Electric lamp and discharge devices – With luminescent solid or liquid material – With gaseous discharge medium
Reexamination Certificate
1998-12-09
2001-03-27
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
With luminescent solid or liquid material
With gaseous discharge medium
C313S486000
Reexamination Certificate
active
06208069
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a low-pressure mercury discharge lamp for tanning purposes, comprising
a lamp vessel which is closed in a gastight manner and filled with mercury and one or more inert gases,
a luminescent screen,
means for maintaining a discharge in the lamp vessel, during operation of the lamp.
Such a low-pressure mercury discharge lamp, hereinafter also referred to as lamp, is known from U.S. Pat. No. 4,703,224. This lamp comprises a lamp vessel formed from an “open” glass having a relatively high transmission for UV-radiation of a relatively short wavelength. The luminescent screen of the known lamp comprises a mixture of cerium-activated strontium magnesium aluminate, europium-activated strontium pyrophosphate and europium-activated barium pyrophosphate. The emission spectrum of this luminescent screen for wavelengths below 400 nm corresponds substantially to the spectrum of sunlight. Since the UV-radiation acting on the skin is situated mainly in this wavelength range, the emission spectrum of the known lamp has biological effects which also correspond substantially to those of sunlight. More particularly, the lamp has advantageous properties as regards tanning and thickening of the skin so that an increased resistance against reddening of the skin caused by over-exposure to sunlight is brought about. A drawback of the known lamp, however, is that the luminescent substances included in the luminescent screen demonstrate a certain degree of optical interaction with each other, so that a part of the light emitted by these luminescent substances is absorbed again by the luminescent screen. This optical interaction causes a relatively low effectiveness of the lamp. Since the degree to which optical interaction occurs depends substantially on the thickness of the luminescent screen, and in practice this thickness varies relatively substantially within a lamp, the optical interaction causes relatively large differences between the emission spectra of different parts of the luminescent screen.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a low-pressure mercury discharge lamp for tanning purposes whose emission spectrum for wavelengths below 400 nm corresponds substantially to the spectrum of sunlight, the lamp also having a relatively high effectiveness, and the difference between the emission spectra of different parts of the luminescent screen being relatively small.
According to the invention the lamp vessel is made of a short-wave UV radiation-absorbing material whose transmission for UV radiation with a wavelength of 312.6 nm ranges between 10% and 50% and whose wavelength range over which the transmission of the UV radiation changes from 20% to 80% is smaller than 40 nm and greater than 28 nm.
The luminescent screen includes a first luminescent material having an emission band with an emission maximum between 300 nm and 330 m and a half-value width of this emission ranging between 15 and 30 nm. and
The luminescent screen further includes a second luminescent material whose reflection for UV radiation of a wavelength above 300 nm is more than 80%, and the material has an emission band with an emission maximum in the wavelength range from 340 nm to 370 nm, the half-value width of which ranges between 35 nm and 80 nm.
The absorption properties of the glass and the emission properties of the luminescent screen jointly bring about that the emission spectrum of the lamp for wavelengths below 400 nm corresponds substantially to the spectrum of sunlight. It has also been found that a lamp in accordance with the invention has a relatively high effectiveness and that there are only relatively small differences between the emission spectra of different parts of the luminescent screen.
It has been found that a very good correspondence between the solar spectrum and the emission spectrum of the lamp for wavelengths belonging to UV-B radiation is achieved if the first luminescent material includes cerium-activated lanthanumphosphate, hereinafter referred to as LAP, and, more particularly, if use is made of a lamp vessel whose transmission for radiation with a wavelength of 312.6 nm ranges between 30% and 40%.
Good results have been obtained with lamps in which the second luminescent material includes lead-activated bariumsilicate, hereinafter referred to as BSP. Good results have also been achieved with lamps in which the second luminescent material includes lead-activated strontium magnesium silicate, hereinafter referred to as SMS.
It proved possible to further increase the correspondence between the emission spectrum for wavelengths below 400 nm of such a lamp and the solar spectrum by adding a third luminescent material whose reflection for UV radiation having a wavelength above 300 nm exceeds 80%, said material having an emission band with an emission maximum between 370 nm and 400 nm and a half-value width in the range between 35 nm and 80 nm. It has been found that lead-activated barium strontium magnesium borate, hereinafter referred to as BMB, can very suitably be used as the third luminescent substance.
The solar spectrum varies with the altitude of the sun and hence with the location on earth where it is measured, and it is also influenced by atmospheric conditions. It has been found that the solar spectrum can very well be imitated under substantially varying conditions by means of low-pressure mercury discharge lamps in accordance with the invention including LAP and BSP, if the quantity of LAP ranges between 10 and 50% by weight of the luminescent screen. For low-pressure mercury discharge lamps in accordance with the invention including LAP and SMS, this proved to be the case if the quantity of LAP ranges between 3 and 40% by weight of the luminescent screen. In either case, the percentage of cerium in the LAP was approximately 10%.
It has also been found that in the case of lamps in accordance with the invention whose luminescent screen comprises LAP, BSP and BMB, a very good correspondence between the emission spectrum of the lamp and varying solar spectra could be achieved if the luminescent screen comprises between 10 and 40% by weight LAP and between 10 and 40% by weight BSP.
REFERENCES:
patent: 4703224 (1987-10-01), Rattray et al.
patent: 4859903 (1989-08-01), Minematu et al.
Jalink Cornelis J.
Justel Thomas
Nikol Hans
Ronda Cornelis R.
Van Der Voort Dick
Halajian Dicran
Patel Nimeshkumar D.
U.S. Philips Corporation
Williams Joseph
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