A therapeutic laser is basically an apparatus to alter light to increase its speed and its energy so it will have the power to penetrate skin. They are termed "cold" lasers because they do not elevate tissue temperature. The two types of low power laser we commonly see in therapeutic use are the infrared which can penetrate to 5-10 centimetres, and the visible-red with a penetration of 12 millimetres. Effects of laser are thought to work on the biological field theory which suggests that a biological energy field exists around every cell. Injury and or disease causes this energy to go out of phase with normal frequency. The resonant effect of low power laser can restore the normal energetic status of the field and so stimulate the repair and healing process. Lasers are primarily used for their analgesic effects and for wound healing. The repair of wounds by laser also seems to result in better strength of repair, and less chance of amount of exuberant granulation.