to measure the temperature of water in fish tanks. Fiber Bragg grating temperature sensors are used in nuclear power facilities to monitor reactor core temperatures and avoid the possibility of nuclear meltdowns. Nanothermometry Nanothermometry is an emergent research field dealing with the knowledge of temperature in the sub-micrometric scale. Conventional thermometers cannot measure the temperature of an object which is smaller than a micrometre, and new methods and materials have to be used. Nanothermometry is used in such cases. Nanothermometers are classified as luminescent thermometers (if they use light to measure temperature) and non-luminescent thermometers (systems where thermometric properties are not directly related to luminescence). Cryometer Thermometers used specifically for low temperatures. Medical Ear thermometers tend to be an infrared thermometer. Forehead thermometer is an example of a liquid crystal thermometer. Rectal and oral thermometers have typically been mercury but have since largely been superseded by NTC thermistors with a digital readout. Various thermometric techniques have been used throughout history such as the Galileo thermometer to thermal imaging. Medical thermometers such as mercury-in-glass thermometers, infrared thermometers, pill thermometers, and liquid crystal thermometers are used in health care settings to determine if individuals have a fever or are hypothermic. Food and food safety Thermometers are important in food safety, where food at temperatures within can be prone to potentially harmful levels of bacterial growth after several hours which could lead to foodborne illness. This includes monitoring refrigeration temperatures and maintaining temperatures in foods being served under heat lamps or hot water baths. Cooking thermometers are important for determining if a food is properly cooked. In particular meat thermometers are used to aid in cooking meat to a safe internal temperature while preventing over cooking. They are commonly found using either a bimetallic coil, or a thermocouple or thermistor with a digital readout. Candy thermometers are used to aid in achieving a specific water content in a sugar solution based on its boiling temperature. Environmental Indoor-outdoor thermometer Heat meter uses a thermometer to measure rate of heat flow. Thermostats have used bimetallic strips but digital thermistors have since become popular. Alcohol thermometers, infrared thermometers, mercury-in-glass thermometers, recording thermometers, thermistors, and Six's thermometers are used in meteorology and climatology in various levels of the atmosphere and oceans. Aircraft use thermometers and hygrometers to determine if atmospheric icing conditions exist along their flight path. These measurements are used to initialize weather forecast models. Thermometers