You've got their resume, you've got the word of their past employers, they've given you some sort of portfolio, isn't that real measure of whether or not they can get the job done? You've got years of work you could go over, but instead you're thinking that in thirty minutes you'll be able to judge a man's skill?

It seems like an interview should be more about getting to understand someone's personality. Are they a good fit for your department? Maybe you could also spend some time clarifying things in their portfolio, trying to understand the work they really did. Giving them a paper test sounds like a waste of time to me.