But the short F20 would have all the light concentrated into the short tube, so too bright.
The need is for lower intensity, larger surface light.
Yes, PMMA were used quite long, but if it needs to diffuse the light, it needs either some difraction pattern (the ridges then tend to hold the dust), or have some "milky paint", which tends to absorb a lot of light.
But if you look at common pricing, the F40T12 cost was about the same or frequently lower than the F20T12, the only reason being the F40 was manufactured and sold at way higher production volumes.
Originally the reduced intensity "residential" ballasts were designed to run the common F40 tubes, just at about 25W. Then dedicated 25W tubes appeared, with just thinner filament, to reduce the cathode heating power consumption when the cathode does not need to be that beefy for the reduced power. It is true the 25W version become a lower volume odd wattage tube (although sharing way more production tooling setup with the main highrunner than the F20T12), but the standard of using the lower power for residential applications was already set at the time the dedicated 25W version tube was introduced..
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