Patent
1980-06-25
1982-12-07
Henry, Jon W.
G02B 718, G02B 508
Patent
active
043623638
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a polygonal mirror, comprising a number of mirror surfaces arranged round an axis of rotation, each of which surfaces is inclined equally to the axis of rotation.
Such a polygonal mirror is known from the published Dutch Patent Application No. 7705514. In that application a process is described for the manufacture of a polygonal mirror to be rotated at high speed by which process an exactly formed multiangular master is provided, an aluminium preform is installed in the master and a thin film of plastics material is centrifugally cast against the aluminium preform in a replica process. With the process described in that application polygonal mirrors can be obtained, of which the mirror surfaces can be inclined at any angle to the axis of rotation. The centrifugal casting takes for instance place with an epoxy resin. After the epoxy resin has cured, the master is removed.
Such a process has disadvantages: due to the curing the epoxy resin shrinks slightly, so that the mirror which is obtained ultimately is not exactly the complementary image of the master. Therefore waste of mirrors which have not shrunk in the way expected, cannot be prevented. The master must be manufactured very precisely, which requires much skill. At a draft-angle of less than 30.degree. the master should even consist of more parts. Stringent measures must then be taken in order to reposition the master parts exactly with regard to each other for each mirror to be manufactured.
It is an object of the present invention to provide a polygonal mirror which can be manufactured in a simpler way but which nevertheless can be made with high precision.
This object is achieved according to the invention in that a reflecting foil is tightened over a number of supporting elements arranged round the axis of rotation.
In this way a light, simple construction is obtained, which nevertheless yields a high precision polygonal mirror. A special advantage is, that the foil is the part with the greatest distance to the axis of rotation. In this way it has been achieved in the first place, that the centrifugal forces on the foil, which forces are proportional to the mass of the foil and to the distance from the axis of rotation, remain small because of the low mass of the foil. In the second place the moment of inertia of a polygonal mirror according to the invention is smaller than that of known polygonal mirrors of the same diameter. Consequently for the same angular velocity the rotational energy of a rotating polygonal mirror according to the invention is smaller than that of known polygonal mirrors. A small moment of inertia is also favourable for the driving and bearing of the polygonal mirror.
A preferred embodiment of a polygonal mirror according to the invention is one in which the supporting elements are installed rotationally symmetrically with regard to the axis of rotation and consist of six first cylinders, all with the same outer diameter, which are placed, each touching its two neighbours and each in contact with a central cylinder the axis of which coincides with the axis of rotation.
In this embodiment use is made of the known fact that a circle of any diameter is encircled exactly by six circles of the same diameter. As the outer diameters of all cylinders are equal to each other, the central cylinder is encircled very exactly and without interspace by the six first cylinders, which touch each other exactly. Thus in a simple way a very exact hexahedral mirror is obtained, the angles between neighbouring mirror surfaces necessarily amounting to exactly 120.degree.. The equality of all outer diameters is achieved by exactly turning a bar of material such as aluminium or silver steel, with generally known means and subsequently dividing it into seven pieces. A first piece serves as central cylinder and the other six pieces form the six first cylinders.
A specially advantageous embodiment of a polygonal mirror according to the invention, in which the foil is shaped as a strip of which both ends are fixed in a groove
REFERENCES:
patent: 4119365 (1978-10-01), Powell
Mulder Bernardus A.
Witteveen Bontko
Henry Jon W.
Johnston Albert C.
Oce'-Nederland, B.V.
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