GB2115544A

GB2115544A - An optical projection system

Info

Publication number: GB2115544A
Application number: GB08304443A
Authority: GB
Inventors: Derek Geoffrey Swingler
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-02-17
Filing date: 1983-02-17
Publication date: 1983-09-07
Also published as: GB8304443D0

Abstract

An optical projection system comprises a light source made up from a plurality of substantially point light sources in the form of light emitting diodes each of which produces a light beam whose solid angle of divergence is less than one steradian and a converging lens or reflector, each point light source being positioned in relation to the lens or reflector such that its light beam is collimated by the lens or reflector. The axes of the light beams from the light emitting diodes are so directed in relation to the principal axes of the optical elements that the collimated output beams have a diverging envelope. An optical projection apparatus advantageously comprises a plurality of such projection systems arranged adjacent each other for example in a grid pattern and also advantageously the lenses or reflectors are formed integrally with one another.

Description

SPECIFICATION An optical projection system The present invention relates to an optical projection system for projecting monochromatic light or alternatively a mixture of light of different wavelengths from a light source to present to a distant observer positioned in a certain angular field of view centered on the light source an apparent image of the light source.

One known form of such a projection system consists of an incandescent source of light comprising a straight filament arranged with its long dimension in a vertical axis the long dimension of the filament being arranged coaxially with a semi-cylindrical lens which possess the property of light convergence in one plane only and which is shaped to wholly or partially surround the filament. In this form the light energy of the filament is only partially collimated into the longer conjugate of the projection system comprising the space between the semicylindrical lens and the location of the observer.

When it becomes necessary to project light of a required colour towards the observer, filters are inserted in the projection system and since such filters absorb light energy of unwanted wavelengths a proportion of the available light energy is dissipated within the projection system.

Another known form of such a projection system comprises an incandescent filament as the light source but with the filament arranged within a relatively small surface area and situated at the principal focus of a concave mirror of particular curvature. In this form the projected light is mostly collimated along the principal axis of the projection system and into the longer conjugate of the projection system comprising the space between the concave mirror and the location of the observer. Again filters have to be used it if is necessary to project light of a particular colour with consequent energy dissipation. Also in order that this projection system may project light into a desired angular field of view it may be necessary to mechanically and usually electrically to rotate the principle axis of the projection system.

Both of the foregoing known projection system are relatively inefficient in converting electrical energy into light energy utilised in the projection system for the reasons stated and additionally in both systems due to part of the electrical energy used being dissipated as heat and not used as visible light.

The object of the present invention is to provide an optical projection system which is economical in its energy consumption as compared with the aforesaid optical projection systems so that it can be operated for longer periods of time using primary or secondary batteries.

According to the present invention an optical projection system comprises a light source made up from a plurality of substantially point light sources in the form of light emitting diodes each of which produces a light beam whose solid angle of divergence is less than one steradian and an optical element possessing the property of light beams convergence by light refraction or light reflection, each said point light source being positioned in relation to the optical element such that its light beam is substantially collimated by the optical element and the axes of said light beams from the light emitting diodes being so directed in relation to the principle axis of the optical element that the collimated beams have a predetermined angular relationship with each other, for example they form a packet of beams having a diverging envelope.

Said optical element advantageously comprises a simple convex lens or concave mirror.

Also advantageously the relative aperture of said optical element is such that its whole area is just filled by the light beam from each light emitting diode.

Since the number of light emitting diodes that may be used in one optical projection system according to the invention, may be limited by the finite space that each light emitting diode occupies and by optical considerations a projection apparatus may comprise a plurality of such optical projection systems arranged for example in a simple grid pattern or a plurality of interlaced grid patterns with the optical elements in each row of one grid pattern staggered with respect to those in an adjacent row of another grid pattern, in order to increase the number of collimated output beams and if desired the field of view angle.

Since each projection system in the apparatus has a plurality of light emitting diodes advantage can be take to produce to the observer the visual sensation of either another colour of light by using light emitting diodes of two appropriate colours of monchromatic light presented alternateiy to visual observation, or the visual sensation of "white light" by using either two or three appropriate colours of monochromatic light presented in sequence to visual observation. As each light emitting diode is electrically switched on and off for pre-determined periods the appropriate adjustment of the electrical energy supplied to each light emitting diode and of the electrical on and off periods may be used to produce the desired visual sensation.

In order to realise the visual sensation of a continuous luminance in the or each projection system while avoiding the continuous consumption of electrical energy by each light emitting diode used in the projection system an electronic solid state circuit can be arranged to control both the quantity of electrical energy available to each light emitting diode and the duration of time that the light emitting diode is supplied with such quantity of electrical energy.

As the visual sensation of a continuous luminance is subject to an optical law attributed to Weber-Fechner the electronic solid state circuit may incorporate at least one sensing element that causes the circuit to make appropriate adjustments of the time that each light emitting diode does not receive a supply of electrical energy or alternatively the quantity of electrical energy provided may be varied or the solid state circuit may incorporate both adjustments to be applied according to the characteristics of the particular light emitting diodes used.

