GB2075736A - Optical display device - Google Patents

Abstract

An optical display device displaying a lit-up symbol has a plurality of light sources, each feeding a group of light guides having their output ends distributed over the display area to define a respective symbol. The output ends of certain guides 3, 4, 5, 6 from one group are brought together with the output ends of certain guides of one or more other groups in a termination. A frusto-conical lens element 2 is arranged to receive light from the termination, the smaller end face of the lens element being disposed adjacent and facing the termination. The dimensions of the lens element are such that the divergence from axis of the beam from the larger end of the lens element is less than that from the end of a light guide and the size of the light spot on the larger end of the lens element is larger than the light spot at the light guide termination. <IMAGE>

Description

SPECIFICATION Optical display devices This invention relates to improvements in optical display devices having a display surface for displaying a lit-up symbol or symbols forming, for example, the message of a so-called secret road sign and in which a plurality of light guides, each of which may comprise a bundle of optical fibres, have their light input ends collected together so that they can be illuminated by a common light source and their light emitting ends distributed over the area of the display surface to define a symbol or symbols. Thus the or each symbol is made up of a number of discrete areas of light hereafter referred to as "light spotes" each of which emanates from a particular light guide.

Known devices of this type are either limited in that each light spot can be illuminated only by one light guide and can hence form part of only one symbol, or suffer optical attenuation due to the multiplicity of optical interfaces involved in the methods used to enable each or some of the light spots to be illuminated by a plurality of light guides.

It is the object of the present invention to enable each or some of the light spots to be illuminated by a plurality of light guides and thus form part of a plurality of symbols and to limit the optical attenuation to a level comparable with arrangements lacking this feature.

More particularly the invention relates to optical display devices as described above and of the kind in which a plurality of groups of said light guides are provided, each group having the input ends of its light guides illuminated by a respective light source and the light output ends of its light guides distributed over the display area to define a respective symbol or symbols and the light output ends of certain light guides from one group being brought together with the light output ends of certain light guides of one or more other groups so that they share a region of the display surface of the device.

It is required that in use the intensity of light emitted by each light spot shall have a polar distribution providing high intensity over a relatively narrow angle generally normal to the display surface and greatly reducing intensities at increasing angles to the normal. This required distribution is much more concentrated than the distribution from a light guide lacking special termination.

It is also required that each light spot should have a light emitting surface of area many times greater than that of light guides of the type employed.

The present invention provides a sufficient integration of the light outputs from a group of light guides, a sufficient control of the polar distribution of light intensity, a sufficient display surface area and low optical attenuation.

In one embodiment the invention comprises a truncated cone of light transmitting material and this form is considered below in detail.

It will be realised that a truncated polygonal prism could be similarly employed and might, in some instances, confer mechanical or optical advantages.

In the embodiment described the output or light exit end of the truncated cone is taken to be flat but it will be realised that convex or concave curvature will modify the output light distribution in accordance with known optical principles. It will be further realised that the visual appearance of the output face can be softened by dimpling, frosting or otherwise modifying the surface texture. A similar softening can be achieved by covering the face of the sign with a sheet of lighttransmissive material having at least one surface dimpled, frosted or similarly treated and this sheet may be in contact with or spaced from the light output surfaces of the truncated cones.

The invention will now be further explained by way of example with reference to the accompanying diagrammatic drawings, in which, Figure 1 shows an optical termination for use in a display device in accordance with the invention, Figure 2 shows a front view of the display surface of the optical display device, Figure 3 is an explanatory diagram, Figure 4 represents a merdional section of a truncated cone of the dimensions shown and additional geometrical construction and Figure 5 is a graph showing the optical performance of one example of truncated cone according to the invention.

Referring to Figures 1 and 2, the display surface ofthe device comprises a matrix of illuminable areas 1 each comprising the larger end face of a light transmitting truncated cone 2. Abutting the smaller end of each cone 2 are the light exit ends of a plurality of light guides 3, 4, 5 and 6 each made up of a bundle of optical fibres. Thus each truncated cone 2 constitutes an optical termination for the light guides 3 to 6.Each of the light guides 3 to 6 abutting the same cone 2 leads to a respective light source (not shown) together with selected light guides from others of the cones 2, that the group of light guides leading to the same light source have their light exit ends distributed between the cones 2 so that when the light source is energised a predetermined symbol is displayed on the display area made up of a number of light spots each of which is provided by the larger end face of one of the cones 2. Thus by selectively energising the light sources different symbols can be displayed, some of the symbols having common light spots, since light guides from different light sources share the same optical termination.

Referring now to Figure 3, this shows one of the cones 2 to a larger scale than Figure 1, the cone having axis A and semi-angle a. For simplicity only one light ray 'L' is shown entering the smaller end of the cone and this will be considered as a ray lying in a meridional plane, i.e. any plane which passes through the axis of the cone. The ray 'L' having entered the cone is transmitted through the cone being reflected three times at the cone surface and emerging from the larger end face of the cone. At each point of reflection, due to the conical shape, the angle of incidence will increase by 2a with respect to the angle of incidence at the preceding point of reflection. Hence any light ray in a meridional plane which enters the cone cannot escape.

