GL-01 — Guidelines for the Measurement of Radio Frequency Fields at Frequencies from 3 KHz to 300 GHz

3. Instrumentation

3.1 Types of Measuring Instruments

A typical RF field or power density measurement device is composed of a probe, leads and metering instrumentation. The probe is used to detect the field. It can either be a conventional antenna or another type of sensor. The performance and the application of the measuring instrument as a whole depend to a large extent on the design and characteristics of the probe. The detected signal is carried by the leads to the metering instrumentation. To reduce the coupling of the leads with the surrounding field in order to minimize any disturbance, the leads take the form of high resistance wires. The metering instrumentation is primarily designed to process and display received field density.

The RF measurement device may be either broadband or narrowband. A broadband device responds uniformly over a wide frequency range and requires no tuning. A narrowband device may also operate over a wide frequency range, but the reception bandwidth is narrow, and the device must be tuned to the frequency of interest. Narrowband and wideband devices have their own advantages and disadvantages depending on the spectral environment and the type of measurements that are projected.

3.1.1 Electric and Magnetic Field Strength Meters

Electric and magnetic field strength meters are narrowband devices. They consist of an antenna, cable(s) to carry the signal from the antenna, and a signal conditioning/readout instrument. Field strength meters may use linear antennas, such as monopoles, dipoles, loops, biconical or conical log spiral antennas, horns or parabolic reflectors. The appropriate field parameters can be determined from a measurement of voltage or power at the selected frequency and at the antenna terminal. The electric (or magnetic) field strength can be derived from information on the antenna gain or antenna factor and the loss in the connecting cable.

3.1.2 Spectrum Analyzers

Spectrum analyzers are essentially broadband tunable receivers whose reception bandwidth may be set over a wide range of frequencies. They are used to measure the power at the antenna terminal at the selected frequency(ies). If used in combination with a narrowband selective antenna, the overall device becomes in concept similar to a field strength meter. However, spectrum analyzers can also be connected to relatively short antennas to produce a broad response over a given frequency range. In this case, the analyzer will display the spectrum of ambient signals and thus will permit to ascertain the frequencies involved and their relative contribution to the overall power density.

3.1.3 Power Density Meters

Power density meters are generally isotropic and broadband devices. However, there are conceptual differences among these devices in the way the fields are detected and processed. The instruments described in the following sections have essentially the same basic components (i.e. a probe, a connecting cable and a conditioning display unit). They are limited to those types which are currently available and which can provide reasonable accuracy in both near field and far field situations.

Measurements conducted with a power density meter may produce erroneous readings when the connecting cables are inadvertently aligned with the electric field. This is due to the fact that high resistance leads carrying the signal act as a more efficient antenna at low frequencies (such as in AM broadcasting band) than the short dipoles in the probe.

3.1.3.1 Diode Rectifiers

Multiple diodes and antenna elements (short dipoles or loops) are arranged in a suitable configuration to sum all three spatial field components independently of polarization and direction of incidence. Three elements, in an orthogonal arrangement, is required for an isotropic instrument which can be used in any orientation with respect to the field. Dipoles respond to the electric field, loops to the magnetic field. To achieve a uniform response over the desired frequency range, the size of the dipole or the loop must be small compared to the wavelength of the highest frequency to be measured.

Schottky diodes, in general, exhibit some photovoltaic effect. Beamlead hybrid types exhibit this effect to a much greater extent and may produce erroneous readings when illuminated by sunlight or strong incandescent light. Therefore, optically opaque shielding is required to eliminate this effect.

Diode instruments are non-linear with respect to field strength. At low levels, the rectified voltage is proportional to the square of E (or H). At higher levels, the rectified voltage becomes directly proportional to E (or H). This change in characteristic requires that the range of operation of the diode be restricted to low levels to provide a true indication of |E|² or |H|² . When the diodes are operated at higher levels, it is required that the output voltages of the individual elements be modified (generally squared) prior to their summation. When diode instruments are used in pulsed fields they usually change from an average to a peak detecting device, hence, measurement errors may be large in fields of high peak to average ratio.

When adapted to broadband operation, the upper frequency range of a diode- based instrument is currently above 12 GHz. The low-frequency limit is below 400 kHz. The burnout characteristics can be in the hundreds of mW/cm² range.

