Convert an image RGB->Lab with python
At the moment I haven't found a good package to do that. You have to bear in mind that RGB is a device-dependent colour space so you can't convert accurately to XYZ or CIE Lab if you don't have a profile. 
So be aware that many solutions where you see converting from RGB to CIE Lab without specifying the colour space or importing a colour profile must be carefully evaluated. Take a look at the code under the hood most of the time they assume that you are dealing with sRGB colour space.
What is the preferred way of doing the conversion using PIL/Numpy/SciPy today?