Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1995-07-19
1997-08-12
Negash, Kinfe-Michael
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
359184, 375238, H04B 1004
Patent
active
056571453
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to signalling techniques, and particularly relates to the transmission of a coded digital signal using pulsed amplitude modulation of a carrier wave. Data transmission using visible light in a building is achieved, according to the invention, which provides a method of sending digital information by modulating the output from gas discharge lamps commonly found in electric lighting installations.
A typical application of such a method would be in the dissemination of information to the individual shelf units of an electronic pricing display system as used in large retail outlets.
Japanese laid open applications numbered 60-32444, 60-32460, and 60-32443 describe a system where in-plant fluorescent lighting may be frequency modulated to form a data transmission path, and mention the use of frequency shift keying as a possible coding method. PCT published application WO 90/13067 describes an electronic price labelling system which features a further application of frequency modulation of fluorescent lighting systems by phase modulation of the whole electrical supply to the lighting. There are also various infrared data transmission methods and applications to be found in the literature that describe optical data transmission applications. For example, U.S. Pat. 4,766,295 by Davis et al describes an electronic pricing display system where optical transmitter heads are installed over the ceiling and are used to signal to infrared sensitive pricing labels.
The present invention seeks to provide a data transmission method which employs amplitude modulation of the light output from a discharge lamp, using a pulse position coding method that overcomes many of the practical problems encountered with the known systems.
The conventional fluorescent lamp used for area illumination is a low pressure gas discharge tube wherein the gas mixture has been chosen to give efficient output in the short to ultra violet wavelengths. This effect is normally achieved by mercury doping. The tube is internally coated with a fluorescing powder that converts the short wavelength emissions into useful light, the fluorescent material having a degree of persistence after illumination. The constituents of the fluorescing powder can be chosen to provide a particular colour of light output. Usually, a mixture which produces as near as possible to white light is chosen.
The electrical characteristic of a gas discharge tube is to exhibit very low or negative resistance when operating, and therefore some method of limiting the current flow through the tube must be employed. With AC mains power supplies this is simply done by incorporating a suitably sized inductance in series with the tube. Nearly always the resulting phase shift is corrected by incorporating a capacitor in parallel with the whole tube and inductance. This current limiting circuit is known as a ballast. A more complex method of current limiting uses an electronic frequency changer (electronic ballast) for each tube, whereby a number of approaches to limit the current flow are then available. Although this hardware is more expensive than the inductance and capacitor installation, the resulting running costs are much lower because the electronic ballast can be designed to operate with very low losses.
The light output from fluorescent tubes driven by these AC circuits consists of two distinguishable elements; a constant, or dc element, and a varying, or `ac` element, where the frequency of the latter is 2f.sub.m (f.sub.m being the supply frequency to the tube). In other words, what appears to the eye to be a constant light output in fact comprises a rectified ac element associated with a constant element. What is happening is that while the current flow reverses every half cycle of f.sub.m, the intensity of the emissions from the gas discharge are, of course, not affected by the direction of flow. In effect the intensity of the emissions from the gas must drop to zero as the current reverses and itself goes through zero. The fluorescing powders used i
REFERENCES:
patent: 3900404 (1975-08-01), Dachs
patent: 4713841 (1987-12-01), Porter et al.
patent: 5424859 (1995-06-01), Uehara et al.
BSC Developments Ltd.
Negash Kinfe-Michael
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