Can the eballast drive high pressure sodium bulbs?

Since the S400DD is an HPS conversion lamp, it's designed to run off of HPS ballasts. From this it is surmisable that the eballast can run an HPS lamp.

The question then becomes, why would you want to? HPS lamps have such poor CRI capabilities, they're practically useless for a projector. This is why lamps like the S400DD exist, it's a high CRI metal halide lamp that will run off of an HPS ballast, so that you can get better colour light without replacing your entire lighting setup.

Although HPS is considerably more efficient.

yes, it is the most efficient lamp tecnollogy I believe.

but is not usefull for as as well, 1800K color temp. (remenber that the halogen yellowish light from an OHP is more like 2900K color), so the 1800K are crap.

Its not the temperature that's the problem its the narrow spectrum emitted. Those things output a ton of orange and not much else. The CRI must be very low.

Oh, and it's not the most efficient lamp technology, Low Pressure Sodium is I believe.

The colour temperature IS the colour emitted.

What the colour temperature is is an idea of the spectrum that would be emitted from a "black body" if heated to that temperature.

The HPS lamps are a low colour temperature, which means that there's a lot of red, but not much blue. Blue requires more energy to produce, which in turn means a higher colour temperature.

This has little or nothing to do with the temperature that the actual lamp burns at. At the kinds of temperatures that we're talking about, glass would vaporise, so it's obviously not that hot. (Remember kelvin is only 273 degrees lower than degrees C, so even 1800K is over 1500 degrees C, and 6200K is way too hot to consider.)

How on earth, then, do you measure the colour temperature of a full spectrum bulb?

There's no such thing as a "full spectrum" bulb, they're all selected spectrums.

The black body at the temperature would not emit pure colours either, it will emit a spectrum of radiation. When we measure the colour tempeature, we compare the visible light radiated by the lamp with the visible light that would be radiated by the theoretical black body, and rate it that way. There's no point in comparing the radio, microwave, IR, UV, Gamma and X-rays that would be emitted by the theoretical black body at those kinds of temperatures.