Measuring Magnetic Fields
Magnetic field assessment involves two distinctly different measurement protocols used for two different purposes. The most common is the measurement of the maximum field strength, used to locate the source of a field, determine its direction and its strength. Typically, power line measurements or general background measurements are of this type. The other is concerned with exposure and is usually associated with the measurement of a particular appliance, like a computer monitor. It is impossible to avoid exposure to low frequency magnetic fields in modern society - there is always some level of background radiation present. This background radiation could be coming from a number of sources and must be taken into consideration when you are attempting to measure fields from a particular appliance or source. Before any assessment of the emissions from a computer monitor or other appliance is possible, it is necessary to first determine the strength of the background field. In some cases, that field will be greater than that which is coming from the monitor, making measurements of the monitor's emissions impossible. In order to determine background radiation levels, turn off the computer monitor to be measured and take a set of readings of the area around the unit. If the background radiation is high, (say, several milliGauss), the contribution of the monitor to the environment may not be measurable.

Fields and Measurement Devices are Directional
Differences among magnetic field meters can be great, and one should know a meter's capabilities and limitations before using it for test and measurement purposes. A well-designed meter will not only tell you the strength of the field, it will also tell you from which direction it is coming.

The meter should measure fields in one direction (or plane) at a time and display the maximum field strength at that location and at that angle. But a person in that location will be exposed to fields coming from all angles simultaneously.

To determine the maximum field strength at a particular location, rotate the meter through all possible angles so that the field which is present can intersect with the sensor in such a way as to display the maximum reading. To determine exposure at that location, take a maximum field strength measurement in three planes (x, y and z) and extract the square root of the sum of the squares of the individual readings. The resulting RMS value (root-mean-square) is part of the MPR II measurement protocol for emissions testing. In order for comparisons to be valid, "before-and-after" measurements must be made without changing the distance between the meter and the source, while maintaining the same angle.

Generalizations about emissions characteristics of specific monitors cannot be made based on the measurement of one, or even several, monitors. Component materials, their placement and other elements affect field levels and even slight variations in these factors will change emissions characteristics from one monitor to the next, even when comparing the same brand and model. While it is generally true that color, high-resolution and large-format displays emit stronger fields, a monitor must be measured directly in order to determine its specific emission levels.

Measuring Electric Fields
Electric fields are the product of and exist between objects which are at different electric potentials, or voltages. An example might be a battery which has a voltage rating of 12 volts. If the terminals of the battery are attached to metal plates at a given distance apart (say, one meter), an electric field would exist between them. To calculate the strength of the electric field between two such objects, divide the potential difference (in volts) by the distance (in meters). The quotient, or answer, is expressed in volts per meter (v/m). In the above example, a field strength of 12 v/m would exist between the plates.

When the voltage difference is constant, the field is said to be static and is called a static electric field. It is important to note that static electric fields are quite common and can be relatively high. Static fields in excess of several thousand volts per meter can be measured regularly in front of a computer monitor. (Static fields can cause skin irritation, dryness of the eyes -- especially for contact lens wearers, and eye irritation.) When the voltage between objects changes over time, the field is called a time-varying electric field, alternating electric field, or an alternating current (AC) field. The rate at which the field changes, or alternates, is called its frequency, expressed in Hertz (Hz) or cycles per second.

The measurement and calculation of such fields is more complex. Assuming the distance between the objects remains constant, the objective of a measuring instrument is to determine the average field strength at the specified distance. Although several methods are available, the usual method accepted by international test protocols is the RMS (root-mean-square) average -- taking a maximum field strength reading in three planes and extracting the square root of the sum of the squares of the individual readings. If the field is alternating or oscillating at a single frequency (60 Hz as is theoretically the case with power lines), an electric field meter can be set such that it has maximum sensitivity at these power frequencies.

If, as in the case of monitors and most electrical appliances, the radiated fields (E-field radiation) are composed of a number of frequencies, there needs to be a limitation of the frequencies which will be "seen" by the meter. The range of frequencies which are allowed to be represented in the RMS average is called the bandwidth of the instrument and is expressed as a range of frequencies (for example, 5 Hz - 2,000 Hz). When choosing an electric field meter, be certain that the instrument is designed to read the specific frequencies to be measured.