Well here’s an interesting use-case for Google Glass.

More than just a mobile HUD that can also record or stream anything the wearer sees, what if the hardware got tweaked to include a second, inward-facing camera? That’s the scenario posited by LinkedIn VP Daniel Shapero, who cites an article in TheScientist that explains how human pupils dilate when the subject experiences happiness. And so Shapero proffers the potential of a Happiness Graph populated by data from Glass.

The information could be tracked and compiled to offer a wealth of features, such as:

Matchmaking: You’re at a bar when you see someone attractive. Glass knows it, and if the other person’s wearing the tech too, it can tell if he or she feels the same way about you. Suddenly walking up to a stranger and buying a drink doesn’t seem so scary.

Crowd-sourced fashion advice: Think you look hot in those jeans? Well, apparently so do other people, says Glass, prompting you to go back to the store and buy a dozen of those denim blues.

Product research: By grabbing a large swath of data from all Glass users, the Happiness Graph could easily pinpoint people’s feeling about a new product. Was it liked? What types of people liked it? When and where did they enjoy it the most? The answers would be like the Holy Grail for tech companies and other manufacturers.

Activity finder: It’s Saturday night and you want to go out, but you’re not sure where. Enter Glass to give you a list of places where the people are happiest. It could even give you recommendations, estimating where you’d be happiest based on your demographic, preferences or social networks. As for the venues, it opens up a hyper-effective advertising tactic: “Come on down to Al’s Bar! Find out why our happiness score is 8.6 right now!”

Virtual parent: You see an amazing skydiving special on Discovery Channel, and suddenly you’re Googling places where you can do a tandem. Fortunately, Glass knows that you hate heights, so it can easily save you from yourself, discouraging you from blowing money on an experience that may sound cool, but would make you sick to your stomach.

Google-Glass2Interesting use-cases? You betcha. And it probably wouldn’t be hard to install a second camera, though a finely tuned one backed by the computing power necessary could be trickier to implement. Not impossible, mind you, but tougher. However, this isn’t the only hurdle.

In the same article in TheScientist, the author explains that all sorts of things can make the human pupils dilate. Sure, happiness does the trick, but so do anxiety, concentration and plain old changes in ambient lighting. (And did we forget drug use? Certain illegal substances can make those peepers crack wide open.) In the past, researchers have attempted to use pupillometry to read people’s hearts and minds — to assess everything from sleepiness to introversion, race bias, schizophrenia, sexual interest, moral judgment, autism and depression — but judging by the laundry list of abandoned studies listed in the article, they’ve probably failed more often than succeeded. But even accuracy may not be the biggest issue.

Some people are already uneasy with Google knowing so much about us as it is. What we search, where we are at any given moment, whom we communicate with and socialize with, the contents of our emails and so much more — all of that is already up for grabs. How would people react if the company also had the ability to directly dig into our psychological states, track our moods and catalog all that in some massive database? What’s really cool and interesting for some would be utterly creepy and invasive for others.

What would you think if Mountain View developed a Happiness Graph using Glass? Would you be excited by the possibilities or unnerved by the company biometrically scanning your feelings?