Teeth may be lightened by one of two techniques. The first involves wearing a custom made tray over the teeth for about 2 weeks. The tray is worn at night and is filled with the whitening agent. The effect lasts for three or four years and can be topped up when required. The second method is power whitening using a plasma light. The procedure is carried out in the chair and takes about one hour to lighten both upper and lower front teeth. This is followed up by wearing the whitening trays at home for three nights to reinforce the effect.