||This lamp is representative of one of the earliest designs of daylight metal halide developed by German Osram. Of primary interest is the long arc tube, which made the lamp quite sensitive to variations in orientation. This /DV lamp is for use vertically ± 45°, but to satisfy the demand for horizontal operation it was joined later by the /DH version. That has a slightly modified dosing to counteract the effects of the arc bowing upwards, and operates at about 10% reduced efficacy.
The long arc and low wall loading implies a rather low cold spot temperature, especially for the DH type, and as a result the efficacy is not especially impressive. Attempts to overcome this have been made by using a vacuum outer bulb.
The daylight spectrum is produced from a dose based on the iodides of dysprosium, which produces a very high density of spectral lines in the visible region, and thallium which serves to balance the colour point and increase efficacy by virtue of its strong green line. However this chemistry is well known to cause severe arc constriction, which generates strong convection currents and brings the risk of arc instability and flicker. The problem is minimised by the addition of a small amount of caesium iodide, an arc-fattening agent which increases the arc diameter to more stable proportions.
In mid 1989 Osram replaced the /DV and /DH with a new universal burning HQI-T 400W/D. The problems of arc bow were solved by reducing its length from 49 to 29mm, and at the same time designing the lamp for use on a sodium ballast thus increasing its rating from 360W to 420W. However overheating problems saw the lamp modified again in 1999 to the current HQI-BT 400W/D design with bulged outer jacket.