Specific absorption rate
From Wikipedia, the free encyclopedia
Specific absorption rate (SAR) is a measure of the rate at which radio frequency (RF) energy is absorbed by the body when exposed to radio-frequency electromagnetic field. It is defined as the power absorbed per mass of tissue and has units of Watts per kilogram [1]. SAR is usually averaged either over the whole body, or over a small sample volume (typically 1g or 10g of tissue). The value cited is then the maximum level measured in the body part studied over the stated volume or mass. It can be calculated from the electric field within the tissue as:
where
- σ is the sample electrical conductivity
- |E| is the magnitude of the electric field
- ρ is the sample density
SAR is used to measure exposure to fields between 100kHz and 10 GHz[2]. It is commonly used to measure power absorbed from mobile phones and during MRI scans. The value will depend heavily on the geometry of the part of the body that is exposed to the RF energy, and on the exact location and geometry of the RF source. Thus tests must be made with each specific source, such as a mobile phone model, and at the intended position of use. For example, when measuring the SAR due to a mobile phone the phone is placed at the head in a talk position. The SAR value measured is then the value measured at the location that has the highest absorption rate in the entire head, which for a mobile phone often is as close to the phone as possible. Various governments have defined safety limits for exposure to RF energy produced by mobile devices that mainly exposes the head or a limb for the RF energy:
- United States: the FCC requires that phones sold have a SAR level at or below 1.6 watts per kilogram (W/kg) taken over a volume of 1 gram of tissue.
- European Union: the SAR limit is 2 W/kg, averaged over ten grams of tissue.
For whole body exposure there is a limit of 0.08 Watt/kg averaged over the whole body.[3]
[edit] See also
[edit] References
- ^ Jianming Jin (1998). Electromagnetic Analysis and Design in Magnetic Resonance Imaging. CRC Press, §5.3.3 pp. 226 ff. ISBN 978-0849396939.
- ^ http://www.icnirp.org/documents/emfgdl.pdf
- ^ http://www.icnirp.org/documents/emfgdl.pdf A whole-body average SAR of 0.4Wkg21 has therefore been chosen as the restriction that provides adequate protection for occupational exposure. An additional safety factor of 5 is introduced for exposure of the public, giving an average whole-body SAR limit of 0.08 W kg21.
[edit] External links
- FCC: Cellular Telephone Specific Absorption Rate
- World Health Organization: Electromagnetic fields and public health
- Cell Phone Radiation Levels, CNET article and comprehensive listings
- An Update on SAR Standards and the Basic Requirements for SAR Assessment
- Motorola's SAR Website
- Nokia's SAR Website
- Sony Ericcson's SAR Website
- Update Nokia full SAR list
