Whether local or general effects result from infrared radiation depends upon

• A. Wavelength of radiation
• B. Frequency of radiation
• C. Intensity of radiation
• D. Not a factor

Answer: (C)

The amount of energy delivered per unit area by infrared radiation determines whether the heating stays focused on a small area or spreads to a larger region. When intensity is low, the energy absorbed by the skin and superficial tissues is enough to produce local heating, increasing blood flow and warmth in that localized spot. As intensity rises, more energy reaches deeper tissues and a larger area can be warmed, potentially producing a broader or generalized heating effect as circulation distributes the heat. Wavelength and frequency can influence how deeply infrared can penetrate, but the key factor that decides local versus general effects is how much energy is delivered per unit area—i.e., the intensity. To target a small area, use lower intensity; for broader warmth, higher intensity or exposure over a larger area.