One way to get the lamp to not cast shadows would be to create a prop from those material zones that make up the lamp and then make the original part of the prop 100% transparent. You create props with the grouping tool. Select the scene prop, open grouping tool. Find the material zones and select them. Hit the Create Prop button. Spawn props will spawn all groups as new props, not what you want here :)

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You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.