3. Common light sources

3.1 Incandescent lamps

Tungsten filaments can be heated up to 2,700 K in a vacuum. To go a little higher, we need to encourage the redeposition on the filament of the tungsten atoms deposited by evaporation on the bulb's quartz. This is achieved by a judicious mixture of halogen gases (bromine and iodine), hence the name "halogen bulbs", which became commonplace around 1980, until the white LED revolution led to them being banned from sales (in 2008) if a substitute existed. Their advantage is a smooth spectrum, extending into the near infrared as far as the cut-off point of the bulb material (i.e. around $\lambda \sim \mathrm{2,5}\mathrm{\hspace{0.17em}à\hspace{0.17em}}\mathrm{4}\mathrm{\hspace{0.17em}\mu m}$...