High intensity discharge lamp with coating on arc discharge tube

Claim 1

In a high intensity arc discharge lamp including an outer envelope and inner, evacuable elongated discharge tube having electrodes disposed through substantially flat pinch-off flanges at opposed ends thereof and containing a discharge medium, one of the ends of each of said electrodes projecting into said medium, the improvement comprising: a reflective coating on said discharge tube, said coating being disposed at, and covering the ends of said discharge tube and extending just beyond the ends of said electrodes projecting into said medium except for an approximately semicircularly shaped portion on at least one of said discharge tube ends, said semicircular portion lying in a plane approximately parallel to the plane containing said pinch-off flanges, and extending toward the nearest pinch-off flange to expose the end of the electrode so that a greater portion of the arc is exposed, whereby the amount of escaping light is increased, while sufficient reflective coating is retained to avoid a decrease in the heat reflective operation thereof.