Low Brightness Lens for Circular Fluorescent Lamps

This invention relates to diffusing lens panels for controlling the light output from gas discharge lamps of circular shape, such as the circline type fluorescent lamps in general use, to direct the widely dispersed light from the lamp into a directional beam of useful light, with reduced side glare and efficiency losses.

Circular fluorescent lamps have been widely used in lampholder fittings, such as those described in my inventions of U.S. Pat. Nos. 4,105,276 and 4,178,535, in which bare, exposed lamps are used as screw-in ceiling fixtures and as sources in portable lamps which use shades to direct and control the light. In my co-pending application entitled A CIRCULAR GAS DISCHARGE REFLECTOR LAMP, Ser. No. 409,999, now U.S. Pat. No. 4,420,799, optical control of the lamp is shown by both reflective and refractive means, a refractive means shown therein being the male prism pattern which faces away from the lamp.

The purpose of the present invention is to provide more precise and effective optical control than is possible with the use of a single optical surface, to essentially produce a spotlight or narrow beam floodlight from a fluorescent source of circular shape, and also to produce a lens for the circular fluorescent source that is adapted for use in fixtures and lampholder fittings which also include light control by means of reflectors.

A transparent lighting panel for circular fluorescent lamps according to the present invention is provided with an inner surface facing towards the lamp, and an outer surface facing away from the lamp, with the panel oriented in a plane parallel to the plane of the circle of a lamp; the inner surface being provided with a fresnel arrangement of a cylindrical toroidal lens with the optical centerline of the torus approximately the same diameter and coaxial with the circular lamp, and the outer surface being provided with an overall pattern of elongated linear and gnerally triangular prisms in a radial pattern coaxial with the toroidal lens of the inner surface. A preferred embodiment provides a lens which is generally convex on the outer surface, and is generally concave on the inner surface.

FIG. 1 is a cross-sectional view of the lens according to the invention, taken along the central axis of the lens and corcular fluorescent lamp, and showing the inner lens surface structure;

FIG. 2 is a plan view of the outer surface of a circular embodiment of the lens according to the invention;

FIG. 3 is a cross-sectional view of a portion of the lens of FIG. 2, taken along line 3--3; and

FIG. 4 is a cross-sectional view of a preferred embodiment of the lens according to the invention, showing a convex/concave configuration.

In FIG. 1 a lens 1 being a generally transparent panel is shown oriented in a plane generally parallel to plane 2 of a circular gas discharge lamp 3 having a central axis 4. Lens 1 has an inside surface 5 which comprises a fresnel arrangement of a generally cylindrical toroidal lens 6 with a circular optical centerline or an optical center cylindrical ring 7 approximately the same diameter as the lamp diameter 8. Divergent rays 9 from lamp 3 are substantially diected towards a more collimated pattern of rays 10.

In FIG. 2 a plan view of outer surface 12 of lens 1 is shown having an overall pattern of generally triangular male prisms 13 arranged in a radial pattern about the central axis 4 of the lens and lamp.

In FIG. 3 prisms 13 on outer surface 12 of lens 1 are shown having included angles of approximately 120.degree. to direct divergent rays 14 from lamp 3 into more collimated rays 15, and to capture stray rays 16 which might emerge from the lens at excessively high angles with respect to the central axis 4 of the lens and lamp.

In FIG. 4 an embodiment of the invention lens 20 is shown having a generally convex outer surface 21 comprising an overall pattern of tapered triangular prisms 22, arranged in concentric rows about the central axis 4 of the lens. The inner surface 23 is generally concave and is comprised of a fresnel arrangement of a generally cylindrical toroidal lens 24 with a circular optical centerline 25. Due to the cnvex configuration shown, the lens area outside the circular optical centerline is closer to the plane of the lamp, and therefore must have a shorter focal length, ie: greater optical power, than the lens area inside the circular optical centerline. This is evident in the greater angular change of incident rays 30 to become collimated compared to that of incident rays 31.

In operation the fresnel toroidal lens generally collimates divergent rays from the lamp in radial planes about the central axis of the lens and lamp, and the triangular prisms generally collimate the divergent rays from the lamp normal to the radial planes, whereby the lens effectively directs widely divergent rays from the lamp into more collimated rays parallel to the central axis of the lamp and lens.