problem with Gamma.setDisplayGamma()

I am trying to use the Gamma class to adjust the brightness of my application. The documentation says the changes should persist until the application exits, however all I get is one flash of the new brightness when my application is starting, then it returns to normal. I make a call to the setDisplayGamma() method once in my GLEventListener.init(). Am I using this class incorrectly?

Thanks,
Mike