British Kinematography (1948)

16 front and rear shutter. Since the effect of the lens is to invert the light beam, the two shutters, though running in the same direction, attack the light beam from opposite sides. The very popular drum shutter is in effect a scissors-type shutter. Provided it is fitted close to the gate, it is surprisingly efficient even in a quite small size, and is difficult to beat at a moderate optical aperture. Its efficiency is not much increased by increase in size, and tends to fall away with increase in angle of the illuminating beam — in effect, it becomes swamped by a wide angle beam — and at apertures above //2 is about the same as that of an ordinary rear shutter of convenient proportions. Even then, however, it has much to commend it, being very compact and enabling a very simple gear train, without the complications and power losses associated with the right-angled drives required by other shutter types. It can be fitted with centrifugal flaps to form a very simple automatic safety shutter, and it is more adaptable to the pull-through soundheads favoured in some overseas markets than is the usual rear shutter machine, the shutter casing of which is apt to come in the way. Reduction of Heat Absorption Minimisation of heat is a matter of great importance, and some overseas countries insist upon fitment of various special cooling arrangements to keep down gate temperature. All useful light sources give out also a lot of non-luminous radiation which adds to the heating effect without contributing to the illumination. We can however " cool " light by using a selective filter to screen out some of the non-luminous radiation. Suitable glass is now available, but it does involve some loss of light, and the problem arises of getting rid of the heat absorbed. In France, the use in the light beam of a glass cell containing an absorbing solution has been compulsory for many years, but the attendant light loss appears to be greater than with the new glasses. Unfortunately, however, the heat problem is most acute in extreme conditions, where the difficulty is to get enough light for the screen anyway, and the loss involved by the use of a filter cannot well be afforded. In extreme cases, special cooling by water circulation or forced air may be necessary, but such arrangements are inconvenient, and much can be done by careful design. Useful steps are the employment of polished reflectors or asbestos screens to reduce heat absorption ; a massive construction to assure rapid diffusion of heat away from potential hot spots, a large external radiating surface, and planned air circulation with draft induced by fan blades on the shutter. The most effective device is adequate screening of ineffective light, and this raises a point of importance relating to the dimensions to which the projector aperture is made. If this is to be in sharp focus on the screen, it must be very close up to the film and fully illuminated. Under these conditions, it is bound to absorb a lot of heat, and being difficult to cool, to get rather hot. It would seem preferable to increase the dimensions of the aperture so that it could be screened effectually without loss of light to the boundary of the frame, and to make use of the masking at the screen to black out the resulting vignetting effect around the projected picture, but this system is not liked, the vignetting being objected to.