2.3.2. Skin Emissivity

Bodies in nature exhibit very different behavior from black bodies and emit only a reduced amount of energy compared to the maximum one. As a consequence, they reflect a certain amount of the energy incident on the surface itself in a complementary way to that emitted (according to Kirchhoff’s law). For this reason, it is necessary to avoid performing measurements in the presence of direct solar radiation on the measurand surface.

To quantify the effect of the surface emissivity, manufacturers of clinical thermometers directly set the emissivity value ε (i.e., the ratio between the energy emitted by the real body and that of the black body at a given temperature) in the instrument and correct the value of the reflected energy, generally considering this is equal to that produced by an environment at 20 °C. However, the monochromatic emissivity trend of real bodies at a fixed temperature should be very different (see Figure 5) due to the constituting material, the surface finish and the wavelength. In fact, total emissivity is not constant with the temperature surface.

Monochromatic emissivity trend of human skin and other surfaces.

Human skin shows a very high far infrared emissivity (regardless of skin color) of approximately 0.98 [6], but numerous parameters can influence its value (e.g., sweat, make-up, lotions, scars, porosity of the skin, hair, etc.) [63]. The emitting properties of the skin are mainly related to melanin which absorbs in the ultraviolet region and also determines the color of the skin. This latter presents a wide absorption and emission band of around 0.275 µm due to aromatic chromatophores. At higher wavelengths (in the visible and near-infrared), absorption peaks at 0.76, 1.00, 1.20, 1.45 and 2.00 µm (due to the presence of water) are present. In addition, other substances present on the epidermis, including keratin, collagen, fats, melanin, make-up and water, affect the skin’s absorption/emission spectrum.

Skin emissivity depends on its humidity. In fact, water has a far infrared emissivity of 0.96, while that of oils and fats is about 0.82. Therefore, in the infrared range, skin emissivity decreases as humidity increases, and the minimum value corresponds to skin completely covered in sweat. The emissivity of clay and earthy pigments, on the other hand, ranges between 0.93 and 0.95. For this reason, the noncontact temperature measurement should always be carried out in a spot of clean and dry (i.e., sweat-free) skin, possibly avoiding the presence of hair, wounds or scars, cosmetics or other products that could greatly affect the measurement. Fever of nearly 1–2 °C could be masked by applying common cosmetics containing solid particles to the human forehead [64].

Figure 6 shows the effect of the skin emissivity variation on the measured temperature.

Error trend depending on skin emissivity.