Electric lamp and discharge devices: systems – Electric switch in the supply circuit
Patent
1982-07-13
1984-03-20
LaRoche, Eugene R.
Electric lamp and discharge devices: systems
Electric switch in the supply circuit
315307, 315360, 315DIG5, H05B 4116
Patent
active
044383746
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
1. Technical Sphere
The invention relates to the technology of picture transmission, in particular to the scanning of originals under utilisation of electric discharge lamps operated by direct current.
2. Fundamental State of Technology
The line by line illumination of the original for scanning purposes is frequently undertaken with fluorescent tubes in picture transmission instruments, copying appliances, telecopiers, reading apparatus, etc. These lamps are preferably operated with direct current to prevent undesirable alternating light modulation.
The charge of these lamps consists of a heavy gas, being mercury vapour as a rule which, upon being energised by the discharge, transmits radiation preponderantly in the ultra-violet range. For its part, this strikes a coating applied on the internal surface of the electric discharge tube, which has the property of radiating light in the visible range. In the case of direct current operation a stage is then reached after a particular period of operation of a fluorescent tube of this kind, when a mercury impoverishment occurs in the vicinity of the anode, because the mercury atoms ionised by collision ionisation are displaced towards the cathode by so-called cataphoresis and accumulate at the same. The result is that the number of non-energised mercury atoms in the vicinity of the anode, and thereby the probability of an ionisation by impact, also become lower. Consequently, the UV radiation generated and thereby too the excitation of the fluorescent substance, are reduced thereat. The tube gets darker at the anode extremity and can no longer fulfil the requirement for uniform illumination of an image line for scanning purposes. The action described depends on time and occurs rather suddenly after a few hours of operation at constant lighting density.
It has been known for a long time, and is also recommended by the lamp manufacturers, to counter this action by alternation of the polarity (e.g. described in "Leuchstofflampen und ihre Anwendung", [Fluorescent lamps and their application], Dr. W. Elenbaas, Philips technische Bibliothek, 1962, pages 111 and 162).
Polarity reversal circuits which are actuated manually by the operative after a definite period are inappropriate for image scanning apparatus because their operation is frequently forgotten. An uncomplicated polarity reversal by means of a time switch is also unusable for image scanning because the probability that this switching action would occur in the middle of a transmission, is too great. The transmission would be disturbed because the lamp is wholly quenched--even if but briefly--during a polarity reversal, as a matter of fact.
Even if the lamp were to have its polarity reversed before every transmission, as a matter of principle, there would be no guarantee that the total service life of the lamp would be split evenly between the two polarities because the number of transmissions during the service life is not too large and the transmission periods in question fluctuate considerably. One polarity is thereby given preference all too easily, and the phenomena described above intervene.
SUMMARY OF THE INVENTION
The invention is based on the task of specifying a circuit which automatically undertakes a polarity reversal before the distribution of lighting density along the tube is rendered irregular by cataphoresis, which assures that the polarity reversal never occurs during a current transmission and which thus makes provision that the lamp is operated evenly in both polarities regarded throughout its service life.
The invention accomplishes this by the fact that the lamp incandescence period during an initial picture transmission in a first polarity is measured and stored, that the polarity is reversed before the start of a second picture transmission, that the lamp incandescence period in the second polarity is equally measured and stored, that the difference between the first and second incandescence periods is calculated and stored, and that a balance of the total incandescence p
REFERENCES:
patent: 3198982 (1965-08-01), Benson et al.
patent: 4100462 (1978-07-01), McLellan
patent: 4327309 (1982-04-01), Wallot
patent: 4337418 (1982-06-01), Walz
Dr.-Ing. Rudolf Hell G.m.b.H.
LaRoche Eugene R.
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