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15GP22 mask transmission 14%
Later tubes (of any type including Trinitron) mask transmission ~ 25%
Ratio 1:1.8
15GP22 area: 88.5 in^2
21 inch round tube area: 255 in^2
Current per unit area ratio: 2.88:1
Brightness ratio (15G 14% / 21 inch 25%), all else equal including beam current: 1.6
So the larger tube had a 38% deficit compared to the 15G, operated at the same beam current.
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