The penetration of infrared radiation into skin tissue depends upon

• A. Wavelength
• B. Source of the radiation
• C. Thickness of glass in the heat lamp
• D. Wattage of the lamp

Answer: (A)

Penetration depth is determined by the wavelength of the infrared light because tissue interacts with light differently at different wavelengths. The skin contains water, melanin, and hemoglobin that absorb IR energy to varying extents depending on the wavelength. Some wavelengths face less absorption and scatter less in the superficial layers, allowing the light to reach deeper tissues; other wavelengths are absorbed quickly near the surface. This makes wavelength the key factor setting how deep the energy can go.

The amount of energy delivered at the surface (wattage) can influence how much heating occurs, but not how deep the energy penetrates. Likewise, the thickness of glass can alter which wavelengths reach the skin by filtering part of the spectrum, but it doesn’t determine the inherent depth of penetration—that’s set by the wavelength itself.