||In 1989 the Philips Spotline series of incandescent reflector lamps was introduced, and marked one of the most significant improvements in the five decades since original blown-bulb paraboloidal lamps were developed. It employs a novel double reflector concept to achieve a major increase in luminous intensity for a given flux and beam angle.
A drawback of conventional blown-bulb reflector lamps is that <40% of the filament's luminous flux impinges on the reflector surface. The rest exits uncontrollably via the front window of the bulb, or is lost backwards via the neck. As a result the intensification of light due to the reflector, i.e. the ratio of the lamp luminous intensity on the optical axis vs the filament luminous intensity in that direction, is typically only about a factor 10. The Spotline system increases the intensification to a factor 15-25 depending on the bulb diameter, thanks to the unique shape which controls 60% of the filament's luminous flux. The net result is a significantly brighter beam for the same luminous flux and beam angle.
Spotline lamps have superior reflectors designed with the entire filament length and shape in mind, whereas earlier lamps treated the filament as a theoretical point source. A key feature is the double reflector concept, where part of the bulb crown is coated so as to redirect to the main rear reflector light rays which would otherwise escape without control. Also the diameter of the transition to the bulb neck is narrowed, to reduce light loss towards the cap. Incidentally the groove shaped indent at this point was later dropped. Originally it had been intended to market some lamps with a metal anti-glare shield clipped to this part of the bulb, but that idea was never commercialised.