Thermometry based on the use of thermographic phosphors utilises the physical properties of the phosphor particles for assessing temperature. The phosphor particles used in thermometry are usually inorganic materials having a powder form. Fluorescence intensity ratio (FIR) technique is widely used for the temperature sensors based on phosphors doped with RE³⁺, providing high detection spatial resolution, precision and excellent sensitivity. Such a phosphor consists of a host material and a doping agent from which the light is emitted. Low temperature sensing using and NIR to visible upconversion (UC) emission could be very useful for biological applications. One of the important conditions an upconversion material should satisfy in order to be used as temperature sensor is that the radiative transitions from a fluorescent level should dominate its non-radiative transitions. This could be achieved only when the host has considerably low maximum phonon energy. This condition is also relevant to achieve UC, since a material which has low maximum phonon energy can be effectively used for RE³⁺ UC. These thermographic phosphors cover a wide range of temperatures, from cryogenic temperatures up to 1700 ºC or higher, making them suitable for many different applications. Each phosphor that is selected is highly sensitive within a specific range of temperatures, exhibiting accuracies in the order of 1-5 ºC. After excitation of the thermographic phosphor, the subsequent emission is imaged onto a detector. The temperature can then be deduced from the spectral or temporal properties of the recorded signal. This technique provides a two dimensional measurements and remote thermometry, as well as a high degree of accuracy. These advantages have allowed thermographic phosphors to be used in a wide variety of applications and in harsh environments. Many of the trivalent lanthanides such as Er³⁺, Tm³⁺, Ho³⁺, Nd³⁺, Dy³⁺ and Eu³⁺ have been used as PL centers of optical thermometry. But the Er³⁺ ion has proved itself to be an efficient UC centre among RE³⁺ ions and tremendous increase in its efficiency when combined with Yb³⁺ ions, acting as sensitizer, makes it the ultimate choice among UC ions. Apart of Er³⁺, Yb³⁺ combination, Tm³⁺ ions can also find their way for a better sensitivity depending on the host characteristics. In view of the above advantages of thermographic phosphors, structural, physical, morphological and temperature sensing characteristics of visible upconversion emission of different hosts namely SrLaAlO₃, BiNbO₄, LaNbO₄ and YNbO₄ doped with rare earth ions at low temperatures and excited by NIR excitation using FIR technique would be presented in detail.