Optics: motion pictures – Shutters – With plural light paths
Patent
1980-03-07
1982-05-18
Henry, Jon W.
Optics: motion pictures
Shutters
With plural light paths
350288, 264 12, 428408, G02B 508, G03B 910
Patent
active
043301835
DESCRIPTION:
BRIEF SUMMARY
The invention relates to reflex devices, preferably on cinematographic and photographic cameras with rotating, oscillating or displaceable retracting mirrors.
The mirrors of such devices or similar devices are optical precision mirrors which are usually made of glass or also of metal. For well known dynamic reasons, such moved mirrors should have as little weight as possible, irrespective of whether they are rotating, oscillating or pivoted mirrors. One can endeavour to obtain a low weight for such mirrors by making them thin but, by reason of the required surface quality for all precision mirrors, this has a limit in the minimum ratio between the mirror thickness and size. In precision equipment such as reflex cameras, constructional reasons often make it necessary not only to balance the rotary mirror shutters statically but, because of the high rotary speeds between 1500 and 6,000 r.p.m., also to balance them dynamically. In the case of one-piece unsymmetrical mirrors this usually involves an additional mass to place the necessary counterweight in such a way that it will not obstruct the optical passage of the beams at the rotating mirror system. Since the balancing masses for dynamic balancing are relatively large by reason of the mirror weights that can nowadays be achieved, the space available for accommodating them between the rotary mirror shutter and the plane of the film is often inadequate, even if the balancing masses are made from tungsten. The larger the balancing masses have to be, the more detrimental their higher inertia will be for the camera drives which are nowadays usually precision controlled. An undesirably high inertia necessitated by an excessively large mirror weight also has a very unfavourable dynamic effect on oscillating or pivoted mirrors.
The invention is based on the problem of improving the reflex device of the aforementioned kind so that a weight reduction of the dynamically operated reflex mirrors is obtained for the same optical surface quality, whereby dynamic balancing is made possible in restricted space conditions particularly for one-piece unsymmetric rotating mirrors and the inertia is considerably reduced in the case of rotating, oscillating and pivoted mirrors.
The solution of this problem by means of the invention resides in that the invention uses for such reflex devices of the kind here in question pure carbon, preferably in the form of glass-like carbon (also known as vitreous carbon), for the mirror carrier material. A reflex device having the mirror made from this carrier material offers a number of considerable advantages which will hereinafter be described. In what follows, the mirror carrier material as distinct from the mirror coating will, for the sake of clarity, be referred to simply as a substrate.
1. The specific weight of the glass-like carbon is about 40% less than the light weight types of glass that are nowadays available and, because of the aforementioned dynamic reasons, this is of decisive importance for balancing purposes and the inertia behaviour of movable mirrors.
2. The mirror substrate of pure carbon, preferably so called glass-like carbon, is much more temperature-resistant than glass and, by reason of its coefficient of expansion at about 3.10.sup.-6 per degree which is on average only half that of glass, it is even less liable to distort than glass. This property of a high temperature resistance is particularly valuable in the case of vacuum vaporisation of complicated dichroic multi-layer coatings such as metal oxide vaporised mirrors because here the mirror substrate is subjected to temperatures up to above 600.degree. K. which destroys the high surface quality of many optical types of glass. In the practical use of professional cameras having reflex devices, considerable temperature fluctuations can occur up to about 130.degree. K.; e.g. in the baggage compartment of an inter-Continental aircraft or in polar expeditions temperatures of about 220.degree. K. are no rarity; in contrast to this very low temperature, temperatures up
REFERENCES:
patent: 3833347 (1974-09-01), Angle et al.
patent: 3900328 (1975-09-01), Parsons et al.
Cowland, F. C., "A New Form of Carbon", Compon. Technol., vol. 4, No. 6, pp. 3-7, Feb. 1971.
Arnold & Richter Cine Technik GmbH & Co. Betriebs KG
Henry Jon W.
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