The optical elements of the apparatus are advantageously formed integrally with each other, for example as a single moulding.

The invention will now be described in greater detail by way of exumple with reference to the accompanying drawings in which: Figure 1 is a vertical sectional view of part of a preferred projection apparatus according to the invention using a plurality of convex lenses.

Figure 2 is a cross-sectional view of the apparatus shown in Figure 1.

Figure 3 is a vertical sectional view of part of an alternative projection apparatus according to the invention using a plurality of concave mirrors.

Figure 4 is a cross-sectional view of the apparatus shown in Figure 3.

Figure 5 is a simplified block diagram of the electronic solid state circuit connected between the source of electrical energy and each light emitting diode used as part of any one of the projection systems in the apparatus.

In Figures 1 to 4 a plurality of projection systems forming part of the total apparatus is shown, the number of such projection systems being extended as desired to provide the desired luminosity and field of view.

Also for convenience the same references have been used in Figures 3 and 4 as in Figures 1 and 2 to designate corresponding parts.

Referring to Figures 1 and 2 the apparatus has a transparent casing 6 which is straight sided as seen in Figure 1 in the plane of the vertical axis of the apparatus and is circumferentially curved in cross-section as seen in Figure 2. On one surface of the transparent casing 6 which is of a material of known refractive index are formed a pattern of convex lenses arranged horizontally and vertically.

The arrangement depicted in Figures 1 and 2 for the transparent casing 6 is thus cylindrical or partcylindrical in form but may alternatively be planar or curved both vertically and horizontally according to the desired projection.

Where the casing surface 6 is planar the projection at an angle to the principle aixs of any lens is accomplished as shown in the arrangement in Figure 1. Where the casing surface 6 is curved, projection may be chosen to be limited to projection on the principal axis of each lens using either the angular displacement or the horizontal displacement of any horizontal line of lens patterns relative to the next adjoining horizontal line of lens patterns or a combination thereof in order to achieve a satisfactory angular relationship between any pair of axes of projection.

A preferred arrangement for the cylindrical casing 6 is shown in Figures 1 and 2 where reference numeral 1 represents the longitudinal axis of the cylinder and reference numeral 2 represents a suitable position for mechanical fixing of and electrical connection to a plurality of light emitting diodes numbered 3.1 to 3.3 and 4.1 to 4.6. The plurality of light emitting diodes may be increased to 3.nth and 4.nth according to the extent of the arrangement. The curved forms 5.1, 5.2 and 5.3 on transparent casing 6 each comprises a convex lens. The plurality of convex lenses may be increased to 5.nth according to the extent of the arpangement. The convex lens may be of constant curvature or it may vary according to the desired projection arrangement.The chain dot lines referenced 7.2, 7.5 and 7.8 represent each principal axis of projection from a respective light emitting diode. The plurality of principal axis may be increased to 7.nth according to the extent of the arrangement. The chain-dot lines referenced 7.1,7.3,7.4,7.6,7.7 and 7.9 represemt each axis of projection oblique to the principal axis of projection. The plurality of such oblique axis may be increased to 7.nth according to the extent of the arrangement.

The positioning of each of the light emitting diodes in relation to its associated lens is such that its output beam is substantially collimated, i.e. it is positioned at or close to the focal point of the lens along its respective axis and the axis of its beam is directed substantially at the centre of the lens.

The light emitting diodes may be of the type having a primary source of luminance at its centre and a surrounding reflected ring~of secondary luminance and produce a beam of 70 to 80 cone angle. The use of several light emitting diodes on or grouped around the principal axis thus form via the lens used a series of circular images that closely adjoin each other.

The type of light emitting diode used has a luminous intensity directed into a solid angle of less than 1 steradian-in fact it is confined within 10 of its axis. Advantages accrue from this as follows: a) the lens or reflector used can be relatively small, 1 2 mm dia. being a useful size.

b) the spherically curved surfaces of the lens or reflector can be of quite large radii i.e. curvature can be gentle, (giving a principal focus at a relatively long distance from the lens or reflector, which in turn means that the relative aperture of the lens or reflector is within the range f2.7 to f3).

c) on account of b), positioning of the light emitting diode relative to the lens or the reflector becomes less criticai to obtain a good collimation of the light rays projected by the lens or reflector.

d) Since the angle of deviation of light from the light emitting diode incident upon the lens or reflector is not very large in the projection systems devised a lens need only have one spherically curved surface, achieving the majority of deviation at that one surface.

e) for a light emitting diode positioned upon the principal axis through the lens (or close to that axis) the effect of spherical aberration caused by the lens is substantially reduced or negligible and because it is a monochromatic light source, the chromatic aberration caused by the lens does not affect the collimation of light rays emerging from the far side of the lens.

f) for a light emitting diode positioned obliquely to the principal axis through the lens the remarks in e) also apply, plus the light emitting diode can be positioned at a distance from the lens that takes account of and corrects the curvature of projection inherent in the lens around its principal focus.

