft are less “stiff,” meaning they generate less energy.
But the surge needed to light an instant-start fluorescent tube would create a significant voltage dip, lasting a few tens of milliseconds, that could play havoc with other aircraft devices.
This is the reason the light has a “soft start.”