function openLightbox() {
    // Abmessungen des kompletten Fensters ermitteln
	var windowWidth = 0, windowHeight = 0, boxWidth = 0, boxHeight = 0;
    if( typeof( window.innerWidth ) == 'number' ) {
		// Non-IE
		windowWidth = window.innerWidth;
		windowHeight = window.innerHeight;
	} else if( document.documentElement && ( document.documentElement.clientWidth || document.documentElement.clientHeight ) ) {
		//IE 6+ in 'standards compliant mode'
		windowWidth = document.documentElement.clientWidth;
		windowHeight = document.documentElement.clientHeight;
	}
	
	// Abmessungen der Box ermitteln
	boxWidth = document.getElementById('lightbox-content').offsetWidth;
	boxHeight = document.getElementById('lightbox-content').offsetHeight;
	
	document.getElementById('lightbox-content').style.left = (windowWidth/2) - (boxWidth/2) + 'px';;
	document.getElementById('lightbox-content').style.top = (windowHeight/2) - (boxHeight/2) + 'px';
	
	document.getElementById('lightbox-background').style.visibility = "visible";
	document.getElementById('lightbox-content').style.visibility = "visible";
}

function closeLightbox() {
	document.getElementById('lightbox-background').style.visibility = "hidden";
	document.getElementById('lightbox-content').style.visibility = "hidden";
}