The invention relates to a discharge lamp equipped with a gastight discharge vessel containing a gas and equipped with electrodes, at least one of the electrodes comprising
a first part that is suitable for connection to a pole of a supply voltage source and that during operation is capacitively coupled to the discharge in the discharge lamp,
a second part formed out of a first dielectric material, said second part being connected to the first part and during operation of the discharge lamp being in contact with the discharge.
Such a discharge lamp is known from U.S. Pat. No. 2,624,858. In the known discharge lamp the first part of both electrodes is formed out of metal or deposited graphite. The second part of the electrodes is relatively thick and the dielectric constant ∈ of first dielectric material is higher than 100. During operation of the lamp the operating voltage that is applied to the first part of the first electrode and the first part of a second electrode is coupled capacitively to the discharge by means of the second part of the first electrode and the second part of the second electrode. Both electrodes form capacitive impedances during the operation of the lamp. These capacitive impedances render the current/voltage characteristic of the discharge lamp positive so a separate external ballast element can be dispensed with. Since the dielectric constant ∈ of first dielectric material is higher than 100, the capacitive impedances of both electrodes are relatively low, so that the lamp can be operated at relatively low frequencies (e.g. less than 500 KHz). An important disadvantage of the known discharge lamp, however, is that virtually each material that has a high dielectric constant also has a relatively high electron affinity. Because of this high electron affinity electrons adhere relatively strongly to the surface of the second parts of the electrodes. This results in a relatively high lamp voltage, a corresponding low efficiency of the lamp and also to blackening of the wall of the discharge vessel in the vicinity of the electrodes.
The invention aims to provide a discharge lamp that during operation is capacitively coupled to a supply voltage source and can be operated by means of a low frequency (less than 500 KHz) supply voltage, with a relatively high efficiency and a relatively low amount of blackening of the discharge vessel.
In accordance with the invention the electron affinity "khgr" of the first dielectric material is negative.
It has been found that the negative electron affinity of the first dielectric material causes the efficiency of a discharge lamp to be relatively high. In practice the dielectric constant of the first dielectric material is very often relatively low, e.g. lower than 10. In order to keep the capacitive impedances of the electrodes acceptably low, it is often necessary to choose the thickness of the dielectric material in the direction of the lamp current relatively small, i.e. smaller than 100 xcexcm, whereas the best results have been obtained thicknesses smaller than 1 xcexcm.
Very good results have been obtained for discharge lamps in which the first dielectric material is chosen from the group formed by diamond, AlN, AlGaN and BN.
Since in practice the second part of the electrode is relatively thin it is often desirable to realize electrical insulation of the first electrode part from the discharge making use of a third part consisting of a second dielectric material having a dielectric constant ∈ higher than 100 and preferably higher than 1000, the third part of the electrode being situated between and in contact with both the first part and the second part of the electrode.
Preferably the first part of an electrode in a discharge lamp according to the invention has flat metallic layer while the second part comprises a sheet of the first dielectric material parallel to the flat metallic layer. In case the electrode comprises a third part, this third part can conveniently be realized in case it comprises a sheet of the second dielectric material parallel to the first and the second part of the electrode.
It has been found in practice that it is desirable for the electrode to comprise a carrier for rendering mechanical strength to the electrode construction, the carrier being in parallel with the second electrode part. The carrier can be a separate part of the electrode but it is also possible that the carrier is formed by the first electrode part.
In case the electrode comprises a third part, the carrier can also be formed by this third part.