referring to fig1 magnetron 1 feeds microwave power through waveguide 3 and slot 5 into microwave resonant cavity 7 . cavity 7 is defined by reflector walls 15 and screen 9 . envelope 17 , which contains mercury as a constituent of a plasma - forming medium , is mounted on stem 13 which is rotated by motor 11 . fig2 depicts the distribution of light - emitting mercury vapor 25 when the envelope 17 is rotated at a relatively low rate . as shown in fig2 the rotation axis 21 of envelope 17 is perpendicular to the axis 27 of the electric field . the convective forces of the electric field act on the plasma 23 within the envelope 17 to produce a relatively thick layer of cool mercury vapor 25 in the polar regions 31 , and a thin layer in the equatorial regions 29 . surface temperature is greatest in the equatorial region . fig3 shows the effect on the distribution of cool mercury vapor of rotating the envelope 17 at a rate which is significantly higher than the rotation rate of the envelope of fig2 . the cool mercury vapor 25 is shown as being distributed substantially uniformly about the inner surface of envelope 17 . under these conditions , the temperature in the equatorial region is reduced to levels close to those in other surface regions . a more uniform intensity of light which is emitted from various regions of the envelope also results from the increased rotation rate . fig4 shows the results of tests run on an apparatus depicted in fig1 to determine the relationship between surface temperature and rotation rate . the apparatus consisted of a spherical envelope having an inside diameter of 28 mm and a fill consisting of argon , mercury , and a metal halide . the lamp coupled 1 . 4 kilowatts of microwave power at 2 . 45 ghz . at rotation rates between about 100 and 1000 rpm there was no significant change in the distribution of cool mercury vapor within the envelope . at speeds of about 2000 rpm the cool mercury vapor became substantially evenly distributed about the inner surface of the envelope . as shown in fig4 the temperature at the area around the polar axis remained nearly constant until a rate between 2000 and 3000 rpm was reached , at which rate the temperature started to increase . the temperature at the equatorial regions remained constant until a rotation rate between 1000 and 2000 rpm was reached at which rate the temperature began to decrease . at about 2000 rpm the temperature was substantially the same at the equatorial regions as at the polar regions . as can be seen from fig4 the rotation rate can be selected to achieve highly uniform temperatures at the envelope surface . although the changes in uniformity of surface temperature do not necessarily produce equivalent changes in uniformity of emission of light , for the system shown in fig1 a rotation rate of about 2000 rpm also produces uniformity of light emission . as noted above , changes in rotation rate also can produce changes in spectrum emitted from different portions of the surface of the bulb . for example , electrodeless lamps designed for visible applications may be filled with a number of metal halides each of which contributes to different parts of the visible spectrum . in operation , some types of metal halides may separate from other types . the result is different color emitted from one area of the lamp compared to another . by selecting the rotation rate at which the additive separation or color separation is minimized , the lamp performance is significantly improved for applications requiring high quality color imaging or projection . the optimum rotation rate , i . e ., the rotation rate which provides the desired heat , light and color distribution , will typically be different with different lamp designs . for example , the optimum rotation rate will decrease with an increase in the diameter of the envelope . other factors which may influence the optimum rotation rate are the microwave cavity dimensions , the microwave frequency , the operating power level , the constituents of the plasma - forming medium , and the orientation of the rotation axis with respect to the axis of the electric field . although the distribution of heat , light intensity , and color is a complex function of many variables , the optimum rotation rate can readily be determined experimentally by rotating the envelope under consideration and measuring the temperatures , light intensities and spectrum at various rates . rotation rates greater than about 600 rpm are typically necessary to have any measurable effect on surface heating , light or color distribution about an envelope . rotation rates in the range of 1500 to 2500 rpm will normally be required to achieve uniformity in these emission properties for envelopes having diameters from 0 . 75 inch to 1 . 5 inch . if the axis of rotation is parallel to or coincident with the axis of the electric field , rotating the envelope will increase the temperature differences between the polar and equatorial regions and increase the non - uniformity in light emission . consequently , it is essential in practicing this invention that the axis of rotation be properly oriented to the axis of the electric field . to increase uniformity between polar and equitorial regions , the angle between the two axes should be greater than 30 ° and preferably close to 90 °. to increase differences between the two regions , the two axes should be close to parallel . the envelope as shown in the drawings is spherical ; however envelopes having shapes other then spherical may be used in practicing this invention , and other variations falling within the scope of the invention may occur to those skilled in the art , and the invention is limited only by the claims appended hereto and equivalents .