Luminaire design and planning of lighting systems. Osram HQI-E 150 W/NDL CL
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7 Luminaire design and planning of lighting systems
7.1 Measuring temperatures, ambient temperature
Outer bulb
7.1.1 General physical conditions for temperature limits for outer bulbs and pinches in metal halide lamps
Foil
Radiation
Heat flow
When the limit values for pinch temperature or outer bulb temperature for hard and soft glass are exceeded, the following can be expected: Capillary
Arc tube
• the foil oxidizes
• the evacuated outer tube collapses and the gas-filled outer tube blows out when the glass turns soft
Fig. 33: Diagram to show thermal coupling of arc tube and outer bulb
7.1.2 2 Measurement with thermocouple
• the cement in screw-base lamps crumbles
Although quartz glass can withstand far higher temperatures than the stated limit values, it is possible for the arc tube to overheat when the stated limit values for the outer bulb are exceeded, with the following effects:
Measurement with thermocouple is a simple, practical method of obtaining measured values.
• change in colour properties
• leaking arc tube
• blackening of the arc tube and deterioration in luminous flux maintenance
Fig. 34: Affixing the thermocouples to the outer bulb and the lamp base
The outer bulb temperature is only an indirect indicator for arc tube load! Outer bulb and arc tube are coupled by radiation and to a small extent by thermal conduction via the leads.
While it is important to limit the outer bulb temperature, it is worthy to note that an inadequately designed reflector can still cause the arc tube to overheat without any major change in the outer bulb temperature
(see also chapter 7.9 “Optical design of reflectors”).
One indication that the design of a luminaire is insufficient comes from comparing the lamp voltage measured when burning freely outside the luminaire, and inside the luminaire after being left long enough to stabilize. The increased lamp voltage measured in lamps < 400 W in the luminaire should not exceed 5 V.
There is no notable difference in the lamp voltage inside the luminaire for lamps with wattage input
> 1000 W, outer bulb and free-burning operation. But soiling of the outer bulb caused by evaporation from luminaire components can cause the lamp voltage over its service life to increase more than in free-burning operation. This increase depends on the level of surface soiling in the lamp and can therefore not be put in figures. To avoid this effect, it is recommended to use temperature- and UV-resistant materials in the luminaire.
Fig. 35: Clamping the thermocouple on the outer bulb using a spring element
Please adhere to the following to properly measure with a thermocouple:
• Good contact with the surface being measured
• Low heat dissipation from the connection point and therefore
– thermocouple wire with a small diameter
– thermocouple wire parallel to the surface being measured
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