The electric, thermal and optical behaviour of an incandescent lamp was studied by a real time
laboratory (RTL) apparatus, using two voltage probes and a light probe. The software STELLA was used
to model the phenomena and to analyse the transient behaviour in absence of thermal equilibrium. We
show how the Joule-heating effect explains the non-linear current/voltage relation and how the
filament thermal capacity affects the phase lag of light peaks with respect to input power peaks.