Interesting stuff! That 1941 Westinghouse ad looks IDENTICAL to two of my 2x90/100 T17 fixtures!
I was reading through the old lighting information and there was a mention about these type of fixtures with the closed ends.
It seems that the larger 100 watt fixtures reflectors with the open ends were having problems with flexing when taken down to be cleaned and breaking the porcelain off of the reflectors.
So the closed end reflectors were made so they would be stronger and reduce the flexing of the reflector.
When the painted reflectors were used, the flexing was not a problem so open ends were ok.
But heat build up from the bulbs can be a problem as the light output decreases above a bulb wall temperature of 120 deg F.
So fixture ventilation is needed.
According to the book, for the 100 watt T17 bulb the ambient temperature for maximum light output is 60 Deg. F.
Once the bulb wall temperature goes above 120 deg F. the light output starts to fall off because the mercury vapor pressure in the bulb increases and the ultraviolet light produced goes out of the proper range for the bulb phosphor to emit light correctly.
I did not know this.
Enclosed bulbs have to have some sort of ventilation or way to conduct heat away from the bulb to maintain full light output.
