Illumination – Light fiber – rod – or pipe – Illuminating or display apparatus
Reexamination Certificate
2000-05-24
2002-09-24
O'Shea, Sandra (Department: 2875)
Illumination
Light fiber, rod, or pipe
Illuminating or display apparatus
C362S260000, C313S117000, C313S635000
Reexamination Certificate
active
06454451
ABSTRACT:
TECHNICAL FIELD
The invention relates to a flat lighting device having a discharge lamp with an aperture, an optical system and an optical conductor plate.
The discharge lamp, optical system and optical conductor plate are coordinated in this case with one another geometrically and arranged relative to one another such that the light from the lamp can be coupled into the optical conductor plate through at least one narrow side (“edge”) thereof (so-called “edge-light technique”). By means of reflection at scattering centres which are applied, for example, to the underside of the optical conductor plate, this light passes through to the outside over the entire front side of the optical conductor plate, and thus acts as a flat light source extended in accordance with the dimensions of the optical conductor plate.
Moreover, the discharge lamp used is, in particular, a fluorescent lamp with a tubular discharge vessel which is sealed at both ends and whose wall is coated at least partially with a fluorescent material. Moreover, in order to increase the luminous density this lamp is provided along its longitudinal axis on the inside or outside of the discharge vessel with a reflector of visible light which is recessed along the longitudinal axis over a defined region. This creates an aperture through which the light from the lamp reaches the outside (aperture lamp). The discharge vessel can be tubular, or also angular, for example L-shaped or U-shaped. In the last-mentioned case, the light from the lamp is coupled into the optical conductor plate via two or three of the edges thereof.
Such lighting devices serve, for example, for backlighting displays, in particular liquid crystal displays (LCDs) but also large-area advertising panels. Liquid crystal displays are used multifariously, for example in control rooms, aircraft cockpits and, increasingly, also motor vehicles, in consumer electronics and communications electronics, and as display screens for personal computers (PCs).
PRIOR ART
U.S. Pat. No. 5,055,978 discloses a flat lighting device having a tubular aperture fluorescent lamp and an optical conductor plate. The diameter of the circular cross section of the aperture lamp is greater than the thickness of the optical conductor plate. However, the width of the aperture is selected to be smaller than the thickness of the optical conductor plate. Arranged between the aperture and the optical conductor plate is a trapezoidal perspex wedge which is intended to reduce the losses when the lamp light is coupled into the optical conductor plate. In this case, the device is designed in such a way that the light can be guided with the aid of total reflection inside the perspex wedge from the lamp to the optical conductor plate.
The width of the aperture, which is relatively slight by comparison with the lamp diameter, is disadvantageous, since the light yield and the luminous flux of the lamp drops distinctly with reduction in the ratio b/D between the aperture width b and lamp diameter D. In the cited prior art, the aperture angle produced by the width of the aperture and referred to the midpoint of the circular lamp vessel cross section is smaller than 45°, but in any case distinctly smaller than 90°.
SUMMARY OF THE INVENTION
It is the object of the present invention to provide an improved flat lighting device. An important aspect in this case is to improve the overall efficiency of the device.
It is helpful to define an optical axis for the purpose of better understanding of the following explanations with reference to the basic idea of the invention. Said axis lies in the plane of the front side of the optical conductor plate and, in addition, forms a right angle with the lamp longitudinal axis.
The light coming from the lamp is thus coupled into the optical conductor plate substantially in the direction of the optical axis and subsequently outcoupled from the optical conductor plate through the front side as useful light.
The starting point of the following considerations is the realization that the requirement for total reflection inside the optical conductor plate must be met for as large as possible a portion of the radiation coupled into the optical conductor plate. The point is that only this portion, together with that portion which in any case strikes directly on the underside of the optical conductor plate after entering the latter can be reflected at the diffuse reflector, arranged on the underside of the optical conductor plate for example, and through the front side, and be relayed beneficially. The remainder is lost for the actual application.
Investigations have now shown that the emission characteristic of aperture lamps without additional measures is very similar to a Lambert distribution, that is to say the angle-dependent intensity distribution of a small subarea of the luminous area of the aperture follows the relationship I(&agr;)=I
0
·cos &agr;, &agr; denoting the angle between the surface normal and the relevant light beam with the intensity I(&agr;) and I
0
denoting the maximum intensity in the direction of the surface normal of the subarea (a=0). In other words, aperture lamps emit the majority of their luminous flux in the forward direction.
This leads, undesirably, chiefly in the case of the use of lamps whose aperture width is comparable to the thickness of the optical conductor plate to the fact that a significant proportion of the radiation experiences no total reflection inside the optical conductor plate, but essentially strikes the narrow surface, opposite the light entrance surface, of the optical conductor plate and is lost at some stage. The curvature of the surface of the discharge tube plays only a subordinate role here, because of the correspondingly large diameter of the discharge vessel, that is to say the surface normals of all the surface elements of the aperture are orientated approximately parallel to one another and to the optical axis.
On the other hand, as already explained at the beginning, the aim is to select the width b of the aperture to be as large as possible. For this reason, the ratio b/D between the aperture width b and lamp diameter D is preferably selected, at least in the case of tubular lamps with a circular cross-sectional surface, so as to achieve an effective aperture angle &thgr; of greater than 45°, particularly preferably of the order of magnitude of approximately 90°, for example approximately 80° or more.
Moreover—for reasons of the targeted high luminous densities on the front side of the optical conductor plate—the ranges of b/d>0.6, 0.8 and 1 are preferred for the ratio of the aperture width b to the thickness d of the optical conductor plate.
It has been shown in this regard that the outside diameter D (in the case of a circular cross section) of the discharge vessel is typically equal to or greater than the 0.8-fold thickness d of the optical conductor plate.
The light loss mentioned further above can be distinctly reduced by specifically varying the distribution of the light which is coupled into the optical conductor plate. According to the invention, for this purpose the proportion of the radiation which otherwise passes directly through the optical conductor plate and is lost for the useful radiation is redistributed onto the proportion which is totally reflected inside the optical conductor plate.
This procedure additionally renders it possible for the first time in the case of the edge-light technique actually to profit from the luminous flux which increases with the lamp diameter D in the case of lamps with electrodes arranged on the wall of the discharge tube parallel to the tube longitudinal axis. These lamps are operated by means of dielectrically impeded discharge, for example by arranging two strip-shaped electrodes diametrically on the discharge vessel wall. To be specific, enlarging the diameter of such lamps also increases the striking distance, the power which can be coupled in and, consequently, the luminous flux. For further details on the notion of the “dielectrically impede
Hitzschke Lothar
Vollkommer Frank
Clark Robert F.
Negron Ismael
Patent-Treuhand-Gesellschaft fuer Elektrische Gluehlampen
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