Diode detectors, depending on design, may be temperature sensitive. Variations in output with ambient temperature will typically be less than 0.05 dB per °C. Diode units also may be modulation sensitive if the square-law region is exceeded, resulting in errors dependent upon the type of modulation.

3.1.3.2 Active Antenna

It is difficult to make accurate, broadband E and H field probes that cover the long wavelength (1000 m) region, using the conventional means described above. In order to provide a flat frequency response and adequate sensitivity in a dipole probe, the load impedance of the detector and the high impedance lead in combination must be greater than the antenna (source) impedance. One solution is to provide a high impedance RF buffer amplifier that is connected directly to a monopole or loop antenna and which acts as the load. This is practical for frequencies between 10 kHz and several hundred MHz. Commercially available magnetic and electric field probes, using active electronics, operate at frequencies as low as 60 Hz.

A second problem associated with probes without active electronics is that of isolating the signal carrying leads from the antenna/detector combination. This problem may become severe below about 100 MHz, and particularly below 10 MHz. This is due to the fact that the typical high resistance signal carrying leads serve as a low-pass filter, and their ability to separate the low frequency detected signal from the RF field being measured becomes more difficult as the two frequencies approach each other. This results in excessive sensitivity and poor antenna patterns in passive probes.

Finally, at frequencies above about 300 MHz, where "free-space" or uniform irradiation conditions exist, both the sensor and the metal enclosure of the survey instrument can be exposed to similar levels of RF, and scattering from the enclosure to the sensor (probe) can cause significant errors.

Active electronic probes eliminate the use of such leads entirely by including the visual display (readout) with the metal box containing the active electronics. A fibre optic data link can be provided for a remote readout.

3.1.3.3 Displacement-Current Sensors

In addition to short dipoles and monopoles, a form of parallel plate capacitor, called a displacement-current sensor can be used to measure electric fields normal to its surface or normal to any large conducting surface. Instruments designed primarily for measuring fields associated with video display terminals, are based on the displacement-current sensor concept.

Displacement-current sensors are typically used at frequencies in the LF and VLF regions (e.g. from DC to a few hundred kHz) but may effectively be used at frequencies as high as a few hundred MHz.

3.1.3.4 Electro-Optical (Photonic) Sensors

This type of electromagnetic field sensor utilizes a non-metallic, passive sensing element (electro-optic modulator) with a very broadband response (DC to 20 GHz) that converts electromagnetic field strength information to instantaneous modulation of a laser beam. The laser energy is transmitted via fibre optics to the modulator.

The modulator impresses amplitude modulation on the laser beam, in proportion to the instantaneous amplitude of the RF electromagnetic field to which the modulator is exposed. The amplitude-modulated laser beam is then carried from the modulator to a photodetector that converts the modulated optical beam to an electrical signal that represents the instantaneous amplitude of the RF field strength. This signal is then detected and processed before being sent to for display.

The above system has been used with electrically small dipoles as an electric field sensor as well as with no antenna (where the electro-optic modulator itself serves as the E field sensor). In addition, conventional antennas can be connected to a commercially available electro-optic modulator via a short lead, to provide a non-metallic, passive RF link to the antenna.

3.1.3.5 Thermocouples

The detection elements are thin-film type thermocouples. Parts of the film perform the functions of the antenna element. Some low frequency probes also use loop antennas terminated with thermocouple detectors. The DC output of the thermocouple is proportional to the square of the electric field strength. The major limitation of the thermocouple type radiation monitors is the burnout characteristic. The burnout characteristic is typically 3 times full scale in terms of average values. Newer designs of thermocouple instruments have burnout ratings of 15 to 20 times full scale. Thin resistive films provide very broad bandwidth.

3.1.4 Shaped Frequency Response

Safety limits for field strength and power density in Canada are frequency dependent. Probe designs, which rely on dipole-diode elements separately or in conjunction with thermocouple elements, may be designed to have a "sensitivity versus frequency" characteristic that is the inverse of the standard. This allows the summation and weighing of multiple frequency signals in conformity with the frequency dependent safety limits. The readout of such devices is in % of the standard. The probes may be tailored to a specific American National Standards Institute (ANSI) or Canadian Standard.

Shaped frequency response probes may cover only a portion of the frequency range of the standard. Additional probes may be used to complement each other and provide a wider measurement range. When complementary probes are used, these should exhibit good rejection of out-of-band signals.

