In message .com,
writes
Looking for an alternative for the 500w halogen floodlight as I am
currently working outside and its stiil getting dark early. Saw this at
screwfix and although nowhere near as powerfull thought that it might
come in usefull,
http://www.screwfix.com/app/sfd/cat/...95515&id=19417.
It says it has a long-life 2D lamp. 38W = 150W GLS. I am assuming GLS
is the equivilent to a standard tungsten bulb but wondered what GLS
meant,

General Lighting Service - basically a 'normal' light bulb.

and why is the energy savng bulb measured in a different way,
i.e why is the output wattage lower for an enery saving bulb or why is
it measured differently?

Because it is the power consumed that is measured not the output.

--
Chris French