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unit. It should be possible to get a sharp spot or a wide flood out of one unit and keep a beam pattern which would always be slightly high in intensity in the centre and taper off at the edges. The equipment should be small in size, light in weight, yet durable enough to withstand handling and frequent moving about.
For incandescent equipment it became apparent that lamps which were absolutely #pre-focussed were an_ essential feature of the design, these lamps to be mounted in holders so that lamps could be changed without disturbing the focus of the unit. In short, it would be desirable to have the efficiency of the parabolic mirror spotlight combined with the beam pattern of the plano-convex condenser spotlight.
The Fresnel Lens
Towards this end several years of diligent study brought forth a spot lamp using the Fresnel type condensing lens which offers the advantage of a short focal length lens without the bulk of glass which is necessary on lenses of short focal length of the plano-convex type. This great bulk of glass decreased the efficiency of the unit and caused high breakage by reason of heat.
The advantages of the short focal length lens is shown in Fig. 4.
f
Figure 4
Pre-focussed Lamps
The need of the pre-focussed incandescent lamp had by this time become very apparent, and the lamp manufacturers produced what is commonly called the bi-post lamp, which has many advantages over the older types.
It was now possible to design a lamp holder and spherical mirror which would always keep the proper position of the filament with relation to the optical system. This lamp has the further advantage of being more compact, which enabled new equipment to be designed in smaller size for a given wattage. The spherical mirror, which picks up the back light of the filament and redirects it through the lens, is so mounted that lamps can be changed from the back, and this mirror pushed back into place without altering the focal position.
Figure 5
Fig. 5 indicates the compactness of the design of this equipment, Fig. 6 showing the beam pattern which indicates that, except for the very hot spot which is procurable from the parabolic mirror spotlight, this lamp in every way meets ithe requirements set forth for a more usable lamp so that the cameraman can have sufficient flexibility in the size of the sources used.
It is now possible to secure these lamps designed to take a 250 or 500 watt lamp, while the next size takes a 1,000 or 2,000 watt lamp, and the largest size takes a 5,000 watt lamp.
Are Lamps for Colour Work
Since it was necessary to design new arc lamp equipment to meet the demands of colour photography at the time when this new incandescent equipment was being designed, it soon became apparent that the Fresnel lens spot lamp offered many advantages for are lighting