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Eye Tracking System Angle Meter NT

Downloads Features Measuring Principle Concept Included Software
Coil Frame Search Coils More Information Specifications List of References

Download Product Brochure (PDF, 689 kB)

The Angle-Meter NT is the second generation of Primelec's search coil systems for the linear detection of 3D eye and head  movements, which was  introduced in 1999. The system was developed in cooperation with the Vestibulo-Oculomotor Laboratory of the University Zurich. The system's very high accuracy and bandwidth ensures effortless recording of horizontal, vertical and torsional eye and head movements of unrestrained laboratory animals.

The system is based on Primelec's enhanced amplitude detection (FFT). A digital signal processor is used to detect the angular orientation of the search coils and to control the unit. The completely digital controlled Angle-Meter NT provides a wide variety of settings, nevertheless the handling is very easy and user friendly.

Discontinued Product Information
The Angle-Meter NT has been taken out of production by Primelec (end of sales in September 2006). Of course we still provide technical support for the system. For new projects please use our latest search coil system CS681.

Features (click here for detailed specifications)

  • Easy to use, straightforward system configuration
  • User friendly menu controlled operation
  • 4 + 4 recording channels (modular construction)
  • Recording range 360 in the horizontal plane, 80 in the vertical plane
  • Output data rate up to 4000 Hz
  • Resolution < 0.05
  • Noise < 0.09 peak-peak
  • Digital synthesized field signals
  • Digital processing of the search coil signals
  • Digital and analog output data (RS-232 interface and analog outputs)
  • Automatic tuning of gain- and offset settings
  • User selectable output format of the readings (linear angular orientation or detected X-, Y- and Z-signals)
  • User adjustable gain and offset correction for the X-, Y- and Z-signals per channel
  • User selectable low-pass filtering
  • User selectable voltage swing of the analog outputs
  • User selectable attenuation of the generated magnetic field
  • Nonvolatile memory to save / recall 2 user specific system configurations
  • Application software included
  • Various coil frames applicable
  • Various search coils applicable
  • Comprehensive customer support by Primelec
  • Two years full warranty
Measuring Principle

The magnetic field search coil technique introduced by Robinson [1] has become the most commonly used method for quantitative studies of eye and head movements in man and in experimental animals. The technique is based on phase-locked amplitude detection of the voltage induced in a search coil in the external ac magnetic field. The angular orientation or displacement of the search coil in three-dimensional (3D) space is detected by using two or three external magnetic fields, which are arranged in space quadrature. Demodulation of the induced signals with respect to the magnetic field directions is obtained on the basis of phase or frequency coding by driving the external magnetic fields in phase quadrature or at different frequencies.

Conventional search coil systems use analog electronics to demodulate the search coil signal. This may cause noise-, phase- and drift problems and requires relatively complex adjustments by the user. External A/D converters are required in order to obtain the angular orientation of the search coil, which increases the total system costs.

The Angle-Meter NT uses an other approach to detect the angular orientation of a search coil. Three digitally synthesized sine wave signals of different frequencies are used to generate a magnetic field. This field induces a voltage in a search coil, where the spectral frequency components of the three field frequencies are proportional to the horizontal and vertical angular displacements of the search coil relative to the system's reference frame. The spectral frequency components are obtained by computing the FFT (Fast Fourier Transformation) of the digitized search coil signal. The use of a high performance digital signal processor allows the computation of the FFT and all required trigonometric calculations in real-time. This approach offers various advantages compared to conventional systems and eliminates most of their above described weaknesses.

[1] Robinson D.A. (1963), A method of measuring eye movements using a scleral search coil in a magnetic field, IEEE Transactions on Biomedical Engineering 10: 137-145


The Basic System of the Angle-Meter NT is capable to process simultaneously four search coils. The use of an Add-On Detector Module allows the simultaneous processing of eight search coils.

Basic System

Basic System plus Add-On Detector Module

Application examples:

Application examples:

  • Simultaneous binocular measurement of horizontal, vertical and torsional eye position
  • Simultaneous binocular measurement of horizontal and vertical eye and head position
  • Simultaneous binocular measurement of horizontal, vertical and torsional eye and head position
  • Very high speed simultaneous binocular measurement of horizontal, vertical and torsional eye position

The Generator Module of the Basic System generates three digitally synthesized sine wave voltages of different frequencies, which are amplified by three power amplifiers within the Generator Module. These field signals are applied via the Matching-Box to the coil frame. The generated magnetic fields (X-, Y- and Z-field) induce a voltage in the search coil(s), which is pre-processed in the external preamplifier.
In the Detector Module, the measuring signal is digitized by an A/D converter and analyzed by a digital signal processor (DSP) in order to detect the horizontal, vertical and torsional orientation of the search coil(s). The complete detection takes less than 250 us per search coil, which allows up to 4000 measurements per second. The output data rate depends on the number of processed measuring channels (user selectable: one, two or four channels per Detector Module). The minimal output data rate (four channels per Detector Module processed) still ensures a high-speed output data rate of 1000 measurements per second.

The user may choose the system's output mode: Angular data or raw data (detected signals X, Y and Z). In the angular mode, the output signals for the horizontal and vertical angles correlate directly proportional to the angular orientation of the search coil(s) in the external magnetic field. Thus, in this mode the system delivers linear outputs and no extra trigonometrical calculations are required. In the raw data mode, the detected signals X, Y and Z are scaled automatically, which simplifies subsequent data processing and ensures always an optimal S/N ratio.

Even though the enormous system flexibility of the Angle-Meter NT, the handling remains very easy and user friendly. The system contains no potentiometers to adjust gains and offsets - these settings are performed automatically by the Angle-Meter NT, where the user of course also has the opportunity to control the settings manually. The menu of the Detector Modules allows quick and easy system settings. In addition, the system may be completely controlled over the serial interface by the included software.

See also the list of references for more information...

Coil Frame

The coil frame consist of three pairs of one-turn field coils (X-, Y- and Z-coil), arranged as a cube, which are building the inductive part of three resonance circuits. An external Matching-Box contains the capacitor networks to tune these resonance circuits. The field signals are applied to the resonance circuits and the resulting currents in the field coils generate the magnetic field.
Primelec offers coil frames in various colors, arranged as a cube with an edge length in the range from 25 cm to 70 cm. Due to the monocoque construction, the restriction of the visual field is minimal. Our coil frames can be de- and remounted, which may be helpful during the building of user specific setups. To ensure optimal mounting flexibility, the Matching-Box is not fixed to the coil frame.

The picture shows a coil frame with an edge length of 30 cm and the connected Matching-Box.

Search Coils

The Angle-Meter NT is capable to process up to eight search coils simultaneously, which is sufficient to fit the needs of almost any application. For example, the recording of horizontal, vertical and torsional binocular eye and head movements requires six measuring channels.

Various search coils are applicable in terms of dimension, geometry, number of turns and material. Of course scleral search coils are suitable. Torsional recordings require a two-in-one search coil (frontal and sagittal plane), i.e. two recording channels are required.

The Angle-Meter NT is compatible with search coils of third-party suppliers. Our partner CHRONOS VISION manufactures and distributes 2D (direction) and 3D (torsion) scleral search coils which are perfectly suited for the Angle-Meter NT.

Read more about search coils manufactured by CHRONOS VISION...


  Product Brochure (PDF, 689 kB)
  User Manual (PDF, 1.95 MB)
  Software (ZIP, 11.16 MB)
More Information

  Included Software
  Measurement of 3D eye movements (Application Note)
  List of References
  Contact us

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