. Electronic apparatus for biological research . 15° 10° 5° 0° 5° 10° 15° 15° 10° 5° 0° 5° 10° 15° Polar diagram candlepower Grid filament spotlight â Ring filament spotlight Floodlight Figure Polar diagrams of the candlepower distribution of internal reflector type lamps Gas-discharge lamps With a tungsten lamp running at a high temperature it is difficult to exceed a source luminance of 5,000 cd/cm^. If a higher luminance is required it is necessary to use some form of arc discharge. The following sources are a few of the many available. Type high-pressure mercury vap
. Electronic apparatus for biological research . 15° 10° 5° 0° 5° 10° 15° 15° 10° 5° 0° 5° 10° 15° Polar diagram candlepower Grid filament spotlight â Ring filament spotlight Floodlight Figure Polar diagrams of the candlepower distribution of internal reflector type lamps Gas-discharge lamps With a tungsten lamp running at a high temperature it is difficult to exceed a source luminance of 5,000 cd/cm^. If a higher luminance is required it is necessary to use some form of arc discharge. The following sources are a few of the many available. Type high-pressure mercury vapour lampâThis consists of a quartz bulb with two tungsten electrodes between which can be formed an arc in mercury vapour under high pressure {Figure ). To protect the user against accidental rupture of the lamp it is enclosed either in a metal box with a tough glass window or in a glass envelope. It runs very hot but it should not be cooled by forced air as this reduces its efficiency. If mounted in a box with a concave reflecting mirror the effective luminance of the arc can be nearly doubled, for the arc is transparent to its own radiations and does not obscure the image formed by the mirror as is the case with a solid tungsten filament. The arc, when it strikes, acts as a negative resistance so that appropriate supply circuits are required. It will operate on either an or supply of 200 V or more, using a choke and resistance to limit the supply current and to provide an inductive starting voltage surge. It takes about 10 minutes for the lamp to reach its maximum light output after starting: if it is then extinguished it wifl not re-strike for another 10 minutes until it has cooled. 344
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