, Maneli Mozaffarieh1 and Hans Bebie2
(1)
Department of Ophthalmology, University of Basel, Basel, Switzerland
(2)
Institute for Theoretical Physics, University of Bern, Bern, Switzerland
Abstract
The standard international (SI) units are summarized in Table 20.1. Alternate units frequently used in medicine are indicated in subsequent comments. Often, units are given with a prefix, such as 1 mJ = 1 milliJoule = 10–3 J. Metric prefixes are listed in Table 20.2. Photometric units are dealt with in a separate section (Sect. 20.2).
20.1 Some Physical Units
The standard international (SI) units are summarized in Table 20.1. Alternate units frequently used in medicine are indicated in subsequent comments. Often, units are given with a prefix, such as 1 mJ = 1 milliJoule = 10–3 J. Metric prefixes are listed in Table 20.2. Photometric units are dealt with in a separate section (Sect. 20.2).
Table 20.1
Some standard international units
Length | meter (m) |
Time | second (s) |
Mass | kilogram (kg) |
Force | newton (N) |
Energy | joule (J) |
Power | watt (W) |
Pressure | pascal (Pa) |
Frequency | hertz (Hz) |
Temperature | kelvin (K) |
Table 20.2
Metric prefixes
peta | tera | giga | mega | kilo | deci | centi | milli | micro | nano | pico | femto | atto |
P | T | G | M | k | d | c | m | μ | n | p | f | a |
1015 | 1012 | 109 | 106 | 103 | 10−1 | 10−2 | 10−3 | 10−6 | 10−9 | 10−12 | 10−15 | 10−18 |
20.1.1 Length
Wavelengths are often given in μm (micrometer, 10−6 m), in nm (nanometer, 10−9 m), or in Å (angstrom, 10−10 m).
20.1.2 Frequency
A frequency denotes a number of events per unit time or the number of cycles of an oscillation per unit time. Unit: 1 Hz = 1 s−1.
20.1.3 Mass
1 g = 10−3 kg.
20.1.4 Force
1 N is the force required to accelerate 1 kg of mass at 1 m/s2. A mass of 0.102 kg has a weight of 1 N (on earth).
20.1.5 Energy
The unit calorie (cal) is sometimes used for heat energy. Formerly, the calorie was defined as the quantity of heat required to raise the temperature of 1 g of water by 1 °C. The conversion into standard units is 1 cal = 4.18 J.
1 electronvolt (1 eV = 1.6 ∙ 10−19 J) is frequently used in atomic/molecular physics and chemistry. It is the energy required to move an electron (or any elementary electric charge) across an electric potential difference of 1 volt (1 V). Photons of light in the visible range have energies between 1.6 and 3 eV.
Energy density (or fluence) is given in units of J/m2 or, in laser applications, often in J/cm2. In typical applications, it is the energy delivered to an absorbing area divided by the area. Example: An energy of 1 mJ absorbed by a spot of 1 mm2 corresponds to a fluence of 10−3 J/10−6 m2 = 103 J/m2 = 0.1 J/cm2.
20.1.6 Power
Power is energy per unit time. An energy of 1 J delivered within 1 s corresponds to a power of 1 W. Very high powers can be achieved by the delivery of moderate energy within a short pulse, such as a laser pulse of energy of 1 mJ and a duration of 1 ns, which corresponds to 1 MW.
Irradiance (power density) is power per area, such as the power of a laser beam divided by its cross-section. Standard unit: 1 W/m2. Example: A laser pointer of 1 mW power with a cross-section of 1 mm2 has an irradiance of 103 W/m2. Sunlight (outside the atmosphere) has an irradiance of 1300 W/m2.
20.1.7 Pressure
The standard unit 1 Pa is a very small pressure, defined by a force of 1 N acting on an area of 1 m2. Frequently used units are:
1 bar = 105 Pa; about equal to the atmospheric pressure on the earth at sea level.< div class='tao-gold-member'>Only gold members can continue reading. Log In or Register to continue
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