294 IEEE SENSORS JOURNAL, VOL. 13, NO. 1, JANUARY 2013 Fiber Optic Displacement Sensor Using Multimode Plastic Fiber Probe and Tooth Surface Husna Abdul Rahman, Sulaiman Wadi Harun, Malathy Batumalay, Faizah Abdul Muttalib, and Harith Ahmad Abstract—The principle of operation, experimentation, and performance of a fiber optic displacement sensor on a tooth surface is presented. The performances of this sensor are inves- tigated by correlating the detector output with different probe configurations and various teeth samples with different surface reflectivity properties. With a multimode plastic bundled fiber probe, the sensitivities for the molar, canine, hybrid composite resin, and acrylic surfaces are obtained at 0.9667, 0.775, 0.5109, and 0.25 mV/mm, respectively, with a good linearity of more than 99% based on the analysis of the front slope. With a multimode plastic coupler, the sensitivities for the molar, canine, hybrid composite resin, and acrylic surfaces are obtained at 0.3538, 0.2951, 0.2069, and 0.1579 mV/mm, respectively. This proves that the sensor is capable of discriminating between different teeth surfaces. Such results are useful as guidelines for tooth surface related research, such as tooth surface profiling and measurement of tooth surface roughness. Index Terms— Fiber optic displacement sensor, multimode plastic fiber probe, tooth surface reflectivity. I. I NTRODUCTION O PTICAL sensor technology has received tremendous amount of attention and are widely used in applications such as temperature, strain, mechanical vibrations, displace- ment and acoustic pressure [1]–[3]. Non-invasive optical meth- ods have been explored and found to be potentially useful in various dental applications. X-ray has long been the dominant method for the acquisition of the tooth surface profile and detection of dental flaws [4]–[6]. One of the emerging optical technique in this area is quantitative light-induced fluorescence (QLF) [7] but its widespread implementation are still hindered by other factors such as cost and complex of operation [8]. Significant advances have also occured in techniques and instruments for colorimetric analysis in dentistry which mini- mizes the subjective variance in the colour matching process. Spectrophotometers are among the newer techniques which measures the spectral reflectance or transmittance curve of a Manuscript received February 8, 2012; revised June 12, 2012; accepted August 15, 2012. Date of publication September 4, 2012; date of current version January 7, 2013. This work was supported by the University of Malaya under PPP Grant PV033/2011A and HIR-MOHE Grant D0000009-16001. The associate editor coordinating the review of this paper and approving it for publication was Dr. M. Nurul Abedin. The authors are with the Photonics Research Centre, Department of Physics, Faculty of Science, University of Malaya, Kuala Lumpur 50603, Malaysia (e-mail: husna232@salam.uitm.edu.my; swharun@um.edu.my; malathy.batumalay@newinti.edu.my; hazeaf.retsehc@gmail.com; harith@um. edu.my). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSEN.2012.2215848 specimen. In comparison to colorimeters, they have a longer working life but are more complex and expensive [9]. Here, we introduce a simple intensity modulated fiber optic displacement sensor as a quantitative technique for various dental applications. The intensity modulated based sensors use the modulation of light power transmitted between the head of the sensor and the target surface. In comparison with the phase and frequency modulation, the light intensity modulation is the simplest method to obtain a high resolution measurement. The intensity modulated sensors compete well with other sensing methods as they are relatively inexpensive, contact less, easy to be fabricated and suitable for employment in harsh environments. A number of studies have been performed with mirrors and metal surfaces as the reflecting object [10]– [12]. Here, we attempt to exploit the correlation of reflected light intensities along the tooth surface. The reflected light from the tooth surface is coupled back into the probe and the intensity of the reflected light is used to determine the distance between the surface and probe. In our approach, variations of surface reflectivity are achieved through the use of different types of teeth surfaces, namely molar, canine, hybrid composite resin and acrylic. Fiber optic displacement sensors constructed from multimode plastic optical fibers offer the benefit of low optical signal transmission loss, low produc- tion cost, compact size and compatibility with optical fiber technology. II. EXPERIMENTAL SETUP Fig. 1(a) and (b) shows the setup for the displacement measurement using fiber optic displacement sensor using mul- timode plastic bundled fibers and multimode plastic couplers, respectively. The sensor consists of a fiber optic transmit- ter, mechanical chopper, fiber optic probe, 4 teeth samples (consisting each of the molar, canine, hybrid composite resin and acrylic sample), a silicon photodetector, lock-in amplifier and computer. The multimode plastic bundled fiber probe is made of two 2 m long PMMA (polymethyl methacrylate) which consists of one transmitting fiber of 1 mm in diameter and 16 receiving fibers of 0.25 mm in diameter, numerical aperture of 0.5, core refractive index of 1.492 and cladding refractive index of 1.402. A red He-Ne laser (λ = 633 nm) is used as the light source with an average output power of 5.5 mW, beam diameter of 0.80 mm and beam diver- gence of 1.01 mRads. The photodetector is a high speed silicon photodiode with an optical response extending from 400 to 1100 nm, making it compatible with a wide range of 1530–437X/$31.00 © 2012 IEEE