What sort of brightness control are you wanting to achieve, sorry, but it is not clear to me from your comments.
Brightness is directly controlled by the forward current, but increased current means a higher junction temperature. There is a negative brightness coefficient due to increasing junction temperature (it is manufacturing specific)This also means that a change in ambient temperature will change the brightness. Do you want proportional control of brightness, or just to reduce the brightness when you reach a chosen threshold temperature.
It also raises the issue of what it is to be used for, since the eye will not detect a small change anyway, so is it just to lengthen the life of the led, for example.
Measuring the junction temperature of the diode is not really possible, junction temperature higher than the external temperature, and the thermal resistance from junction to heat sink may not be known reliably, and there is also the thermal time constant to deal with.
Junction temperature is directly related to forward voltage, but you have to do a bit of maths, solving an equation (a micro could do it) to determine the relationship between the temperature and voltage, this would be the approach to take, since it takes into account ambient changes as well. The voltage could then be used to control the forward current.
If you explained a little more just what you hope to achieve it might help.
Controlling brightness is more often done by monitoring the light output, making sure that the current does not exceed a specified amount, taking into account derating curves for power dissipation if the current gets high enough.
As Dean said, there is a lot of info on the web,
cheers,
Richard
