Optical: systems and elements – Single channel simultaneously to or from plural channels – By surface composed of lenticular elements
Patent
1998-10-13
2000-03-07
Ben, Loha
Optical: systems and elements
Single channel simultaneously to or from plural channels
By surface composed of lenticular elements
359668, 359711, 359799, 359800, 359868, 359619, G02B 2710
Patent
active
060348224
DESCRIPTION:
BRIEF SUMMARY
The invention concerns a device for distributing light emitted by a virtual point source on the surface of an image frame, comprising focussing means for projecting said light towards an image point of the source, a divergent optical element suitable for transforming the convergent beam emanating from the focussing means into a divergent beam, and a condenser lens collecting said divergent beam to concentrate it towards the image frame. The invention applies more particularly to the case where the image frame has a dimension exceeding 8 cm.
Xenon arc lamps with a power of at least 1600 W are used in large-format slide projectors. In these projectors the xenon arc lamp is used as a virtual point light source, that is to say the light emitted by a reduced volume, whose dimensions are only a few millimeters, is collected and transmitted by various optical elements. In the interior of this small volume there is a large luminance gradient between the anode and the cathode. The very high luminance (hot point) close to the cathode is translated into a hot point at the centre of the slide. To avoid burning the latter, a known solution consists in spreading the beam, which results in the loss of a large part of the light. Furthermore, the illumination of the surface of the slide remain very heterogenous, the illumination being nearly twice as weak at the edges than at the centre.
The object of the invention is to remedy the stated drawbacks and at the same time improve the efficiency of light collection on the slide, or any other image frame, and the uniformity of its distribution on the latter.
This problem arises more particularly in the case of a stationary frame, that is to say one which remains immobile for a prolonged period, or is moved at slow speed, as opposed to a cinematographic film which runs continuously and rapidly.
The invention concerns in particular a device of the type defined in the introduction, and makes provision for the divergent optical element to comprise a central part traversed by the axis of the beam and an annular peripheral part surrounding the central part, the latter having a higher optical power than that of the peripheral part, so that the light rays which pass through it are more or less distributed over the same area of the image frame as the totality of rays passing through the divergent optical element.
Optional features of the device according to the invention, supplementary or alternative, are stated below: concave and has a radius of curvature which is smaller in a central region than in the peripheral region surrounding the central region, said central region and said peripheral region defining the central and peripheral parts of the divergent element; arranged in succession along the axis of the beam and extending radially over different distances from said axis, the lens with the smaller radial extension defining the central part of the divergent optical element; concave face; projecting on a screen having sides of several metres an image formed on said image frame.
The features and advantages of the invention will be explained in more detail in the description below with reference to the attached drawings, of which:
FIG. 1 is a partial schematic representation of a slide projector according to the invention;
FIG. 2 corresponds to an enlarged part of FIG. 1, for a variant of the device;
FIGS. 3 and 4 are diagrams illustrating the distribution of light on a screen, obtained by means of a projector according to the invention; and
FIGS. 5 and 6 are diagrams similar to those in FIGS. 3 and 4, relating to a known projector.
The projector illustrated in FIG. 1 uses a xenon arc lamp 1 as a light source. Such a lamp emits virtually white light from a very reduced volume comparable to a point P. However, the luminance varies widely in the interior of this small volume. For example, in the case of a lamp sold by the OSRAM company under the designation XBO, with a power of 2500 W, the luminance varies from a maximum value of 360000 cd/cm.sup.2 in the immediate vicinity of the centr
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Ben Loha
Hardware
Spencer George H.
LandOfFree
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