3.1.5 Combined Electric and Magnetic Field Probes

Devices described in Sections 3.1.3.1 to 3.1.3.5 employ separate probes to measure the electric and magnetic field components. In the near field region of an RF source, the relative values of the electric and magnetic fields vary considerably with respect to one another, depending on the distance from the source.

Also, in typical situations, fields may vary rapidly with time. To measure both electric and magnetic field strengths, which vary over time and space, one must place an electric field probe and then a magnetic field probe at exactly the same point. However a measurement uncertainty results, since the field under study may change during the finite time that elapses between the successive measurements.

A broadband isotropic probe system to measure the electric and magnetic fields simultaneously can be produced with a set of three mutually orthogonal dipole elements and a set of three mutually orthogonal loops that are physically located within a very small (compared with the shortest wavelength) volume. These elements are described in Section 3.1.3.1. Since the lengths of the dipoles, or the diameter of the loops, are kept small for uniform frequency response, the electric field pickup will be negligible. Thus, the mutual coupling between any of the probe elements is minimized by the use of electrically small antennas. Detectors based on the use of square law operated diodes or thermocouple are used to provide a signal to the electronic circuits to perform summing, data processing or conversion.

3.1.6 Induced Current Meters

Induced current meters display the amount of current induced through the body to ground when an individual is standing in an electric field created by a high-power transmitter. These currents can provide an indication of energy absorbed by the body.

Induced current meters are generally stand-on devices that measure the induced current flowing through the subject's feet to ground. The stand-on baseplate is made of two stainless steel plates and is, in fact a capacitor/resistor network. The meter reads the current flowing through the resistor connected between the capacitor plates. The size of the baseplate is kept small to minimize any pick-up of electric field from the sides of the baseplate.

There are also the clamp-on induced current meters that can measure directly the induced current in arms and legs using clamp-on sensors. Typical frequency range of commercial meters is from 10 kHz to 100 MHz.

3.1.7 Contact Current Meters

Contact current meters display the amount of current through the body caused by contact with a 'hot' metallic object located in the vicinity of a high-power transmitter.

Contact current meters generally feature an insulated contact probe for contact with the 'hot' object. Together with a stainless steel baseplate and internal circuitries, the measured current simulates the equivalent induced current by a barefoot individual gripping the 'hot' metallic object. Typical frequency range of this type of meter is from 3 kHz to 30 MHz.

3.2 Desirable Performance Characteristics

There are certain characteristics, which are desirable in a survey instrument. They can be arranged in two classes: physical characteristics and electrical performance characteristics.

3.2.1 Physical Characteristics

3.2.1.1 Portability

The instrument should be lightweight and small to permit convenient operation under restrictive conditions. The weight should be kept as low as is practical in keeping with good engineering practice. The volume should be convenient for hand held operation.

3.2.1.2 Durability

The display and other components of the device should be durable and able to withstand shock and vibration associated with transportation and handling under difficult conditions. A storage case should be provided.

3.2.1.3 Effects of Temperature, Humidity and Pressure

The accuracy of the device should be specified in terms of effects of temperature, humidity and atmospheric pressure. The extent of the effect of these parameters should be taken into account.

3.2.1.4 Display

The markings should be large enough to be easily read at arm's length. For shaped frequency response probes with an the analog type readout, the applicable safety standard should appear within the central one-third of the full scale reading of the dial. If more than one range of sensitivity is provided, the full-scale value of the selected range should be indicated. In any case, the analog or digital readout should provide a clear indication of the units being displayed.

3.2.1.5 Adjustments

The device should have a minimum number of controls. The functions associated with the controls should be clearly labelled and the operating procedures relatively simple.

3.2.1.6 Simplicity

Complicated operating procedures should be avoided. The information provided in the instruction manual should be sufficient to make accurate measurements.

3.2.2 Electrical Performance Characteristics

3.2.2.1 Power Supply

The instrument should be battery operated. The battery should be easily replaceable or rechargeable. A test switch or some other means should be provided to indicate their condition. The instrument should be capable of operating within its rated accuracy for at least eight hours before replacement or recharging of the batteries becomes necessary.

3.2.2.2 Polarization Factor

Probe antennas based on multiple dipoles or loops will respond to all polarization components of the electromagnetic field. A device based on a single antenna may respond to the same field by physical rotation of the single antenna about its axis.