Thus with a projection system described a physically quite small array of projection systems can be used together to permit the projection of images, with the images adjacent to each other or alternatively overlaid if it is desirable to intensify the illumination at the image.

On the other hand by arranging each projection system at an angle to the adjoining systems and around the axis of a circle, images could be projected into a very wide field of view concentric with the circle.

Combinations of these arrangements permits "tailoring" the image pattern to the desired extent.

In Figures 3 and 4 the corresponding arrangement using mirrors rather than lenses is shown, the curved forms 5.1 etc. thus representing shallow concave mirrors rather than lenses and reflect the light from the light emitting diodes.

Figure 5 is a block diagram representing the principal part of the solid state electronic control circuit for the plurality of light emitting diodes used in the projection systems of each apparatus.

Block 11 represents a sensor responding to the ambient lighting and is coupled to block 12 which represents a clock pulse generator whose repetition frequency may be partially controlled by the sensor 1 The output of generator 12 is supplied to a plurality of electronic gates 1 3 and also to a counter/divider 1 6. The gate 13 controls the supply of clock pulses to the circuit 1 5. The electronic gate 13 is enabled by a sensor 14 responding to the ambient lighting. The circuit 1 5 comprises a plurality of electronic gates followed by a plurality of inverting stages. The output of each gate is supplied to an inverting stage which in turn supplies a current switch comprising one of the plurality forming block 1 7.The outputs of counter/divider 1 6 are supplied to the plurality of electronic gates in circuit 1 5 which itself may be controlled by a sensor responding to the ambient lighting and represented by block 1 9.

The current output peaks of the plurality of current switches are limited to the extent determined by the current control of block 1 8.

The output of each current switch is supplied to one or a plurality of light emitting diodes positioned in the optical projection systems.

The positive and negative symbols within circles indicate in the case of negative symbols a common connection to the supply battery negative terminal. In the case of the positive symbol this denotes either direct connection to the battery positive terminal or connection to a voltage regulated position supply from the battery as may be appropriate within the circuit.

Comparison can be usefully be made between a slide projector and one of the projection systems described above using one light emitting diode plus lens, in terms of the illuminance at the image plane and the relative electrical consumption.

The slide projector using a 100 watt lamp would produce white light of 1000 times the illuminance of a RED light emitting diode plus lens at the same projection distance. However, the introduction of a RED filter in the slide projector's light path plus the reduced sensitivity of the eye to RED light would considerably narrow the difference.

The slide projector consumes 100 watt of electrical energy continuously, whereas the RED projection system is pulsed on/off fast enough to give the visual sensation of continuous light, but nevertheless not consuming electrical energy for an appreciable part of unit time period. Typically it would be about 0.05 watt per second as against the 100 watt per second of the slide projector.

A consideration of the use of LED's whose light energy is directed broadly (into more than 1 steradian say) shows that associated lenses or curved reflectors would need to be physically larger, with small raddi of curvature. Such systems would be unlikely to meet the advantages described above.

Claims

1. An optical projection system comprising a light source made up from a plurality of substantially point light sources in the form of light emitting diodes each of which produces a light beam whose solid angle of divergence is less than one steradian and an optical element possessing the property of light beam convergence by light refraction or light reflection, each said point light source being positioned in relation to the optical element such that its light beam is substantially collimated by the optical element and the axes of said light beams from the light emitting diodes being so directed in relation to the principal axis of the optical element that the collimated beams have a predetermined angular relationship with each other.

2. An optical projection system according to Claim 1, wherein the predeterminal angular relationship is such that the collimated beams together have a diverging envelope.

3. An optical projection system according to Claim 1 or 2 wherein said optical element comprises a convex lens or a concave mirror.

4. An optical projection apparatus comprising a plurality of projection systems according to Claim 1, 2 or 3 arranged adjacent each other.

5. An optical projection apparatus according to Claim 4, wherein said optical projection systems are arranged in a single grid pattern or a plurality of interlaced grid patterns.

6. An optical projection apparatus according to Claim 4 or Claim 5 wherein said optical elements are formed integrally with each other.

7. An optical projection apparatus according to Claim 6, wherein said optical element are convex lenses and are formed as a transparent casing part of the apparatus.

8. An optical projection system or an optical projection apparatus according to any preceding claim, wherein light emitting diodes producing monochromatic light outputs of different colours are used.

9. An optical projection system or an optical projection apparatus according to any preceding claim, wherein each light emitting diode is arranged to be controlled so that it is intermittently energised.

10. An optical projection system or an optical projection apparatus according to any preceding claim, wherein the peak intensity of current supplied to each light emitting diode is controlled.

11. An optical projection system or an optical projection apparatus according to Claim 9 or 10 wherein each light emitting diode is controlled in dependence upon ambient light level.

12. An optical projection system or an optical projection apparatus substantially as hereinbefore described with reference to the accompanying drawings.