Halogen Lamps - Voltage Control x Wavelength

Having the lamp filament on standby decreases to time needed for the lamp output to stabilize when switching to the higher voltage.

I suspect the equipment manufacturer has determined empirically that the time allowed (you say about 3 seconds, but it's probably quite precisely defined) is sufficient for reaching a stable output, and the reading is actually made after the delay.

I used a similar system in some optical densitometers, also using a halogen lamp as the light source, and obtained stable, consistent results (to better than 0.2%) after about 2.5 seconds of warm-up time (from standby). In more recent machines I've added light output monitoring and feedback, which has improved the stability, but it still needs about 2 second to steady.