@nanasusan You will need to look at the ratings for the red light device you bought, then read whatever RESEARCH material you can find on treating your specific condition with red light therapy. That does not include any ads/literature by sellers or on web sites.

Here is an article specifically evaluating red light therapy in rotator cuff injuries:
https://onlinelibrary.wiley.com/doi/full/10.1111/wrr.70043
Note that they were evaluating specific wave-length pulsed lights. All subjects in this small (20 subject) study were under age 60. The specific treatment was effective for most in lowering pain, but not necessarily improving function. Functional use improved with a multi-prong approach "...Another study by Eslamian et al. [18] found that incorporating PBM into a combination treatment of exercises, along with heat therapy, ultrasound and TENS, resulted in greater pain relief than the combination therapy alone in a group of 50 patients with RC tendinitis. Overall, multiple evidences determine the effect of PBM in reducing pain, improving ROM of the shoulder, and quality of life. Contradictorily, a few studies showed no change in ROM after PBM therapy, although pain was improved after a month's follow-up [19, 20]..."

Here is a deep-dive:
https://pmc.ncbi.nlm.nih.gov/articles/PMC8946909/
Their conclusion: "Overall, IR appears as a safe and effective complementary therapy for a number of musculoskeletal conditions, including knee OA, FM, and chronic myofascial pain. Its efficacy remains debatable when it comes to low back pain and muscle damage in sports injuries. Given the diversity of therapeutic means and applications, further research is necessary in order to establish the optimal indications and treatment plan for the clinical use of IR in musculoskeletal medicine."

So effective? Big maybe when used alone. Best to see a rehabilitative physical therapist for a multi-modal approach.

(PS With several tears, my body did not respond to therapy, and I ended up with extensive rotator cuff surgery and therapy. I am doing greatnow, about 2 years later.)