Is a headlight required to emit light to the sides? The California Vehicle Code says:


  (d) A bicycle operated during darkness upon a highway, a sidewalk
  where bicycle operation is not prohibited by the local jurisdiction,
  or a bikeway, as defined in Section 890.4 of the Streets and Highways
  Code, shall be equipped with all of the following:
  
  (1) A lamp emitting a white light that, while the bicycle is in
  motion, illuminates the highway, sidewalk, or bikeway in front of the
  bicyclist and is visible from a distance of 300 feet in front and from
  the sides of the bicycle.


I have a pretty bright Cygolite headlight, but it emits practically no light from the sides, if you look at a 90 degree angle you can't see any light at all from the headlight.

Is this strictly legal under the law? I do supplement with a blinky light that  provides some side illumination (though I'm not sure you could see it from the sides at 300 feet from the bike), but I'm wondering if my headlight meets the letter of the law? If I'm ever in an accident, I don't want the other driver to be able to say "How could I have seen him, he didn't even have the headlight required by law!"

I've looked at a few other states but they didn't have any requirement for side visibility of the headlight.
What the code is saying is that your headlight has to be visible on the sides. That's why most modern bicycle headlights have those little clear plastic channels on the side for the light to seep through and be seen from the side. ( The idea isn't to blind people GordonM )

Maybe it really reaches 300 feet, maybe it doesn't, the traffic police probably aren't going to pull you over and measure. But if you do get in a crash, the other person's lawyer may very well get clever and bring this up, so if you're worried about that, make sure your supplemental blinky light reaches far enough to satisfy the law.

The bottom line is that you want other bicycles and cars to be able to see you so that they don't hit you. If you are in an accident with a car or truck, there's a chance you'll be fatally injured and whether or not you adhered to the law won't mean anything. So be safe and make sure your lights are bright enough for other people to see you.