Deminishing light output from LED

[amtrakuk]

Just having a discussion on ATL about how after a few years and without really noticing the light output of LED lamps drops. I was told with my like for like replacement from Philips the new generation produce more light and compared with a two year old one and yes it is noticeable. It would be interested if anyone has done any historical light measurements as the LED promise seems to be questionable.