For many years, the compact fluorescent lamp was promoted as the ideal replacement for inefficient incandescent bulbs, but it was far from popular with the public at large because of its strange appearance and inability to immediately provide full-power illumination. The design limitations of what is basically a fluorescent tube also presented physical fitting difficulties in some instances. The arrival of LED lighting technology changed all that and LED lamps are now available in a bewildering range of different versions, including straightforward replacements for some earlier types. One criticism of LED lamps was that they failed to produce the familiar 'daylight' white light but warm-white versions are now available. The white LED is in fact formed from a combination of different colours. Selecting a suitable LED bulb can pose a few problems and the first thing to establish is the fixing type, by examining either an existing bulb or the fixing socket itself. For new installations, a choice of suitable fixings can be found. The power output is usually expressed in watts and bears little similarity to earlier bulbs. Lumen output may also be stated. LED lamps are available as replacements for almost all types, including halogen lamps, but large fluorescents employing ballasts will probably require rewiring.