function grijzeBalk() {
    var windowHeight = 0;

    if(window.innerHeight) {
        windowHeight = window.innerHeight;
    } else if (document.documentElement.clientWidth) {
        windowHeight = document.documentElement.clientWidth;
    } else if (document.body.clientWidth) {
        windowHeight = document.body.clientWidth;
    }
    if (navigator.userAgent.indexOf('MSIE') != -1) {
        document.getElementById("mainContent").style.height = Math.round((windowHeight / 2) - 195) + "px";
    } else {
        document.getElementById("mainContent").style.height = (windowHeight - 195) + "px";
    }
    
}