The relieve is temporal and mild. It is not deep seated. it is a superficial relief. if it is non-liminous infra-red type you are using means it should be used for chronic pain. but, the luminous type is for acute pain. The deeper pain relief electrotherapy modality is preferably the Short-Wave diathermy.
Infrared therapy has soothing effect comes from improvement of circulation it causes. With pain, as in low back pain, the cause of pain could be due to muscle fatigue and accumulation of metabolites (wast products of the metabolism) which stimulate the pain receptors. Washing these metabolites through improving circulation relieves pain. So it is integral when clinically take decision about the appropriate intervention to consider the underlying cause of patient's symptoms. It is also advisable to check the presence of any contraindications for the though modality. In addition, considering the patient's preference and patient's response is also of high importance.