Even small devices with a weight of roughly three kilos produce a very bright picture. The secret lies in lamp technology. These video projectors use metal vapour lamps, so-called UHP lamps, which are highly pressurized and have an excellent degree of efficiency. They convert about 70 % of electrical energy into visible light, whereas for halogen lamps it is only 20 %. The rest is radiated as heat. UHP lamps have a very long life ranging from 1,000 to sometimes 3,000 hours. Compared to this, the halogen lamps of slide projectors only had a lifetime of 35 to 200 hours. Data projector lamps, on the other hand, are distinctly more expensive. According to the chosen type, they are about 300 to 700 Euros. Normally, high-pressure lamps last forever, although their luminosity will drop over the years. This is why manufacturers always specify the lamp life and the time after which the lamp is reduced to 50 % of its original brightness. Most data projectors have an operating hours counter to control this factor.
In the medium-term UHP lamps will be replaced by laser or LED systems as UHP lamps contain the poisonous mercury. LED system are actually only considered for home cinema projectors as they are not yet bright enough. Laser system nearly always use a phosphor wheel in order to turn blue light into green light. However, green lasers are still lower quality than blue or red lasers. The advantage of such a system is its long lifetime which is usually 20,000 hours. After this time the system is still in working order but will only have 50 percent of its original brightness.
There are data projectors with a luminous flux between 500 to 20,000 lumens and more. This brightness value is measured in nine fields distributed across the projection image and output as a mean value. In order to convert the luminous flux into lumens the measured brightness (Lux) is multiplied by the projection surface (m²).
The required projector brightness depends on the projection environment and the desired image size. In well blacked-out rooms with a luminous flux of 1500 lumens an image width of more than 5 m is possible. The larger the projected picture, the darker it gets. If you divide the luminous flux by the size of the projected picture, you receive a value in lumens per square meter. The following rule of thumb with reference to the ambient light applies:
less than 100 lumens/m² 
allows only a limited projection quality even in dark rooms
100 to 250 lumens/m² black out room well enough
250 to 400 lumens/m² room need not be completely dark
more than 400 lumens/m² allows daylight projection although room should be protected from direct sunlight
In practical life, brighter images are subjectively perceived as better quality. A projector with extra capacity in brightness will be able to cope much better with unexpected interfering light sources, such as emergency escape lighting or light leaks in darkening systems.
See also