Difference between infrared heating tube coatings?
Alrighty, I did some digging around and found some information. Briefly...
Tube coating/reflectors
Applied directly to the quartz tube (reflectors often only coated on top-half to radiate heat from bottom-half.

Clear - No coating, spreads heat evenly, high temperature.
White - High temperature applications, Moderate reflection.
Gold - Very effective reflection for directional heating, lower temperature.
Ruby - Filters visible light while maintaining IR output (doesn't reflect).

Wavelength
Specifies the range in the IR portion of the electro-magnetic spectrum.

Short/Near - Doesn't physically feel hot i.e. TV remote.
Long/Far - Physically feels hot, like sun light.
Medium/Mid - A mix of the two depending on how the spectrum is divided.

Resources
Helios Italquartz s.r.l. have some good literature on IR quartz heating. They describe these coatings as reflectors:


GOLD REFLECTOR is a gold layer, fixed directly on the quartz tube. It’s able to reflect more than 90% of IR radiation and if properly
cooled it can reach the working temperature of about 600°C;

WHITE REFLECTOR is a with ceramic layer, fixed directly on the quartz tube. It’s able to reflect about the 70% of IR radiation, so it
is less effective compared to the gold one , but it can resist up to a
working temperature of about 900 - 1000°C;

RUBY REFLECTOR is used to mitigate the IR radiation intensity of the filament. It is usually fixed all over the quartz tube in
combination with another reflector;



I also found a data-sheet (PDF) by Osram Halotherm Infrared Lamps
They describe the intended use for their ruby products:

Lamps made of ruby quartz predominately emit infra-red heat radiation
with reduced transmission of visible light. They are used where
typically visible light would interfere, or in applications where only
heating is required, e.g. outdoor and indoor zone or space heating and
auto/marine service or repair shops for drying/curing paint.

Finally, HowStuffWorks &quot;How Infrared Grills Work&quot;
Describe the infrared spectrum well (page 2), and heating by radiation vs convection (page 3).