Both the fixture and the bulbs are E26 standard medium socket; so they definitely fit.
And I'm pretty sure the wattage is fine (42w < 50w).
I'm just worried about causing a safety hazard… Would combining these non-"R20 Halogen bulbs" into this "R20 Halogen bulb only" fixture create an unsafe lighting installation?
I should mention that the fit is rather tight, and that although 100% of the bulb and 50% of the base of the bulb are uncovered, the bottom half of the bulb's base is tight against the edge of the fixture.
Please let me know if additional information is needed.
Best Answer
One consideration is current draw. CFLs have a power factor of about 0.5, meaning they draw twice as much current as a similarly sized halogen or incandescent bulb, which have power factor of 1.0.
So if your fixture is rated for a 50W halogen bulb, that means: 50 = 120*current*1.0 (Power = voltagecurrentpower factor), so the fixture is rated for 0.42 amps.
A 42W CFL would demand: 42 = 120*current*0.5, current = 0.7 amps.
The rating of the lamp takes many different things in to account, not just current draw, but also cooling of the bulb and the fixture itself, but this is just one consideration.
Another consideration is that CFLs, although they run at a much lower temperature, will also fail prematurely at a much lower temperature, so if the fixture is not properly ventilated and the base isn't allowed to cool, it could fail long before its time.