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November, 1950
ROSS : NEW STILL PROCESS PROJECTOR
161
is 85 lumens per watt, and for white light perfectly filtered 220 lumens per watt). It is calculated that this combined heat-filter and lens absorbs only 10 per cent, more of the light than does an ordinary lens. This is, however, recovered, for it is unnecessary to use a spatter glass because the cool quartz window is not severely spattered by the arc, and is, in any case, not difficult to replace.
One face of the second lens is aspheric and this lens, together with No. 1 lens, images the crater into No. 3 lens.
The theoretical function of the relay condenser lenses is to provide sufficient variables in the design to fulfil all the required conditions — to accept the maximum amount of light, to fill the gate aperture and obtain the optimum distribution, and to fill the objective lens. In practice, the primary function of No. 3 lens is to throw on to the gate an image of a surface near No. 2 lens ; by moving No. 3 lens nearer the gate the light is spotted, i.e., reduced in area and increased in brilliance, and moving it towards the arc floods the gate. No. 4 lens also has an aspheric surface, and its primary function is to converge
Fig. 2. Schematic diagram of ths optical system, incorporating a combined heatabsorbing filter and water-cooled No. I lens, relay-condenser lenses, and an f/2 objective.
No 3 LENS
No
GATE
LENS
DIM -OUT SHUTTER
OBJECTIVE
IO INCH f}2
the beam of light so that it may pass through the objective lenses, which are of various focal lengths. t
The mounting of each lens is designed so that it may be easily removed tor cleaning and yet be dropped back in exactly the correct position.
Dim-Out Shutter. ,,_'.,* r
Between Nos. 3 and 4 lenses a control is fitted for altering the brightness ot the picture on the screen, comprising two curved blades of thick polished aluminium. It is, in effect, a two-bladed iris diaphragm. It is important that this shutter should not alter the distribution of the lighting over the screen while reducing its overall brightness, and its position is chosen so that it works equally well with all the objective lenses of different focal lengths, from 6.4 to 16 ins. It is possible to reduce the light on the screen by any amount down to one-eighth of the maximum without producing non-uniformity in the light over the screen of as much as ±10 per cent. The light may also be dimmed right out. The shutter is not damaged in any way by the heat developed on it.
Objective Lenses.
The objective lenses were specially designed by Mr. A. Warmisham andhis staff, and made for this type of projector by Taylor, Taylor and Hobson Ltd. ; they are believed to be unequalled in performance. The range available is given in Table I, and against each is noted the focal length of a camera lens