By effectively increasing the size of the thermometer bulb, the convection transfer coefficient is reduced and the effect of radiation is proportionally increased. The measurement is affected by air movement because the measured GT depends on both convection and radiation transfer. M R T 4 = T 1 4 F p − 1 + T 2 4 F p − 2 +. Because the sum of the angle factors is unity, the fourth power of MRT equals the mean value of the surrounding surface temperatures to the fourth power, weighted by the respective angle factors. Most building materials have a high emittance ε, so all surfaces in the room can be assumed to be black. Therefore, it is necessary to measure those temperatures and the angle factors between the person and the surrounding surfaces. Since the amount of radiant heat lost or received by human body is the algebraic sum of all radiant fluxes exchanged by its exposed parts with the surrounding sources, MRT can be calculated from the measured temperature of surrounding walls and surfaces and their positions with respect to the person. There are different ways to estimate the mean radiant temperature, either applying its definition and using equations to calculate it, or measuring it with particular thermometers or sensors. This is done with effective design of the building, interior and with the use of high temperature radiant cooling and low temperature radiant heating. Maintaining a balance between the operative temperature and the mean radiant temperature can create a more comfortable space. The operative temperature, which is a more functional measure of thermal comfort in a building, is calculated from air temperature, mean radiant temperature and air speed. The MRT is controlled by enclosure performances. What we experience and feel relating to thermal comfort in a building is related to the influence of both the air temperature and the temperature of surfaces in that space, represented by the mean radiant temperature. MRT also has a strong influence on thermophysiological comfort indexes such as physiological equivalent temperature (PET) or predicted mean vote (PMV). This is meaningful as long as the temperature differences of the objects are small compared to their absolute temperatures, allowing linearization of the Stefan-Boltzmann Law in the relevant temperature range. The MRT is simply the area weighted mean temperature of all the objects surrounding the body.
MRT is a useful concept as the net exchange of radiant energy between two objects is approximately proportional to the product of their temperature difference multiplied by their emissivity (ability to emit and absorb heat). The mean radiant temperature ( MRT) is defined as the uniform temperature of an imaginary enclosure in which the radiant heat transfer from the human body is equal to the radiant heat transfer in the actual non-uniform enclosure.