Get Screen Position of an Object (Model View Projection Matrix)

Hi all,
I would like to get the screen position of a light source for a shader that I'm implementing.
How would you go to achieve that? Probably I should get the Model View Projection Matrix but am not sure from where to get it.
I remember that jit.gl.camera gives us some matrices, but I'm not sure it gives also the one for screen transform.
If somebody can help would be appreciated, thank you!