{HEAD ~ the diffuser elements} have {TAIL ~ an average space therebetween} of 50 μm to 300 μm, 
have
{ HEAD ~ the diffuser elements } have { TAIL ~ an average space therebetween } of 50 μm to 300 μm ,
HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH