Donwill explained :
Now you can dim filament bulbs by using an astable multivibrator and varying
the mark to space ratio, relying on the chopping frequency and the time
constant of the filament to provide a non flickering and dimmable light.
Doing that to an led would cause flickering of the light but would the
relativly slow response of the eye smooth out the flicker? as in film movies
which I think have to be in excess of 25 fps in order to fool the eye.
In the interests of battery life, are there any dimmers of this type
available for leds?

It works, but for LEDs you need to increase the switching frequency to
be much higher - say around 1Khz. The filament in a lamp also takes
some time to heat up and cool down, giving it some persistence.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk