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¹ Department of Family and Community Medicine, St. Michael's Hospital, 30 Bond St., Toronto, Ontario, M5B 1W8, Canada.

² Kent Chiro-Med Wellness Clinic, 563 Gladstone Ave., Ottawa, Ontario, K1R 5P2, Canada.

³ Firefly Community Service, 1 Meno Ya Win Way, Sioux Lookout, Ontario, P8T 1B4, Canada.

⁴ Department of Physiotherapy, Haliburton Highlands Health Services, 7199 Gelert Rd, Haliburton, Ontario, K0M 1S0, Canada.

⁵ Department of Physical Therapy, University of Toronto, 500 University Ave., Toronto, Ontario, M5G 1V7, Canada.

⁶ Princess Margaret Cancer Centre, Toronto Medical Discovery Tower, 101 College Street, Room 15-310, Toronto, Ontario, M5G 1L7, Canada.

¹ Department of Family and Community Medicine, St. Michael's Hospital, 30 Bond St., Toronto, Ontario, M5B 1W8, Canada.

² Kent Chiro-Med Wellness Clinic, 563 Gladstone Ave., Ottawa, Ontario, K1R 5P2, Canada.

³ Firefly Community Service, 1 Meno Ya Win Way, Sioux Lookout, Ontario, P8T 1B4, Canada.

⁴ Department of Physiotherapy, Haliburton Highlands Health Services, 7199 Gelert Rd, Haliburton, Ontario, K0M 1S0, Canada.

⁵ Department of Physical Therapy, University of Toronto, 500 University Ave., Toronto, Ontario, M5G 1V7, Canada.

⁶ Princess Margaret Cancer Centre, Toronto Medical Discovery Tower, 101 College Street, Room 15-310, Toronto, Ontario, M5G 1L7, Canada.

Forty volunteers were measured for skin color and skin-fold thickness at a standardized site near the elbow. Transmittance, reflectance and skin temperature were recorded for energy doses of 2, 6, 9, and 12 Joules using 635 nm (36 mW) and 808 nm (40 mW) wavelength laser diodes with irradiances within American National Standards Institute safety guidelines (4.88 mm diameter, 0.192 W/cm ² and 4.88 mm diameter, 0.214 W/cm ² , respectively). The key factors affecting reflectance to an important degree were skin color and wavelength. However, the skin color effects were different for the two wavelengths: reflectance decreased for darker skin with a greater decrease for red light than near infrared light. Transmittance was greater using 808 nm compared with 635 nm. However, the effect was partly lost when the skin was dark rather than light, and was increasingly lost as tissue thickness increased. Dose had an increasing effect on temperature (0.7-1.6°C across the 6, 9, and 12 J doses); any effects of wavelength, skin color, and tissue thickness were insignificant compared to dose effects. Subjects themselves were not aware of the increased skin temperature. Transmittance and reflectance changes as a function of energy were very small and likely of no clinical significance. Absorption did not change with higher energy doses and increasing temperature. Skin color and skin thickness affect transmittance and reflectance of laser light and must be accounted for when selecting energy dose to ensure therapeutic effectiveness at the target tissue. Skin heating appears not to be a concern when using 635 and 808 nm lasers at energy doses of up to 12 J and irradiance within American National Standards Institute standards. Photobiomodulation therapy should never exceed the American National Standards Institute recommendation for the maximum permissible exposure to the skin. Lasers Surg. Med. 50:291-301, 2018. © 2017 Wiley Periodicals, Inc. Joensen J, et al. Photomed Laser Surg. 2011 Mar;29(3):145-53. doi: 10.1089/pho.2010.2793. Epub 2011 Jan 10. Photomed Laser Surg. 2011. PMID: 21219241 Grandinétti Vdos S, et al. Lasers Med Sci. 2015 Jul;30(5):1575-81. doi: 10.1007/s10103-015-1755-0. Epub 2015 May 19. Lasers Med Sci. 2015. PMID: 25987340 Haslerud S, et al. Photomed Laser Surg. 2017 Oct;35(10):567-575. doi: 10.1089/pho.2017.4269. Epub 2017 Jun 30. Photomed Laser Surg. 2017. PMID: 28677985 Salehpour F, et al. Photobiomodul Photomed Laser Surg. 2019 Oct;37(10):581-595. doi: 10.1089/photob.2019.4676. Epub 2019 Sep 25. Photobiomodul Photomed Laser Surg. 2019. PMID: 31553265 Fekrazad R, et al. Photomed Laser Surg. 2016 Nov;34(11):533-542. doi: 10.1089/pho.2015.4029. Epub 2016 Apr 12. Photomed Laser Surg. 2016. PMID: 27070113 Plec EMRL, et al. Codas. 2024 Aug 2;36(5):e20230333. doi: 10.1590/2317-1782/20242023333en. eCollection 2024. Codas. 2024. PMID: 39109706 Free PMC article. Neto RPM, et al. J Biophotonics. 2024 May;17(5):e202300501. doi: 10.1002/jbio.202300501. Epub 2024 Jan 23. J Biophotonics. 2024. PMID: 38262071 Aldalawi AA, et al. J Lasers Med Sci. 2023 Aug 29;14:e28. doi: 10.34172/jlms.2023.28. eCollection 2023. J Lasers Med Sci. 2023. PMID: 37744010 Free PMC article. Colvin L, et al. Biosensors (Basel). 2023 Aug 4;13(8):788. doi: 10.3390/bios13080788. Biosensors (Basel). 2023. PMID: 37622875 Free PMC article. Shanshool AS, et al. Materials (Basel). 2022 Aug 18;15(16):5696. doi: 10.3390/ma15165696. Materials (Basel). 2022. PMID: 36013828 Free PMC article.