I use According to Gospel Therapy Light Pads and they have a beautiful explanation:
LED light therapy is really a way to help the body heal itself. We use specific wavelengths of light that are absorbed by a photo acceptor, cytochrome c oxidase within the mitochondria of the cell. The energy (photons) from the lights increases the energy within the cell and that speeds up the healing process. The lights must be specific wavelengths and must be delivered at a specific dosage. We use two wavelengths in our light therapy pads, a visible red and a near infrared wavelength. Using both red and near infrared lights is beneficial because the wavelengths are absorbed by different depths of tissue. The combination of the two work in concert to provide benefits for soft tissue injuries, inflammation, ligament soreness, tendon problems, sore backs, splints, strains, stifle issues, sprains, swelling, shoulder pain, hip pain, sore backs, sore necks, salivary gland problems, wounds, cuts, scrapes, arthritis pain and for trigger points and acupuncture points. Visible red light (660nm) is absorbed by skin layers very efficiently and is best for uses such as stimulating trigger and acupuncture points and for treating wounds and infections. Near infrared light (850nm) penetrates to a deeper level and has been used to treat concerns of tendons, ligaments, bones, joints and muscle.