Cesium label blurred

I created a Cesium label, using the following code:

        var label: Cesium.LabelGraphics = new Cesium.LabelGraphics({
            text : lab,
            verticalOrigin: Cesium.VerticalOrigin.TOP,
            horizontalOrigin: Cesium.HorizontalOrigin.RIGHT,
            font: '15px Helvetica',
            fillColor: Cesium.Color.WHITE,
            outlineWidth: 2,
            style: Cesium.LabelStyle.FILL,
            pixelOffset: new Cesium.Cartesian2(20, 20)

But it is blurred…

I would like to have a clearer label. Is this image you can see inside the red rectangle the real label. In the blue rectangle is the label with a zoom In. In the green rectangle is how I would like it to be. enter image description here

Is there some way to make the label cleaner?