This invention relates to a low light miniature flash light, and in particular it relates to a flash light which is sufficiently economical in cell life to allow the unit to be discarded when 5. the energising cells are exhausted.

Flash lights of many kinds are of course well known and these usually comprise a lamp having a filament energised by one or more cells held in a case and including switch means to allow 10. the filament to be energised at required times, the cells being replaceable when exhausted.

Such flash lights are generally of relatively high brilliance and may be provided with reflectors or otherwise to help concentrate the beam from 15. the light source, and more recently transistorised flash lights using a fluorescent tube are available.

It is known to use a low intensity light source for night- ime viewing, see for instance the specification of United States Patent No. 20. 4,409,643 of Livermore and Edwards assigned to the United States Department of Energy which uses a diode and lens, also.

The specification of United States Patent No. 4,808,261 of Polakoff which uses a light emitting 25. diode electrically connected across a battery.

The object of the present invention is to provide a small low intensity flash light which can be used for map reading, examination of areas where a strong beam is not required, and which 30. is of relatively low cost and rugged to be reliable

under adverse conditions, which can be used in the field and is waterproof. The object can be achieved by using a throw-away construction which allows the torch to be discarded when the energising 5. cells are exhausted.

Such a flash light which can be built to have a selected intensity when used in complete or semi-darkness whereby to minimise image retentivit in the eye, and can be used without attracting _1Q attention to make it useful to army or other personne examining maps or the like which requires minimum scattering of light. The device is also useful for signalling.

The invention comprises a low-light miniature 15. flash light comprising a light source, battery means, and switch means connecting the light source with the bat-tery means, characterised as a throw-away package in which the light source is a light emitting diode connected by a resistor to the battery means 20. all housed in and held by a resilient shrink fit casing orranged to allow the switch means to be actuated by pressure on the casing, the light emitting diode having a light-transmitting face projecting from the casing. The flash light has 25. a resilient supportive case and a simple switch.

Thus the objects are achieved according to this invention by using, instead of a filament type light source, a light emitting diode (LED) energised by cells through switch means and a

30. resistance to control the current flow through the LED so that by selecting a suitable value the light intensity is selected. The LEDs used are high efficiency types which give longer battery life for a given light output. Also LEDs are

35. . inherently rugged as well as economical, and the

light-emitting face can be polished to a flat surface with minimum detection from the side of the flash light.

Such a flash light can be variously constructed, 5. but to enable the invention to be fully appreciated an embodiment will be described with reference to the accompanying drawings in which :-

FIG. 1 is a perspective view of such a flash light using two energising cells, and

10. FIG. 2 is an enlarged side elevation of same with the outer case longitudinally sectioned.

In the form illustrated in the drawings the device comprises a pair of cells 1 and 2 which are oppositely orientated so that the positive 15. terminal of one is at the end opposite to the positive terminal of the other.

The two cells are connected at one end to the LED 3 through a cathode lead 4 from the negative end of one of the cells, the lead 4 being

20. soldered or otherwise secured permanently to this end of the cell, the anode lead 5 of the LED 3 being connected to the positive end of the other cell so that there is a connection from the negative end of one cell to the positive end of the other

25. cell via the LED 3.

The negative end of the cell 2 is connected by a lead 6 to a resistor 7 the other end of which has its lead bent at the end to form a switch 8. The cell 1 at the area of the switch 8 has 30. its sheath removed to expose the positive outer wall of the cell, the lead being so bent that

it can be deflected onto the body of the cell 1 to form a switch to make the necesary flow connectio to energise the LED 3 through the resistor 7. The cells 1 and 2 can have an insulating separator 5„ 11 between them, and the positive core of the cell 1 is exposed at 12 to be engaged by the lead 8.

After this assembly is completed a "Thick dual wall heat-shrink tubing with meltable adhesive"

10. inner wall 9 is placed over the complete assembly and is shrunk into position to lock the assembly tightly together and to provide the necessary support for the LED 3 from the cells 1 and 2, while at the other end it is preferred to allow τ_5. the tube 9 to extend beyond the cells 1 and 2 and to place an eyelet 10 through the wall of the tube. However, before the heat shrink tubing is applied it is advantageous to build up the LED with two layers of white heat shrink tubing

2o. to ensure an effective seal between the LEDs and the outer heat shrink tubing 9, and In this way a very small flash light is provided which is permanently encased in the tubing 9 after it is shrunk into position and, as well as holding the

25. components together, and thus a completely sealed unit is provided which can be operated underwater and in other adverse conditions. The two layers of white heat shrink material are designated 13 and 14, but it will be realised that a single

30. white heat shrink tube could be used.

As referred to earlier herein to ensure that the light is not observable from the side of the flash light the face 15 is polished to be flat, but for some uses this may not be necessary.

However, the flat face allows ready cleaning and ensures that even under adverse conditions, the maximum light is emitted.

The light of such a flash light is satisfactory 5. because of the economic operation of a LED 3 as opposed to filament energisation as used hitherto, and light intensity and cell life is readily selectabl during construction of the flash light.

As an example of the type of cell which

10. can be used, these can be miniature alkaline cells as used for instance as a group in the well known 9 volt batteries. Two of these cells are required to produce the correct operating condition for the LED (red, yellow or green) i.e., each alkaline

15. cell produces 1.5v (per cell). Two of these cells connected in series produces 3_v which is of sufficient magnitude to operate red, yellow or green LEDs (the requirement being that the excitation voltage be greater than 1.6v for red LED and greater than

20* 2.0v for yellow or green). Alternatively, only one lithium cell would be required as these produce 3v nor- cell. These cells have the advanta ^g e of very long shelf life 5 to 10 years (compared to 1 to 2 years for alkaline cells) but have the disadvantage of ^5, higher initial cost. The modest current requirement is due to the high efficiency nature of these LEDs as a light emitting source.

As an example the LED can be arranged by appropriate use of resistances to have a different 0, current consumption, for example in the optimum range, of between 1 milliamp and 5 milliamps, and it will be realised that LEDs are available in various colours, thus making the colour of the emitted light readily selectable and, as said,

that by using appropriate resistances the light intensity can be controlled and from this it follows that battery life is also controllable. It has been found that by reasonable use such a flash c light can have a life of up to twelve months.

Moreover, because the unit is completely sealed and is of small dimension, such a flash light can be readily carried and is generally proof against shock or the ingress of moisture _Q particularly as the switching means are also within the enclosing casing which because of its resilience, allows the switch to be actuated by simple pressure o the wall of the heat-shrink tubular casing.

It will be realised that these cells can ,c be of larger capacity where a larger flash light is required, as higher power LEDs are being developed but generally the object is to provide a relatively small completely sealed unit with a light intensity such that it minimises image retentivity in the 20. eye at night, making it suitable for use, for instance, where the user is avoiding detection. The effect of image retentivity of eye at night is less severe if longer wavelength light is used, the eye recovering quicker from red than from 25. green or yellow light.

From the foregoing it will be realised that the invention comprises a flash light having a light source connected through switch means to a cell or cells arranged to energise the light 30. source, the flash light being characterised in that the light source is a LED and is connected to energising cell means through switch means and if necessary a resistance arranged to control flow through the LED to regulate the light emission.

According to the preferred construction described the pair of cells are oppositely orientated with a light emitting diode connected across one end of the cells and the other end of one cell 5. is connected to the bared case of the other cell through a resistance by a switch in the form of the tail of the resistance, bent so that it can be pressed against the negative outer case of one of the cells.