SoLux is a patented new light bulb that simulates daylight far better than any other light bulb on the market. It is a high performance halogen type bulb (rated to last 4000 hours) that currently comes in the low voltage (12 volt) MR-16 format (2 pin socket). SoLux uses a multilayer thin film coating on the reflector to reflect the desired daylight spectrum forward and pass unwanted light out the back yielding the closest match to daylight.
Below is an image showing a comparison between a solux 4700K 50w globe, a D50 (theoretical daylight) light source and a typical Fluorescent light source. The Solux is in blue, the spectral coloured area is the D50 curve and the green curve is the fluorescent. There is a tight correlation between the SoLux and D50 light sources, demonstrating that the SoLux is an excellent daylight simulator.
The green curve below shows a fluorescent light source with it's typical spiky spectrum which can produce unwanted colour shifts when viewing prints.
It is also worth considering that when we make profiles, out default setting for viewing lights is a D50 light source. The closer your viewing light is to a D50 when comparing your print to a screen proof, the better your match is likely to be. A solux viewing light teamed with a profiled quality screen like an Eizo makes for a great proofing setup.
Light quality is often quoted in terms of its Colour Rendering Index (CRI). Daylight has a CRI of 100 and other light sources are compared relative to that. Below is a screen grab from i1 Share, an application from Gretag MacBeth used to measure light. It shows again the comparison of daylight to SoLux but also shows the CRI value at the top left.
This is from my Solux setup next to my EIZO. I run my Solux at 13.8v and this gives me almost exactly 5000K and a CRI of 97. Any CRI over 95 is considered excellent.