Also, for such rays the entrance angle of the ray 0a will be greater than the exit angle e2 and hence the light rays emerging from the cone will be collimated with respect to the light rays entering the cone to provide a relatively narrow beam angle from the cone as compared with the large beam angle of about 70 emanating from a bundle of optical fibres.

For any meridional ray it can be shown that: ra sine; + d1 tan a cos 61, = r2 sin 02 + d2 tan a cos 62' 0 where r1 = the radius of the smaller end of the cone 61, the angle of refraction of the ray at the smaller end face of the cone d1 = the distance from the axis A at which the light is incident on the smaller end of the cone r2 = the radius of the larger end of the cone 02 = the angle of incidence on the large end surface of the cone d2 = the distance from the axis A at which the light leaves the larger end face of the cone Equation (3 can be converted to external angles 61 and 62 by the application of Snell's Law and completely defines the behaviour of the taper.

It can be shown that the maximum possible angle of incidence on the smaller end of the cone for which lig ht will be transm itted throug h the cone wiil be determined by the numerical aperture.

where flc - the refractive index of the cone n1 = the refractive index of the medium from which light enters the cone.

n2 = the refractive index of the medium surrounding the conical surface.

The above sets out the optical principles apply to the cone. However, equation 0 is very difficult to apply in practice and the following design procedure has been evolved for practical cases in which the simplifying approximation a 0i is is not valid.

This design procedure gives an adequate description of the polar distribution of output intensity in terms of the polar distribution of the input intensity and the geometry of the cone.

Symbols used: I in Average intensity of a set of rays within a selected range of angles of the input polar diagram I out Average intensity of set of exit rays corresponding to a selected range of angles of the output polar diagram.

ir Intensity contribution of rays suffering rtotal internal reflections.

1r - 1 Intensity contribution of rays suffering r - 1 total internal reflections J Is the proportion of rays suffering internal reflection which exit within the selected range of angles corresponding to the definition of I out.

Then I out = I in x K x J for each of a series of selected ranges of internal entry angles where the series is:- 0 to 6;, 6; to 8;, 02 - 03 etc., the angles being those illustrated in Figure 4.

In the design procedure the input polar diagram is divided into said ranges chosen so that rays entering within each range may occupy two subsets differing by no more than one in the number of internal reflections suffered.

The analysis determines for each range a factor K relating the intensity contributions of rays of the said subsets.

The analysis further determines for each subset a factor J relating the average output intensity to the average input intensity of that subset The output polar diagram is then constructed.by graphical summation of I I out = (for subsets suffering r reflections) and C I out = I in X(1 - K)XJ (for subsets suffering r + 1 reflections), By way of example the range of internal entry angles 63 to 64 contains rays which suffer one reflection and rays which suffer two reflections.

For the subset undergoing one reflecftion

Where angles are for one reflection subset i.e. 6' out = 6' in - 20', For the subset undergoing two reflections

Where angles are for two reflection subset i.e. 6' out = 6' in - 46; There are certain factors determining the size of the cone. The smaller end of the cone must be sufficiently large to accommodate the desired number of light guides abutting against it. However, the number is limited since the further a light guide is away from the axis of the cone the greater the distortion of the output beam profile. The optimum cone angle is determined by the method described above.From this it follows that the length of cone and the area of the larger end face of the cone are a compromise between fitting the cone into a reasonable space, providing a desired size of light spot, and limiting the light loss in the cone due to absorption in the cone material.

By way of example a truncated cone of material having refractive index 1.5 and larger diameter 24 mm and smaller diameter 6 mm and length 68 mm has an optical performance illustrated in Figure 4.

Figure 5 shows in curve (1 ) the section of polar distribution of light emanating from the exit end of a fibre optic light guide of 2 mm active diameter the entry end being illuminated by commercially available lamp/reflector unit.

Figure 5 shows in curve (II) the stepwise approximation to the light output from the truncated cone. Curve (III) represent the smoothed output.

It will be apparentthat the axial intensity has been more than doubled and the unsatisfactory 'bright ring' output from the light guide reduced to a beam of the required pattern.

Claims (2)

1. An optical display device having a display surface for displaying a lit-up symbol, a plurality of light sources, a plurality of groups of fibre optic light guides, each group having their light input ends illuminated by a respective light source and their light output ends distributed over the display area to define a respective symbol or symbols, the light output ends of certain light guides from one group being brought together with the light output ends of certain light guides of one or more other groups to form a light guide termination, characterised in that afrusto-conical lens element or similar truncated and diverging side wall lens element is arranged to receive light from said light guide termination, the smaller end face of the lens element being disposed adjacent and facing the light guide termination and the dimensions of thelens element being such that the angle of the beam output from the larger end face of the lens element is less than that from the light output end of a said light guide and the area or size of the light spot on the larger face of the lens element is larger than the light spot at the light guide termination.

2. An optical display device substantially as hereinbefore described with reference to the acccompanying drawings.