3.2.2.3 Display Units

The instrument should indicate one or more of the following parameters: 

1. average "equivalent plane-wave" power density in milliwatts per square centimetre (mW/cm²)
2. mean-squared electric field strength in volts squared per metre squared (V²/m²);
3. mean-squared magnetic field strength in amperes squared per metre squared (A²/m²)

Some instruments display "equivalent plane-wave" power density as derived from the field quantities (E field and H field) being measured.

Instruments with a shaped-frequency response should indicate in terms of "percent of exposure limit" based on the appropriate safety standard.

3.2.2.4 Frequency Range

The manufacturer should specify the frequency range of the device. The dynamic range for flat frequency response probes should be at least 10 dB below the lowest value and 5 dB above the highest value of the safety standard. These limits should also apply to shaped-frequency response instruments.

3.2.2.5 Coupling and Response to Other Radiations

The probe should only respond to the field component being measured (i.e. a dipole antenna should respond to the electric field and should not interact with the magnetic field and vice versa).

The specified accuracy of the instrument should take into account effects such as ionizing radiation, artificial light, sunlight, etc.

3.2.2.6 Shielding

The housing of the instrument and antenna cables should be designed to reduce or eliminate electromagnetic interference. The shielding should be effective under conditions in which the maximum coupling or "pickup" occurs for the unintentional receiving elements.

3.2.2.7 Out-of-Band Response

The manufacturer should specify the out-of-band response characteristics of the instrument to assist the user in selecting an instrument for a particular application.

3.2.2.8 Modulation

The device should indicate RMS value of the electromagnetic fields. However, the device may also be equipped with a switch for CW and amplitude-modulated continuous wave (AM-CW) modes. The device should also be able to average the narrowest pulse-modulated envelope of a non-continuous wave field that is expected to be encountered by the surveyor.

3.2.2.9 Static Electricity

Static charges are often induced on the probe of the survey instrument. The device should not indicate false levels due to a response to static charges. Windy or dry conditions may also influence the reading of the survey instrument.

3.2.2.10 Recorder Output

It may be desirable to equip the instrument with a recorder output. This will enable the measurement of hazardous fields without endangering the operator. It will also facilitate spatial and time averaging.

3.2.2.11 Response Time

The response time is generally defined as the time required for the instrument to reach 90% of its final value when exposed to step function CW RF energy. The user should be made aware of the response time of the instrument.

3.2.2.12 Special Features

The following is a list of options that can be provided with the instrument: 

1. A "peak-hold" circuit. This is useful when the amplitude of the field is changing during the measurement;
2. An alarm or test switch to indicate that the preset level has been exceeded. Also a means should be provided to alert the user that the measured signal is overloading the instrument;
3. A data-logging function, which can provide an average, maximum and minimum values of the field components being measured. This function could provide a real-time average of the measured fields with an averaging time specified by the user (e.g. six minutes).

3.2.2.13 Stability

The instrument should be able to operate continuously for 10 to 30 minutes without the need to reset the device. Automatic electronic reset circuitry can be used to avoid the necessary shielding of the sensitive probe from ambient RF fields during the reset process. This is a desirable feature, particularly when performing RF surveys in situations where broadcasting or other major communication towers are involved. In such environments, RF-free locations may not be available. The instrument should not be sensitive to thermal variations within the range of normally encountered temperature extremes. The manufacturer should specify the maximum zero drift for each range.

3.2.2.14 Measurement Uncertainties

The accuracy of the measured field strength levels can be affected by the uncertainties of the actual measurement and the uncertainties of the instrument used to perform the measurement (see Ref. [13]). Actual measurement uncertainties can be minimized by following proper measurement procedures and instrumentation uncertainties can be reduced by correct calibration and careful selection of the instrument. To reduce instrumentation uncertainties, additional procedures may be taken (e.g. conducting single frequency measurements). To increase the confidence level of compliance to the safety standards, the sample size of measurement points (locations) of a site may be increased to provide a better indication of field strength values at the site. In any case, the survey report should include the specifications of the instrument as provided by the manufacturer. The instrument specifications should also address the instrument's ability to respond to amplitude-modulated (AM) fields such as pulsed radar signals as well as a multiplicity of signals, which might simultaneously illuminate the probe. The instrument readout should permit resolution of the measured field strength to within at least 5% of the full-scale value. Appendix 5 provides further information on the issue of measurement uncertainties.

