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11)
effective way of doing this is to prevent the heat from reaching the parts in question, that is, by preventing direct radiation from the light source from reaching them. This can be done by making the fitting into a double walled enclosure. In arc lamp units it is usual to lag the space between these walls with heat insulating material. In incandescent units no such lagging is used (since the heat produced by the lamp is relatively small) but it is effective to have the inner (facing) walls polished and the outer wall (facing the open air) painted. The wall facing the lamp must also be painted black to reduce stray light. This double wall construction is shown in Fig. 13 and it is interesting to note that the temperature of the hottest part of the sloping surface near the top of the unit is 60 degrees C. (ambient temperature 25 degrees C). If the double wall is replaced by a single skin painted black and screwed up to the casting, this temperature rises to 85 degrees C.
In the fitting shown in Fig. 13 the lead screw driving the focusing mechanism is also shielded from direct radiation by being enclosed in a metal tube. This prevents binding due to overheating and obviates the need for frequent greasing.
Fig
I I. Schlieren photograph of air streams in studio illuminator (b) Baffled ventilating system
(a) Efficient ventilating system
The construction conventionally used for incandescent lamp fittings consists of an aluminium alloy casting, suitably relieved to reduce weight, and filled in with sheet metal. This construction lends itself admirably to the double-wall requirement as does another with a pressed aluminium body looking almost identical to that shown in Fig. 13. It is interesting to note, however, that one fitting now on the market is a complete break-away from this design. The fitting is made of U-shaped bent metal pieces on the principle shown in Fig. 12. This ; design leads to a very light weight fitting, but it gets much hotter than the more normal design. It is novel and it will be interesting to know whether the saving in weight proves, with use in the studios, to be worth the sacrifice of coolness.
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12. Principle of sheet metal design : Section