with high emissivity would be asphalt, with an emissivity coefficient of .98. A black body is a theoretical object with an emissivity of 1 that radiates thermal radiation characteristic of its contact temperature. That is, if the contact temperature of a thermally uniform black body radiator were , the black body would emit thermal radiation characteristic of . An ordinary object emits less infrared radiation than a theoretical black body. The fraction of its actual emission to the theoretical emission (of the black body) is its emissivity (or emissivity coefficient). In order to make a temperature measurement of an object using an infrared imager, it is necessary to estimate or determine the object's emissivity. For quick work, a thermographer may refer to an emissivity table for a given type of object, and enter that value into the imager. The imager would then calculate the object's contact temperature based on the value entered from the table and the object's emission of infrared radiation as detected by the imager. In order to get a more accurate temperature measurement, a thermographer may apply a standard material of known, high emissivity to the surface of the object. The standard material might be as complex as industrial emissivity spray produced specifically for the purpose, or as simple as standard black insulation tape, with an emissivity of about 0.97. The object's known temperature can then be measured using the standard emissivity. If desired, the object's actual emissivity (on a part of the object that is not covered by the standard material) can then be determined by adjusting the imager's setting to the known temperature. There are situations, however, when such an emissivity test is not possible due to dangerous or inaccessible conditions. In these situations, the thermographer must rely on tables. Difference from infrared film IR film is sensitive to infrared (IR) radiation in the range, while the range of thermography is approximately . So, for an IR film to work thermographically, it must be over or be reflecting infrared radiation from something that is at least that hot. Night vision infrared devices image in the near-infrared, just beyond the visual spectrum, and can see emitted or reflected near-infrared in complete visual darkness. However, again, these are not usually used for thermography due to the high temperature requirements, but are instead used with active near-IR sources. Starlight-type night vision devices generally only magnify ambient light. Passive