Heat is a factor in enclosed fixtures. I do not know if a 72 watt halogen generates higher temperatures in an enclosed fixture than does a 75 watt incandescent. But I doubt that it does. The halogen capsule within the lamp does operate at higher temperature. But that capsule is somewhat isolated within the lamp.
Just a guess, but the 72 watt halogen should not damage the fixture more than does a 75 watt incandescent.

Thank you, bus-driver. Like you, I don't know if a 72 watt halogen generates significantly more heat than an 75 watt incandescent. Heat would be my main concern.

Each vertical fixture has two 7-inch glass cylinders--one facing up and the other facing down. The cylinders are open at each end and are wide enough for me to fit my hand inside to turn the bulb. I hope the opening is wide enough to let any extra heat dissipate.

For now I can use the 75 watt incandescent provided with the fixture, but I think the 72 watt halogen would produce more light. In the future when 75 watt incandescent bulbs are no longer available, I would prefer to continue using a 72 watt halogen rather than switch to a lower wattage halogen. These fixtures will have dimmers and won't be on all the time, so energy efficiency is not as great a concern here as it might be in some spaces. There are times when I will need full light, however, so my goal is to find the brightest option possible.

As long as you cannot actually see the halogen capsule it should be fine.

If you CAN see the halogen capsule you need some filtering on the glass.

The spectrum is much wider and can damage things (like make plastic brittle, make cloth brittle).

The light intensity is very high, even after filtering out the small amount of UV produced.

Measure the temp with the OE lamps and then with halogens. (Stick a thermometer into the cylinders.) I bet the temperature will be lower with the halogens installed if you find any difference at all. The halogen lamp is a lower power. Halogen produces more light and less heat. Since both types produce mostly heat, the difference in heat output will be less noticeable than the difference in light output (3.5% vs 2.1% of total power in visible light).

I know that the 72 watt halogen will probably produce similar light but less heat that its equivalent 100 watt incandescent, but I am just not sure how it compares to a 75 watt incandescent.

If the fixture calls for a 75 watt maximum incandescent, does that mean I have to use the 53 watt halogen that is equivalent to a 75 watt incandescent, or can I go ahead and use a 72 watt halogen (equivalent 100 watt incandescent)?

When the manufacturer specifies maximum wattage, I assume it is to control amount of heat generated. The fixture is not installed yet so I can't test directly with a thermometer. If I can get enough light by using the higher wattage halogen, I might be able to get along with two fixtures in the vanity area (other lighting in the room is some distance from the vanity). If I can't get enough light in that area from just these two fixtures, then I need to ask the electrician to put in a can or some other light in the ceiling. I'd rather avoid that if possible. The electrician is coming this week, so I have to try to figure this out without being able to do any direct testing before he arrives.

72 Watt halogen > (more) light than 75 Watt standard incandescent.

72 Watt halogen =/Why not call the manufacturer?

I will call the manufacturer on Monday.

In the meantime, I went to a couple of lighting stores today and got 3 different answers to my question. One store consultant said, "Yes, of course you can substitute a 72 watt halogen for a 75 watt incandescent--no problem." Another said, "Definately not. The heat might damage the fixture so you must use the lower 53 watt halogen when replacing a 75 watt incandescent." The third said he had heard both answers from various sources, but he would err on the side of caution and use the 53 watt. I came home no wiser than when I set out.

I just hope the manufacturer can give me an accurate answer.

The original post did not mention the open cylinders. So it is not an enclosed fixture.
"Each vertical fixture has two 7-inch glass cylinders--one facing up and the other facing down. The cylinders are open at each end and are wide enough for me to fit my hand inside to turn the bulb. I hope the opening is wide enough to let any extra heat dissipate. "
If a cylinder is vertical and open at each end, how can it be considered to be facing only up -or down?

My expectations for customer support for issues like this are always low. They might answer that it has not been tested. If they answer no, and you follow up with "why", they will probably verbally "shrug" or say that it has not been tested. That will probably be after a month after an engineers' meeting to discuss the problems that have come up in the past month.

Bus_driver, I had the same problem that you did. My best guess is that the cylinders are cylinders are horizontally oriented and not completely closed on the curved surface. They are more like a "C" in cross section.

Sorry for not describing the fixture clearly. It is the Hudson Wall fixture from Tech Lighting, model # 700WSHUD2

The fixture can be mounted either horizontally or vertically.

The back plate is 4 inches square with a 2 inch band of metal in the middle. There are two sockets, one on each side of the center band. A round cylinder of glass fits over the bulb in each socket. The cylinders are 7 inches long x 3.5 inch diameter. I meant to say that each cylinder it open at one end. When mounted vertically, the upper cylinder is open on the top and the lower cylinder is open at the bottom.

The final decision is up to you, but I don't see any problem with
using 72watt halogens there. They will generate less total heat than a 75 watt bulb (by a trivial amount). You may want to put it on a dimmer (which will itself cut the light output by a small amount).

http://www.build.com/mediabase/specifications/tl_hudson_wall_111610.pdf

I might go for the CF version myself.

That is a nice looking fixture. (But I am a fan of contemporary design.)

The 72-watt halogen bulbs would be fine. Looks-wise, the higher color temp of halogen would even be an advantage. However if it is a highly used fixture, more than say 1-3 hours/day, I would suggest simply using LED A19 bulbs and never worry about it again.

Energy is producing either heat or light, so if a bulb that consumes the same amount of power (watts) generates more light (higher lumens), it must be generating less heat.