70w 3000k ceramic metal halide lamp. Specially produced for engineering testing and measurement with transparent Yttrium Aluminium Garnet arc tube.

Normal ceramic metal halide & high-pressure sodium lamps are made from sintered polycrystalline alumina. These arc tubes appear opaque due the light scattering effect of the crystalline grains that make up the structure of the ceramic material.
This is troublesome for lamp engineers who wish to make certain diagnostic measurements of the arcstream and electrodes of the lamp during operation.
Forr this reason special lamps with transparent arc tubes are sometimes produced.
High-pressure sodium arc tubes can be made transparent by growing the arc tube out of a single crystal of alumina instead of sintering but this is not possible for metal halide lamps which require complex shaped arc tubes.
A special ceramic material, Yttrium Aluminium Garnet is used whose crystal grains are so tiny that they do not scatter the light and a transparent ceramic arc tube can be created. This lamp was used for measuring electrode temperatures at full power.
As this process is more than 50 times more expensive than normal ceramic lamps, for obvious reasons they have not been commercialized.

Special thanks goes to James for this. :)

Lit pic:
http://www.lighting-gallery.net/gallery/displayimage.php?pos=-109117