Approximately 90% of the energy used by an incandescent lamp is wasted as heat, which is why they have the poorest efficacy of any lamp. They are, however, very cheap to manufacture and are easily dimmable and work well with all modern dimming switches.
Incandescent lamps put out visible light at all frequencies, including many infra-red wavelengths which we experience as heat, thus their inefficiency. Because they put out all colours of visible light, they are considered a very good white light source for colour discrimination and generally have a very high CRI rating. If a change of colour is desired, then a filter or coloured glass bulb can be used to block all unwanted wavelengths and allow only the desired wavelengths out.
Incandescent lamps are often a “warm” white light source and can range from around 2400 K to 3100 K, depending on their wattage ratings and construction.
There are two main categories of Incandescent lamps: simple Tungsten Filament Lamps, and the slightly more advanced Halogen Lamps.