a question re: b & w......

[Karen Clanin]

can anyone tell me what those little round pieces of thick blue acrylic are 
called?  you look through them to make everything look like a monochrome to 
see values etc which are not as obvious when you look at things in color 
and take it in b & w.

i have my husbands but want to get one for a friend and don't even know 
what to search for.

karen