The idea occured to me when I first saw how thin the wire in a bulb is and how bright the light was. Also, how could the (average) thickness of lightning be found out?

Take a picture of lightning. Scan through the picture counting the pixels occupied by lightning. Apply the formula GSD = range * IFOV. IFOV is the instantaneous field of view for a single picture element of the camera which is easy to figure. the range is a little more problematic. You could use the rule that lightning travels about 1 Km in 3s. But that's 1) only a gross approximation and 2) varies with air pressure and moisture. You need a better estimate of range.

Possible algorithm:

0. Acquire two movie cameras and two shelters (houses).

1. Set the cameras pointing out the windows of the house in such a way that the cameras are at either end of an hypotenuse of a right triangle, each looking towards the vertex opposite the hypotenuse.

2. This will allow you to come up with a better estimate of the distance of the lightning from each of the cameras.

If you don't feel comfortable with a lot of geometry, I recommend waiting until you get a bit of lighting that is very close to the vertex opposite the hypotenuse, and working from there.

This solution is very simple, but it assumes that the height of the lightning is much smaller than the projection of the actual range onto the ground.

A much more accurate system would be to use synchronized microphones on the ground and back-compute the range based on the time differentials. This is a very trivial problem in analytic geometry - unfortunately I haven't done this calculation in about 30 years, so I can't recall off the top of my head. This is essentially the same thing the UK military's HALO system uses.

http://www.scienceagogo.com/ubb/ultimatebb.php?/topic/1/488.html

(Same thing used by the US military in their UTAMS system http://www.arl.army.mil/main/main/default.cfm?Action=49&Page=132 )

You would still need to use something like this http://www.measure.demon.co.uk/Acoustics_Software/speed.html

to figure out what speed of sound to use. This solution assumes that the lightning is not so high in the atmosphere that the speed of sound changes significantly.

OTOH, I think you might save a good deal of time and effort if you read the following paper:

http://64.233.161.104/search?q=cache:SX2TFDafVSwJ:cam.arts.usf.edu/McCollumPDF/25.Uman10.pdf+%22diameter+of+lightning%22&hl=en&lr=lang_en

A bolt of lightning is a plasma channel perhaps a centimeter in diameter while current is flowing. The large current is zeta-pinched.

The visible lightning bolt is anywhere from a meter to ten meters in diameter. But this is almost entirely light emanated by the charged air surrounding the actual plasma channel, which, as noted, is pretty small.

I mean the actual bolt, ignore the light.