High intensity discharge (HID) lamps such as mercury vapor, metal halide and high pressure sodium lamps, because of their high luminous efficacy, excellent lumen maintenance, relative low cost of light, good optical efficiency and ease of installation have been in general use for many years and are in increasing demand today. It has recently been publicized that, under certain conditions, these HID lamps may present a potential health hazard.
The light emitting member of these lamps, namely, the quartz arc tube containing mercury vapor or metal halide, and the alumina high pressure sodium discharge tube, all contain mercury as at least one of the constituent fill components. The mercury vapor lamp arc tube uses only mercury as the fill component (except for argon starting gas) and the resulting lamp discharge yields the well known mercury high pressure line spectrum with infrared, visible and ultraviolet radiation. The metal halide tube uses mercury plus combinations of various metal halide compounds as the fill components in addition to argon starting gas. The resulting spectrum will be characteristic of the metal introduced, augmented by the mercury line spectrum. The high pressure sodium lamp is filled with mercury and sodium in addition to starting gases of argon, xenon or neon or mixtures thereof. The spectrum of the discharge of this lamp is characteristic of high pressure sodium augmented by the line spectrum of mercury. Therefore, although ionized and excited mercury atoms are not the primary light producing species in metal halide and high pressure sodium arc tubes and lamps, sufficient mercury ionization and excitation occurs to produce visible and ultraviolet radiation of the characteristic mercury spectrum.
The characteristic mercury spectral lines produced by the discharges of the foregoing types of lamps produce ultraviolet radiation in the 200-297 nanometer range. Ultraviolet radiation in this range is potentially harmful. For example, conjunctivitis, an inflammation of the conjunctivae, will cause visual incapacitation and is caused by exposure to 250-297 nanometer radiation. Conjunctivitis when inflicted by exposure to the ultraviolet radiation is insidious as its symptoms do not appear until 21/2 to 12 hours after exposure to such radiation. Numerous cases of ultraviolet radiation exposure causing abiotically produced cataracts of the eye lens have been reported. Even when such ultraviolet producing sources are viewed from considerable distances eye injuries can occur by ocular absorption.
Hermetically sealed outer glass envelopes are usually used to surround the light emitting tubes of HID lamps. This is done for three main reasons: (a) to obtain proper warm up and operating vapor pressures of the fill components by providing an inert gas or vacuum atmosphere between the discharge tube and the outer envelope, (b) to prevent the slow deterioration, due to oxidation, of the discharge tube lead-in wires and (c) to prevent the lamp from radiating the harmful ultraviolet energy produced by the inner tube.
With respect to point (c), the glass composition of the outer envelope is chosen so as to achieve absorption of the ultraviolet range causing known harmful effects. Therefore, when the outer glass envelope is intact, the harmful ultraviolet radiation emitted by the discharge tube is absorbed. When, for one reason or another, the glass envelope is broken the hermetically sealed light emitting discharge tubes of these lamps will continue to operate for tens to hundreds of hours and will not emit their harmful ultraviolet radiation to the surrounding areas, thus creating a health hazard to persons in those areas. An increasing number of HID lamps are used indoors where lamps, if operating with broken outer envelopes, will be of particular dnager becuase of the likelihood of lamp installations in close proximity to people.
Various solutions to this problem have been proposed by the prior art and these solutions can broadly be defined as: (1) means sensitive to an increase in oxygen in the outer envelope, (2) means sensitive to a change in pressure in the outer envelope, and (3) spring switch means held together by the actual condiguration of the glass outer envelope.
Examples of proposed solutions under item (1) above included U.S. Pat. Nos. 3,262,012 and 4,208,614 wherein an oxidizable filament is employed in the outer envelope which will burn through in the event the outer envelope breaks and admits air.
An example of an item (2) pressure sensitive device can be found in U.S. Pat. No. 4,143,301 in which a bellows switch is used in conjunction with an oxidizable filament.
An item (3) contact switch is disclosed in U.S. Pat. No. 4,156,830.
While all of the above-described solutions will work to a greater or lesser degree, problems exist with all of them.
The oxidizable filament is a fragile component which is subject to breakage from shocks.
The previously disclosed pressure sensitive devices are bulky and expensive and also employ, in conjunction therewith, an oxidizable filament.
The contact devices do not guarantee operation if the outer envelope is merely punctured at a spot remote from the switch.