I find it difficult to see in under my router to where the bit is cutting. There's no light in there. For years I've been wearing a headlamp to try to get enough light in there to see what I'm doing. It doesn't work very well. I finally found a way to mount lighting to the underside of the router body. Here's a pic of the improved router, showing plenty of light.
overall.jpg
The key component in this upgrade is a device called a ring light or a halo light. Here's the one I used. It is about 2 9/16" outside diameter. The 15 little yellow blobs are LEDs. Ring lights are used to dress up cars. They outline and accent headlights. This one cost about $5 from www.superbrightleds.com.
ringlight.jpg
To mount it to my router, I made a plywood adapter. It fastens to the router through screw holes that were already in the router. The ring light is inlaid into the plywood, and held in place with a cover of packing tape. The plywood and tape offer some protection against flying chips, and the wrenches used to tighten the collet.
ringunlit.jpg
Here's a pic of the ring light lit.
ringlit.jpg
The light wants 12 VDC. I velcro'd a 12 volt rechargeable battery to the top of the router. It can be removed for charging. The battery is rated at 1800 ma-hr, which provides a day's worth of light. I really need only a couple of hours, so this battery is overkill, but it was the smallest I could find. The battery plus charger cost $18 off Ebay and Amazon and the like.
withbattery.jpg
Much better!