3.3 Calibration

To ensure the safety of personnel, compliance with safety guidelines and to provide a basis for comparing the results of RF hazard, it is recommended that calibration be performed on instruments used for measuring various RF fields.

Existing calibration methods are based on the premise that a known field strength can be established through measurements, calculation, or a combination of both. The device to be calibrated is placed in this standard field and the meter indication is compared with the known field value. There are three basic approaches for producing a standard calibrating field: the free-space standard field method, guided-wave method and the TEM Cell method. The selection of technique is dictated by the nature of the probe under investigation, the frequency range, the accuracy required and the available hardware for calibration.

3.3.1 Methods

3.3.1.1 Free-space Standard Field Method

The objective is to establish a reliable and known calibration field by the free-space method. In most experimental setups, a microwave transmitter is employed to generate the reference field. The power density at a point is related to the power delivered to the transmitting antenna, the effective gain of the antenna on the on-axis distance of that point from the antenna.

It is to be noted that this arrangement is valid only when the device being calibrated is sufficiently small and far enough away from the transmitting antenna so that the amount of energy reflected back into the transmitting system is insignificant.

It is recommended that calibration should only be performed by qualified personnel in a laboratory furnished with proper equipment, and for field measurements, the calibration of the measuring instruments should be verified at the site using a portable TEM cell.

The main sources of error in the free-space method are multi-path interference from components that are part of the experimental setup and uncertainties in the antenna gain determination. Errors may be also be caused by misalignment of the transmitter antenna axis to the measuring probe.

3.3.1.2 Rectangular Waveguides

Rectangular waveguides will provide sufficiently uniform fields to be considered for calibration purpose. These fields are also predictable. The probe to be calibrated is inserted into the waveguide through a hole in the sidewall and positioned in the centre of the guide where the field is nearly uniform. The access hole is kept as small as possible to minimize its effect on the field distribution. The equivalent power density at the centre of the waveguide can be determined in terms of the square of the electric field.

When compared to the free-space standard field calibration method, the rectangular waveguide takes considerably less space and requires little electromagnetic power. The disadvantage is that the maximum transverse dimension of the waveguide must be less than the wavelength at the highest calibration frequency to avoid higher order modes that result in complicated field distributions. Hence, this method is generally useful only for frequencies below 2.6 GHz.

3.3.1.3 Calibration using TEM Cells

The transverse electromagnetic cell, commonly known as TEM cell is another guided wave method for calibrating electromagnetic field probes. The basic TEM cell is a section of two-conductor transmission line operating in the transverse electromagnetic mode, hence the name. The main body consists of a rectangular outer conductor and a flat central conductor located midway between the top and bottom walls. The dimensions and the tapered ends of the TEM cell are chosen to provide a standard 50 Ω characteristic impedance along the entire length of the cell. In the centre of the calibration zone, halfway between the centre conductor and the top or bottom wall of the cell, the electric field is vertically polarized and uniform. The wave impedance (E/H) will be close to the free-space value of 377Ω.

TEM cells may be made in various sizes to accommodate particular needs and frequency ranges. However, since the width must be less than a half-wavelength to avoid higher order modes in the cell, the useful upper frequency of a TEM cell is approximately 500 MHz. There are several factors that need to be considered in more detail when designing or using a TEM cell, IEEE Standard (C95.3) entitled "Recommended Practice for the Measurement of Potentially Hazardous Electromagnetic Fields, RF and Microwave" provides valuable and detailed information on electrical characteristics, standing waves, size of the probe to be calibrated with respect to plate separation, etc.

3.3.1.4 Magnetic Field Generators

At low frequencies the axial magnetic field (in A/m) at the centre of a circular loop wire is simply the current (in amperes) divided by the loop diameter (in meters). For a single-turn coil in free space, a loop becomes self-resonant when the circumference approaches the free-space wavelength. For multi-turn coils, the resonant frequency is lower because of capacitance between the turns. Using a coil with a total wire length less than λ/10, the input impedance is very low but the field strength value can easily be calculated. This type of coil is useful for probe calibration purposes up to about 30 MHz.

