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Improved fluorescent lamp dimming Disclosure Number: IPCOM000201637D
Publication Date: 2010-Nov-17
Document File: 4 page(s) / 108K

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Improved fluorescent lamp dimming

The conventional method of Amplitude Variation dimming of fluorescent lamps, usually by means of frequency variation, has the disadvantage of not-well controlled lifetime at dim levels below 10% of the lamps nominal operating conditions (< 10% dim). Thus the range of dimming below 10% of the nominal luminous efficacy is excluded according to common fluorescent lamp standardization rules, which is related to high cathode fall values in deep dim circumstances as electrode heating is not optimal. Moreover problems like light inhomogenities, instabilities etc. may occur due to leakage currents etc in deep dim circumstances due to an increased lamp impedance. Besides, the conventional method results also in colour shifting of the emitted light towards the blue.

Pulse Width Modulation dimming, as applied in some applications lamps nowadays, has the disadvantage of lower efficacy compared with amplitude variation, because the wall load remains high during the current pulse. Moreover there may arise problems with HF IR disturbances due to fast switching effects of the noble gas emission with the high current amplitudes.

Improved fluorescent lamp dimming is achieved by combination of Amplitude Variation (AV) and Pulse Width Modulation (PWM) i.e. for dimming above certain value Dim_s (e.g.10% dim) AV is applied and below Dim_s (e.g.10% dim) PWM is applied.

Of course gradual transition may also be applied in other range e.g from 10-20% dim. The improved dimming scheme is shown in fig 1.

Advantages of the improved dimming are:
1 high efficacy for "normal" dimming, above 10% dim (extra energy saving); and
2 well controlled, long life for deep dimming (below 10% dim, down to <1% dim), in particular of importance for e.g. RGB applications.

As the amplitude is around the 10% level the IR disturbance will be an order of magnitude smaller compared with PWM at 100% amplitude level. It is advantageous to control the heating of the electrodes between the current pulses, leading to an improved life

Furthermore a reduction in colour shift is possible i.e. by keeping the current amplitude constant and reducing the duty cycle.

With this improved dim method the colour shift due to a lower discharge current is avoided to a certain extent. However the colour shift due to a lower Hg pressure (lower wall temperature during dimming due to reduced lamp power) is still present.

This remaining colour shift can be further reduced by increasing the discharge current amplitude and simultaneously by an extra reduction in duty cycle, so as to realize the reduction in lam...