HQI deals with the composition of the bulb rather than the shape of the bulb.
here is a HQI description from Sylvania
"SYLVANIA HQI® metal halide lamps combine excellent
brightness, long life and high CRI in a compact package
for optimal lamp performance.
The HQI family of metal halide lamps is manufactured with
the addition of rare earth metal halides to improve color
rendering and luminous efficacy. With their compact size
and very short arc discharge length, these lamps come very
close to being a point source of light allowing luminary
manufacturers to design highly efficient fixtures. The very
high color rendering, white light, compact size, long life and
high efficacy make these lamps ideal for replacing tungsten
halogen in certain retail applications."
here is some More on the physical design
"• Excellent CRI, up to 93
• High luminous efficacy
• Available in warm white and cool white correlated
color temperature
• Compact single ended or double ended design for
optimal fixture efficiency
• Low thermal output
• Long life
• Double-ended contacts allow for exact
fixture alignment
• Operate in enclosed fixtures
• UV-Stop outer jacket"
Now where the confusion comes in is that every tom dick and harry has called DE bulbs (double ended) HQI bulbs which is kinda true, they are one form of HQI bulb but not the only one, you can get them in PAR formats and SE (single ended) mogal isn't really a proper term as it only describes the north American standard size socket. if you check a Ushio or BLV or AB SE bulb closely it doesn't fit perfectly into the mogal sockets and there is an actual # for a different screw in socket that is slightly different than a mogal size that fits them properly.
the difference is that the HQI ballast (ANSI M80) actually mimics European specs so it drives all the bulbs based on European design better than the north American standard. I have found that my SE AB's burned about 30 to 40% more intense on the M80 ballast and also lasted about 1.5 times longer. and even at a time of 2 years My AB bulbs still put out more PAR than a brand new SA 250 watt bulb on the same ballast (it was close though
)
the main problem comes in as the industry is making them out to be different from each other so they can sell you new reflectors and bulbs as an upgrade. In my opinion going to a M80 ballast is an upgrade but switching to DE is a down grade unless you have a shallow narrow tank as they are limited in the foot print they can cover. a SE bulb on an HQI ballast will be limited only by the design of the reflector as there increased size and length allow it to illuminate a larger area.
Steve