Far Field

The term “Far Field” generally refers to the range of objects located beyond the focal distance of a particular infrared device, up to an infinite distance. The smallest measuring spot is reached exactly at the focal length. This means that 90 % of the energy emitted by the objects is focused on this spot size. The Distance to Spot Size Ratio (D:S) describes this geometric behavior and is typically included in the datasheets of Pyrometers. A higher D:S ratio indicates better quality and more effort put into the optics of a unit. The D:S value is always at its peak at the focus distance of a device. As the spot size extends rapidly beyond the focus distance, the D:S ratio significantly decreases. The accompanying illustrations demonstrate that the D:S ratio reduces significantly in the Far Field as opposed to the focus distance for a specific device. For an accurate temperature reading, the object being measured should fill the spot size at the specified distance. This means that in the Far Field—beyond the focus distance— the targets have to get larger and larger the further away they are. This flexibility is in practice difficuilt to realize, because object size and distance might be fix. The customer has the possiblity to use advanced measurment devices with adjustable focus ranges