LED defect ?????

Hi!! I have received a LED and i don't know if you have already seen this , on the beamshot of the LED i have a darker stain.

Look at the pic....

I think that i have my response here : http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=4198906

Led "seconds". What light and where did you buy it?