A few months ago I got sick of moving my desk lamps between my work spaces and the copy stand I use for macro photography. So I headed on up to a local Staples to pick up an extra halogen desk lamp. Browsing through the lamp aisle I spotted this fluorescent magnifying clip-on lamp (Item# 612507, Model# 13464-US) and thought it could make a nice ring light. So I bought one for $25, a bargain for a 3.5" diameter ring light ($26.99 April, 2011).
Converting the lamp is one of the simplest hacks I've ever done. You start by following the included instructions for replacing the bulb. Take out the three screws holding the bulb cover and diffuser and pull off the diffuser. Gently lift the bulb off the metal retaining clips while pulling out the connector. The bulb and connector are not rigidly attached so be careful not to break the two apart.
The lens is held in the lamp by three small metal plates underneath the bulb clips. Remove the three screws, pull out the bulb and lens clips and, then the lens. With the lens out reassemble the clips with the screws but reverse the direction of the lens clips so that they don't cause light reflections outside of the diffuser. Make sure you position the tall sides of the bulb clips away from the center hole like they where originally.
Reinsert the bulb and reattach the diffuser with its three screws. The last step is to permanently remove the hinged lens cover. Simply grab the cover and twist it off the housing leaving hinge bumps that can be cut off with a utility knife.
The result is a good ring light that fits well on my copy stand.
Now I can can use my ring light alone or in combination with one or more halogen lamps to easily get a lighting effect I like.
The much cooler operation of the fluorescent compared to the halogen lamps got me using it with my microscope which only has built in halogen lighting. I quickly got tired of sharing the lamp between the microscope and copy stand so, I bought two more lamps one for the microscope plus one for spare parts in case I need them.
Here's a good article with information about Choosing the Correct Illumination.