" a purposeful optimism"

[Ngoc_Trong_May]

Dear teacher,
i cant understand the meaning of this sentence .Can you help me?
"Jeff Wall’s large color transparencies mounted on electric light boxes fill 10 galleries at the Museum of Modern Art with a pulsating and purposeful, if slightly sedate, optimism. "
purposeful means " knowing clearly what you want" .Is it the adj of " optimism"? why does optimism have a clearly purpose ?I tried to translate it into vietnamese but i couldnt let everyone accept its meaning .I myself cant grasp the ideas of the author ,too.Not only so ,but i cant figure out why has "a" as the article for optimism while optimism is an uncount noun as well.

[Anglika]

a pulsating and purposeful, if slightly sedate, optimism. "

Classic critical writing, this!

A pulsating and purposeful optimism - the content and appearance of the photographs state a view of the world that is not negative; A specific optimism - one that is pulsating and purposeful [positive]

if slightly sedate - the images do not contain violent events and are not "exciting".