Viewers need them to see Van Gogh painting (6)
I believe the answer is:
irises
I believe this is a double definition.
'viewers need them to see' is the first definition.
'van gogh painting' is the second definition.
(Other definitions for irises that I've seen before include "Plants with sword-shaped leaves" , "Flowers found in the eye" , "Flowers and parts of eyes" , "blooms" , "Coloured membranes in eyes, or showy flowers" .)