From WordNet (r) 2.0 (August 2003) [wn]:
diffuser
n 1: baffle that distributes sound waves evenly [syn: diffusor]
2: optical device that distributes the light of a lamp evenly
[syn: diffusor]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Diffuser \Dif*fus"er\, n.
One who, or that which, diffuses.
[1913 Webster]