Electric lamp and discharge devices: systems – Periodic switch in the supply circuit – Silicon controlled rectifier ignition
Patent
1982-03-10
1985-04-02
Chatmon, Saxfield
Electric lamp and discharge devices: systems
Periodic switch in the supply circuit
Silicon controlled rectifier ignition
315306, 315307, 315311, 315DIG7, 378 91, H05B 3702, H05B 3904, H05B 4136
Patent
active
045089957
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
Personnel working with X-ray film viewing cabinets often complain of tiredness and headaches, resulting from flickering light emitted by the flourescent strip lighting with which the X-ray pictures are illuminated. This is noticed most when examining underexposed parts of X-ray film, where it is necessary to decrease the light strength in order to obtain the requisite contrasting effect. In this case, flickering becomes troublesome after only a short time.
One serious, indirect disadvantage with flickering of the flourescent tubes, is that the X-ray technician or the doctor is often unable to make a positive diagnosis on the basis of an existing X-ray picture, and hence it is necessary to take a new picture, with the patient being subjected to a further dose of X-rays as a result thereof.
OBJECT OF INVENTION
The present invention relates to a method of eliminating the flickering of fluorescent tubes in X-ray cabinets with the aid of simple means, thereby eliminating the aforementioned serious disadvantages.
BRIEF SUMMARY OF INVENTION
According to one aspect the invention resides in a method of eliminating discomforting flickering when examining X-ray film in a light cabinet with fluorescent strip lighting supplied with mains voltage via thermionic transformers, for regulating the light intensity. The method is mainly characterized by stabilizing and rectifying the mains voltage, and by energizing the fluorescent tubes subsequent to increasing the frequency to at least twice the mains frequency, suitably to at least 10 kHz and preferably at least 20 kHz. By increasing the frequency of the lighting supply voltage, flickering is reduced or eliminated, which is advantageous, particularly when the light intensity of the lighting is reduced so as to enable underexposed parts of X-ray films to be examined.
A clear and, in many cases, acceptable effect can be obtained by increasing said frequency to approximately twice that of the mains frequency, i.e. to about 100-120 Hz. A fully satisfactory effect is normally obtained, however, when the frequency reaches about 10 kHz. The higher the frequency, the lower the risk of the human ear discerning generated sound oscillations, which risk is considered to be substantially non-existant at frequencies above 20 kHz. Because of this, and for other reasons, a frequency of this order of magnitude is preferred. In addition hereto, a completely flicker-free light is obtained from fluorescent lighting at such frequencies, irrespective of the light intensity.
According to a preferred embodiment of the invention, the increase in frequency is effected by means of an oscillator which co-oscillates with one or more power drive transistors for supplying the primary side of an output transformer whose secondary side delivers a voltage which corresponds to the nominal mains voltage and which is applied to the fluorescent tubes via current-limiting chokes.
These cokes impede the current in accordance with current specifications for existing fluorescent tubes. Chokes intended for frequencies of from 20-25 kHz are considerably smaller in size and cheaper in price than the present day conventional chokes for frequencies of 50 Hz.
The utilization of a pulse width modulated oscillator enables the light intensity of the fluorescent tube to be readily regulated via a variable resistance.
According to a further embodiment of the invention, the mains voltage is stabilized prior to rectification via a transformer, by registering on the secondary side of the transformer any deviation from the nominal mains voltage, and correcting said deviations by switching a bidirectional thyristor arranged on the primary side of the transformer. In this way the mains voltage is stabilized reliably, which is necessary if the subsequent rectification and increase in frequency is to lead to the desired result.
According to a second aspect, the invention relates to a flicker-eliminating arrangement for a light cabinet having fluorescent tubes supplied with mains voltage via a filament transfor
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patent: 3969652 (1976-07-01), Herzog
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patent: 4170746 (1979-10-01), Davenport
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patent: 4277728 (1981-07-01), Stevens
patent: 4378513 (1983-03-01), Yoshikawa et al.
patent: 4388562 (1983-06-01), Josephson
AO:s Metall & Mek. Verkstad AB
Chatmon Saxfield
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