Chart lights are attached adjacent to the overhead panel and are used to illuminate, in particular the chart holders attached to the yoke during night time operations. There are two lights, one on the Captain-side and the other on the First Officer-side.
LEFT: Chart lights removed from a Boeing 737-800 NG airframe. Colour, appearance and design is different to the the older style lights used in the classic airframes (click to enlarge).
The light from the unit can be channeled and focused from a wide angle to a narrow beam by twisting the focus ring at the front of the light. This allows only the chart holder to be illuminated for night time approaches.
To my knowledge, there are two styles of chart lights used, those that are used in classic series 737 airframes and those in the Next Generation airframes.
LEFT: Chart light removed from a Boeing 737-400 airframe. The light has a differing focus ring, appearance and colour to the NG style (click to enlarge). I believe this style of chart light is also used on the B747 aircraft.
The main aesthetic difference between the older 737 classic airframe chart lights and the newer NG style is that the older lights are squatter and a little fatter in shape; the NG style is longer, more slender-looking and has a smaller footprint.
Both style lights have an ingenious design to allow the light to be focussed. Removing the rear plate of from the older style light reveals the inner side to be a circular reflector dish (see image) which evenly distributes the throw of light when the unit is set to wide angle.
LEFT: Chart light showing reflector dish on inner side of end cap. This style is the older light type used in the 737 classic airframes (Click image for larger view).
The newer Next Generation style lights use an aperture blade which either enlarges or contracts as the focus ring is turned. This design is identical to how a camera aperture works.
Both styles operate on either 12 or 28 Volt bulbs and connection is direct to the power supply; an interface card is not required.
The switch to turn the map on and off is located on the side wall of the flight deck
Put bluntly, you cannot achieve a more realistic end product than when using a real aviation part. Genuine parts, although at times difficult to find, are built to last; if they can withstand the continue abuse of pilots in a flight deck then they are more than adequate for home simulation use.
LEFT: The NG style chart light. A blade aperture controls the amount of light that is reflected onto the thick lens glass (click image to enlarge).
It's true that while some parts appear "used" with faded and missing paint, they can easily be cleaned up with a fresh coat of paint. Personally I prefer the worn-appearance.
I still have a day job and as most of my flying is at night, overhead map lights are essential.