The idea is very simple, and can be found described in many web sites, and also in the manual of Fractint.
Click to enlarge
I used a trick to avoid slow computation time, since the Lyapunov exponent is as
one can use the properties of the logarithm to transform the expresion above into
so that the expensive logarithm operation has to be computer only once after the iterations, and not once per iteration. This of course has the problem of precission, since the multiplication of derivatives can quickly get huge.
I made some experiments back in 2001, and the image above is one of the results. I guess it must be nice to extend it to 3d.