Comparison of methods used for measuring the electrostatic field of video display terminals.

Comparison of methods used for measuring the electrostatic field of video display terminals. Scand J Work Environ Health 13 (1987) 255-257. The electrostatic field exposure of video display terminal (VDT) operators was measured with different methods. Using the Wilson plate method the field strength at different locations at the face of three human subjects working with a VDT was determined. The strongest fields were found at protruding facial features. Measurements were also made with a measuring instrument known as a field mill. Comparative measure ments were made with the field mill as such and combined with a metal plate or a hemisphere. The measured field strength was highly dependent on the method of measurement. A conducting hemisphere (diameter 0.20 m), used in combination with the field mill, was found to be a useful model of the human head in electrostatic measurements.

The operator of a video display terminal (VDT) is exposed to electrostatic fields. The field strength at the operator's position varies between different VDT models from zero to tens of kilovolts per meter (3,5,10,15). The biological significance of electrostatic fields is not known, but it has been assumed that it may be associated with the facial skin rashes of some VDT operators (I, 3,7,9,12,14).
Different methods have been used to mea sure the electro static fields of video display terminals. In two studies (4,6) the field strength was measured with a field mill positioned centrally in front of the screen. The field mill was grounded, and the field strength was noted. Another method is to place a large grounded metal plate parallel to the screen and to measure the field strength at the surface of this plat e (5) or thro ugh a hole in the plate made for the sensor of the field mill (10). Neither of these methods gives a realistic estimate of the electric field strength on the skin of the oper ator.
When a conducting object (such as a human body) is placed in the electric field of a terminal, the field is perturbed. The electric field strength at the surface of the conducting object depends strongly on the geometry of the object (5,11). When a longitudinal object such as a field mill is introduced into the field , a stronger field is measured than at the surface of a flat plate. The field strength around a human head is probably between these two cases. ' . -_._------head. In this report, electro static field strengths measured with this model are compared to those measured on the skin of VDT operators with the Wilson plate method . In order to facilitate comparison of the field strengths measured with different methods, also results obtained with the field mill as such and in combination with a flat metal plate are reported.

Methods
The electrostatic field strength was measured with an Eltex Q 475/C field mill. The calibration of the field mill was checked between two (0.25 x 0.25 m) metal plates. The sensor of the field mill was inserted through a hole in one of the plate s, and a high voltage of 2.5 kV was applied to the other plate. When the distance between the plates is small compared to the dimensions of the plates, the electric field is nearly homogeneous, and the field strength can be calculated as voltage per distance (8). The field mill was found to give correct readings . A hemisphere (diameter 0.20 m) made of steel grid was used as a model of the human head. A circular hole was cut at the center of the hemisphere for the sensor of the field mill. Both the hemisphere and the field mill were grounded during the measurements.
For compari son, measurement s were also made with the field mill in combination with a flat metal plate .
A 0.25 x O.25-m aluminum plate was used, and the sensor of the field mill was inserted through a circular hole at the center of the plate .
The Wilson plate (13) was used for measuring the electro static field at the skin surface of VDT operators. The Wilson plate is a simple instrument for measuring the field intensity on a grounded plane. It consists of an insulated metal plate connected to an electrometer. The plate is screened by a grounded plate and is grounded for a momen t. When the screen is removed, the reading of the electrometer (in coulombs) is proportional to the field intensity. We used a 30 x 30-mm copper plate made of printed circuit boar d connected to a Keithley Instruments Model 610C electro mete r. A larger plate (70 x 50 mm) with a handle was used for screening the measuring plate. This measurin g system was calibr ated in a homogeneous electrostatic field. Dur ing the measurem ents the VDT operator was grounded. Every measuremen t was repeated several times , and the average was calculated.
The field mill in combination with the hemisph ere was used for measurin g the electros tatic field of several VDT mod els. Th e measurements were made in usual office room s. The operato r was not present when these measur emen ts were performed . Durin g the measur ements the temper ature was between 18 and 22°C, and the relative humidity varied from 10 to 22 0,10.

Results
Th e measured electrostat ic field in front of a video display terminal is highly dependent on the method of are between these two extremities.
With the Wilson pla te the stro ngest fields were measured at protruding facial features (table 2). Still higher values would probably be measured at the tip of the nose, but the size of the Wilson plate used did not permit such measurements. The field mill app eared to give the most realistic estimate of the average electro stat ic field intensity on the face when it was used in combination with the hemisphere.
Th e electros tatic field measur ed with the field mill and the hemisphere varied from 0 to 40 kV1m at the distance of 0.30 m in fron t of vario us VDT models (table 3). As expected, these values are higher than tho se (0-30 kV1m ) measured at the surface of a flat metal plate (5, 10).

Discussion
The results of the measurements with the field mill demonstrate the importance of measuring geometry in estimating the electrostatic field expo sure of VDT operators. The relative difference between the thr ee method s of measur ement increases with increasing distance from the te rminal. Accordi ng to Pau lsson et al (10) the electrostatic field is propo rtional to r -I " close" to the screen and to r-3 at larger distan ces (r) from the screen. However, at the intermediate distances used in the present investigation, there is no simple relationship between field strength and distance. The exact relationship depends on the shape and size of the measuring system a nd the screen.
At present, not much is known of the biolo gical effect of electrosta tic fields. It has been assumed that the skin problems possibly associated with video display terminal s are due to enhanced aeroso l deposition on the operator's face because of the electrostatic field (4). In this case the exact value of the electrostatic field is perhaps not so important as the physicochemical properties of the aerosol particle s and possibly the elec- trostatic charge of the operator. In this study, stro nger field s were measured at the forehead and chin than at the cheek, which is the usual location of the reported skin problems of VDT operators.
In the case that exact measurement of the electrostatic field exposure is nece ssary, the Wilson plate is probably the most exact method. However, the measuring procedure is sensitive to disturbances, and very careful operations are neces sary to get reproducible results . A limitation of this method is that it can be used onl y on grounded surfaces, ie, the potential of the VDT operator must be zero. In reality, the potential of a sitting person can be some tens or hundreds of volts (or even thousands for short durations), positive or negative (4,6).
A realistic estimate of the electrostatic field str ength at a VDT operator's face can be obtained with a field mill equipped with a conducting hemisphere th at operates as a model of the human head. The measuring system can be brought to any electrostatic potential to simulate real exposure conditions. Th e field mill without the hemisphere gives readings that are too high. It could pos sibly be used to estimate the wor st case exposure, but different models of field mills would give different results. The field mill in combination with a flat metal plate gives readings that are too low in comparison to the values measured at the forehead and chin of VDT operators. However, if one wants to mea sure the field emission from a video display terminal (instead of the exposure of the operator), it may be useful to estimate the potential of the screen surface . One can make thi s estimation by using a flat metal plate at a close distance (in order to obtain a near homogeneous field) and calculating the approximate potential by multiplying the field strength by the distance (I) .
In conclusion , the field mill combined with a conducting hemisphere (diameter 0.20 m) appeared to give a relatively good estimate of the average electrostatic field str ength at the operator's face. Because of th e simplicity of the measurement, it could be used as a standa rd method in measuring the electrostatic field exposure of VDT operators.