There is also another coil arrangement named Helmholz. It consists of two flat coils on the same axis, both carrying current in the same direction. This type of coil system generates a more uniform magnetic field over a larger volume than the single coil. Helmholtz coils are useful up to about 10 MHz. This frequency limit is dictated by the dimensions of the coil which must be small compared with the wavelength.

3.3.1.5 Standard Probe Method

This method is the simplest, and may be the best method of calibrating hazard meters for general field use. The object is to have a stable and reliable probe that has been calibrated accurately (by one of the previously discussed techniques) for use as a "transfer standard." The standard probe is used to measure the field strength produced by an arbitrary RF field-generating device (e.g. antenna or TEM cell) over a particular region in space (or in a waveguide system). Then an uncalibrated probe is placed at the same location in the field that the standard probe occupied, and the uncalibrated probe's meter reading is compared with the known, measured value of the field, based on data obtained with the standard probe. The transmitter and field-generating device used during this process must generate a field that has the desired magnitude and which is constant with time and the field should be uniform over the region where the unknown probe is placed. Accuracies of about ± 2 to 3 dB are readily attainable with this method. The advantages of this approach are convenience, reliability, and simplicity. A potential source of error when using the transfer standard to calibrate another probe is the difference in the receiving patterns of the two probes. Also, in the near field of a radiator, the size of the probe's sensor is important. Ideally, the standard and unknown probes should be nominally identical and the calibration should be conducted in a field relatively free of spatial variations due to multipath interactions between the probe, the radiator, the anechoic chamber and other field generating components. In TEM cells or parallel plate transmission systems, capacitive coupling between the probe and the centre plate and the walls of the cell can create calibration errors. The transfer standard probe should be stable, rugged, and not easily burned out; it should have a large dynamic range, cover a broad frequency range, and possess an isotropic response.

3.3.2 Evaluation of Survey Instruments

Survey instruments have to be evaluated to determine the uncertainties or errors that may occur when the instrument is used to make field measurements. This evaluation also permits the development of procedures that can minimize errors in measurements. The following is a list of parameters that should be investigated:

1. Absolute Calibration. Should be performed at field levels that produce indications that equal or exceed the mid-scale readout of the instrument.
2. Linearity of the Instrument. In order to establish the linearity of the instrument, measurements should be made at field levels that produce indications of 25, 50, 75 and 100% of full scale, on each range of the readout device.
3. Frequency Response. The frequency response of the instrument over the band of interest should be established. The response should be relatively flat over the specified frequency range (± 1 to 3 dB).
4. Out-of-band Response. The sensitivity of the instrument as a whole should be evaluated for fields at frequencies outside the specified frequency range of the instrument.
5. Near Field Response. The magnetic field response of an electric field instrument and vice versa should be evaluated.
6. Polarization. Any variation of the readout as the probe is rotated about the axis of the handle should be noted.
7. Lead Pickup. Variations in response as the probe handle is rotated through the E plane or any extraneous pickup should be noted and quantified.
8. Temperature Response. Changes in the response of the instrument to a given field over the temperature range of interest should be determined.
9. Supply Voltage Response. For battery operated RF survey meters, the overall accuracy of the instrument should be tested for deviations from the nominal voltage rating of one or more of the batteries.
10. Drift and Noise. Short and long term stability of the instrument should be determined with respect to full scale on each measurement range of the instrument, in the absence of electromagnetic fields.

3.3.3 Practical Measurement Accuracy

Several methods for calibrating meters have been discussed and the uncertainties associated with each method were estimated. It is important to understand that one cannot expect to achieve the same accuracy when using the meters for practical measurement applications. Some of the reasons are as follows: 

1. Meters are usually calibrated in nominally plane wave or uniform fields. Such fields are not always encountered in practice, and the sensor may not respond in the same way to non-planar fields (fields with large spatial gradients).
2. With most calibration methods, only the sensor (probe) is exposed to the field while, in practice, the complete system, including the indicating unit and connecting cable, is immersed in the field. Errors can also result from spurious responses from other parts of the instrument including readout meter (case) and cable. The overall uncertainty added by the above factors is difficult to assess and will vary with the type of meter and usage situation. However, if good measurement procedures are followed, accuracies of ± 1 to 3 dB can be expected in practice, with greater uncertainties in near field situations and at higher frequencies (shorter wavelengths), or in areas where large reflecting objects are present.