# A High-Resolution Magnetic Field Imaging System Based on the Unpackaged Hall Element Array
## 1 Introduction  
Magnetic field imaging is widely used in many fields such as non-destructive testing (NDT), metal detection, biomedicine, and circuit diagnostics [1; 2; 3; 4], etc. Also associated with magnetic field imaging is eddy current imaging, which has promising applications in several areas [5; 6]. Magnetic field imaging can visualize the distribution of magnetic fields in magnetic materials or magnetized metals, which can help study their properties. [PERSON] et al. [2] designed a removable magnetic detection system using the Hall linear array, which was used to image the shape of ferromagnetic objects within floors or walls. [PERSON] et al. [7; 8] developed a subsurface magnetic imaging system using a magnetometer inside a smartphone to probe the geometry of buried objects. They then also assembled a magnetic camera using several AMR triaxial magnetometers to detect the effect of hidden objects on the ambient magnetic field and imaged the magnetic field of cables loaded with 50 Hz alternating current. [PERSON] et al. [9; 10; 11; 12] used solid-state magnetic induction probes and arrays to develop a magnetic scanning system that detected and imaged rebars within concrete. The experimental results demonstrated the ability of the system to image embedded metal objects, as well as its potential applications in biomedical, surveillance, and security fields. Magnetic field imaging also maps the leakage magnetic field at defects on the surface of magnetized metal samples to obtain defect information, which has been the subject ofmuch research in NDT of metallic materials. [PERSON] et al. [13] employed the magnetic camera to image surface defects in soft magnetic composites (SMCs) and investigated the effect of defect size and material permeability on the magnetic field through numerical analysis. [PERSON] et al. [14] proposed a moisture separator reheater (MSR) piping system for nondestructive testing, which used the Hall sensors array to detect artificial defects on the piping. [PERSON] et al. [15] employed a gradient Hall sensors array to predict the shape of cracks on the rail surface and assessed the depth of the cracks by a BP neural network algorithm. In addition, magnetic imaging can detect the magnetic field radiation of electronic devices and household appliances to determine whether they meet electromagnetic compatibility standards [16], and it is also used to measure and analyze the magnetic field of rock samples [17]. The key sensing unit to realize the magnetic imaging technique is the magnetic sensor. Currently, there are many types of magnetic sensors, such as AMR, GMR, induction coils, and Hall sensors [18; 19; 20; 21]. Hall sensors are characterized by high accuracy and good linearity, small size and low power consumption, thermal stability, and low price, which makes them very suitable for measuring magnetic fields. A single Hall sensor can spend much time scanning and detecting magnetic fields, so using a Hall sensor array to detect magnetic fields can significantly speed up the process. Several packaged Hall sensors are currently used to form linear or faceted arrays [2; 22]. However, the packaged Hall sensors have low detection resolution due to the presence of package housing that results in a larger sensor volume, making the spacing between neighboring Hall sensors larger. Using unpackaged Hall elements avoids the increased volume and packaging costs associated with packaged housings and effectively improves detection resolution. However, the research on unpackaged Hall element arrays is still relatively small. This paper describes a high-resolution magnetic field imaging system in which the magnetic sensing probe consists of a linear array of 256 unpackaged Hall elements, which are fabricated in GaAs material and have a spacing of 255 \(\upmu\)m between the centers of adjacent elements. The detection distance between the magnetic sensing probe and the samples can be less than 1 mm.