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There are several factors that affect the over-all result for brightness obtained with a given phosphor change. It depends not only on the power output of the phosphor, but also on its exact color, which affects the amount needed to get white. Therefore it also depends on what white color is chosen. A paper by A. Hardy in 1968 indicated that at a 9300K white, the vanadate rare earth red provided 8% greater brightness than the all-sulfide tube, and the oxysulfide tube provided 30% greater brightness. Also, the red/blue current ratio was improved from 2.2:1 in the 15GP22 and 221AXP22 to 1.5:1 in the all-sulphide tube, then about the same in the vanadate tube (1.6:1), and finally 1.1 in the oxysulfide tube.
Individual samples of vanadate phosphors from 6 suppliers showed variations in birghtness of +9% to -10% compared to a reference. The average oxysulfide from RCA measured +46% compared to the reference vanadate in these tests.
The brightness of a phosphor depends on its physical preparation as well as its chemical composition. In the 70's (if I recall correctly) Sylvania discovered that a very thick phosphor slurry would increase the brightness. I recall at Motorola the tube engineers studied how to duplicate this without getting swirl marks due to uneven deposition,
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