OK, here is the closest I've determined for the LL65K T15 400W (available here) bulb's center arc when the bulb is screwed into a LumenLab mogul base.

Please let me know if anyone has come up with a different measurement?

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Just saw this via a search for actual arc height on the new ll56k t15 bulb... does the arc height differ by how it's mounted? I'm doing a vertical mount so I'm imagining the arc is an upside-down U. I haven't measured it while the bulb is lit. I measured my arc at 6.5 inches with mogul. The mogul tilts the bulb and I haven't adjusted for that yet. I need to double check these values from home...

The character of the arc changes with orientation. Not all lamps may be run in all orientations. I believe your lamp is "universal" and can burn in any orientation. The "center" of the arc - with respect to the lamp housing - will move around a bit depending on lamp orientation; however, running a lamp in a vertical orientation will not result in an "upside-down U" if I understand your description. The arc will exist as a line between both electrodes and its precise shape will depend on lamp orientation.

Edit: Here's a pic of my arc with the bulb running horizontally.

Click to view attachment

ahh...that's my fault for not remembering how the electrodes are placed... so a vertical burn will probably have an arc in the shape of the letter I... but with a bulge near the top...

The measurements in the first post are for the original bulb that was something like 10.5 inches long overall - the new LL bulbs are shorter at around 9 inches overall - sans mogul... assuming that it's shipping now?