The GLS is the original ‘General Lighting Service’ bulb-shaped light source, which has barely changed in its appearance from the time of its invention in the late 19th Century to today. In its incandescent form the GLS bulb has a tungsten filament at its centre, which glows hot when voltage is applied. In common with all filament bulbs, the colour temperature is also the physical temperature of the filament. EU legislation has largely phased out production of incandescent GLS bulbs in recent years because of their inefficiency as a light source.
Despite the decline of the original GLS incandescent bulb, the classic and familiar shape of the GLS lamp is still in mass production using the three main alternative technologies:
- Energy-saving halogen is the cheapest replacement for a regular incandescent bulb, and is the more reliable technology for accurate colour rendition. Because it radiates a continuous spectrum, halogen scores a CRI 100 rating by default for colour rendering, despite being a naturally warm source of light (typical 3000K colour temperature).
- Fluorescent GLS bulbs offer substantial energy savings and significantly increased lifespan over incandescent or halogen GLS lamps at a lower cost than LED.
- LED GLS bulbs are the ultimate energy-saving choice of lamp, and also offer long lifespans. Because LED light is naturally directional, GLS lamp designs usually involve the use of multiple LEDs for omnidirectional output. The ‘yellow’ bulbs sometimes seen use a remote phosphor to create white light; the yellow disappears once the bulb is switched on.
Standard GLS bulbs are widely used in table lamps, desk lamps, and many types of shaded ceiling pendant. In more compact light fittings a similar but smaller ‘golf ball’ bulb is typically used, and in open pendants the more decorative candle bulb is preferred.
A classic ‘household bulb’, the GLS bulb is also sometimes manufactured with a tougher glass and a reinforced filament for industrial use. This variation of a GLS bulb is known as a ‘rough service’ lamp, or bulb, and is designed to better withstand vibration and shock.
Large bayonet (B22); Edison Screw (E27).