Showing papers in "Applied Optics in 2009"

HomER: a review of time-series analysis methods for near-infrared spectroscopy of the brain.

Abstract: Near-infrared spectroscopy (NIRS) is a noninvasive neuroimaging tool for studying evoked hemodynamic changes within the brain. By this technique, changes in the optical absorption of light are recorded over time and are used to estimate the functionally evoked changes in cerebral oxyhemoglobin and deoxyhemoglobin concentrations that result from local cerebral vascular and oxygen metabolic effects during brain activity. Over the past three decades this technology has continued to grow, and today NIRS studies have found many niche applications in the fields of psychology, physiology, and cerebral pathology. The growing popularity of this technique is in part associated with a lower cost and increased portability of NIRS equipment when compared with other imaging modalities, such as functional magnetic resonance imaging and positron emission tomography. With this increasing number of applications, new techniques for the processing, analysis, and interpretation of NIRS data are continually being developed. We review some of the time-series and functional analysis techniques that are currently used in NIRS studies, we describe the practical implementation of various signal processing techniques for removing physiological, instrumental, and motion-artifact noise from optical data, and we discuss the unique aspects of NIRS analysis in comparison with other brain imaging modalities. These methods are described within the context of the MATLAB-based graphical user interface program, HomER, which we have developed and distributed to facilitate the processing of optical functional brain data.

Thermal and near infrared sensor for carbon observation Fourier-transform spectrometer on the Greenhouse Gases Observing Satellite for greenhouse gases monitoring.

Abstract: The Greenhouse Gases Observing Satellite (GOSAT) monitors carbon dioxide (CO2) and methane (CH4) globally from space using two instruments. The Thermal and Near Infrared Sensor for Carbon Observation Fourier-Transform Spectrometer (TANSO-FTS) detects gas absorption spectra of the solar short wave infrared (SWIR) reflected on the Earth's surface as well as of the thermal infrared radiated from the ground and the atmosphere. TANSO-FTS is capable of detecting three narrow bands (0.76, 1.6, and 2.0 μm) and a wide band (5.5-14.3 μm) with 0.2 cm−1 spectral resolution (interval). The TANSO Cloud and Aerosol Imager (TANSO-CAI) is an ultraviolet (UV), visible, near infrared, and SWIR radiometer designed to detect cloud and aerosol interference and to provide the data for their correction. GOSAT is placed in a sun-synchronous orbit 666 km at 13:00 local time, with an inclination angle of 98 °. A brief overview of the GOSAT project, scientific requirements, instrument designs, hardware performance, on-orbit operation, and data processing is provided.

Calibration-free wavelength-modulation spectroscopy for measurements of gas temperature and concentration in harsh environments

Abstract: We present a practical implementation of calibration-free wavelength-modulation spectroscopy with second harmonic detection (WMS-2f) for measurements of gas temperature and concentration in harsh environments. The method is applicable to measurements using lasers with synchronous wavelength and intensity modulation (such as injection current-tuned diode lasers). The key factors that enable measurements without the on-site calibration normally associated with WMS are (1) normalization of the WMS-2f signal by the first harmonic (1f) signal to account for laser intensity, and (2) the inclusion of laser-specific tuning characteristics in the spectral-absorption model that is used to compare with measured 1f-normalized, WMS-2f signals to infer gas properties. The uncertainties associated with the calibration-free WMS method are discussed, with particular emphasis on the influence of pressure and optical depth on the WMS signals. Many of these uncertainties are also applicable to calibrated WMS measurements. An example experimental setup that combines six tunable diode laser sources between 1.3 and 2.0 mum into one probe beam for measurements of temperature, H(2)O, and CO(2) is shown. A hybrid combination of wavelength and frequency demultiplexing is used to distinguish among the laser signals, and the optimal set of laser-modulation waveforms is presented. The system is demonstrated in the harsh environment of a ground-test scramjet combustor. A comparison of direct absorption and 1f-normalized, WMS-2f shows a factor of 4 increase in signal-to-noise ratio with the WMS technique for measurements of CO(2) in the supersonic flow. Multidimensional computational fluid-dynamics (CFD) calculations are compared with measurements of temperature and H(2)O using a simple method that accounts for the influence of line-of-sight (LOS) nonuniformity on the absorption measurements. The comparisons show the ability of the LOS calibration-free technique to gain useful information about multidimensional CFD models.

Recent progress in three-dimensional information processing based on integral imaging.

Abstract: Recently developed integral imaging techniques are reviewed. Integral imaging captures and reproduces the light rays from the object space, enabling the acquisition and the display of the three-dimensional information of the object in an efficient way. Continuous effort on integral imaging has been improving the performance of the capture and display process in various aspects, including distortion, resolution, viewing angle, and depth range. Digital data processing of the captured light rays can now visualize the three-dimensional structure of the object with a high degree of freedom and enhanced quality. This recent progress is of high interest for both industrial applications and academic research.

Design of an optical see-through head-mounted display with a low f-number and large field of view using a freeform prism

Abstract: It has been a challenge to design an optical see-through head-mounted display (OST-HMD) that has a wide field of view (FOV) and low f-number (f/#) while maintaining a compact, lightweight, and nonintrusive form factor. In this paper, we present an OST-HMD design using a wedge-shaped freeform prism cemented with a freeform lens. The prism, consisting of three freeform surfaces (FFSs), serves as the near-eye viewing optics that magnifies the image displayed through a microdisplay, and the freeform lens is an auxiliary element attached to the prism in order to maintain a nondistorted see-through view of a real-world scene. Both the freeform prism and the lens utilize plastic materials to achieve light weight. The overall dimension of the optical system per eye is no larger than 25 mm by 22 mm by 12 mm, and the weight is 8 g. Based on a 0.61 in. microdisplay, our system demonstrates a diagonal FOV of 53.5° and an f/# of 1.875, with an 8 mm exit pupil diameter and an 18.25 mm eye relief.

Long-range time-of-flight scanning sensor based on high-speed time-correlated single-photon counting.

Abstract: We describe a scanning time-of-flight system which uses the time-correlated single-photon counting technique to produce three-dimensional depth images of distant, noncooperative surfaces when these targets are illuminated by a kHz to MHz repetition rate pulsed laser source. The data for the scene are acquired using a scanning optical system and an individual single-photon detector. Depth images have been successfully acquired with centimeter xyz resolution, in daylight conditions, for low-signature targets in field trials at distances of up to 325 m using an output illumination with an average optical power of less than 50 microW.

Extremely high-definition full-parallax computer-generated hologram created by the polygon-based method

Abstract: A large-scale full-parallax computer-generated hologram (CGH) with four billion (2(16) x 2(16)) pixels is created to reconstruct a fine true 3D image of a scene, with occlusions. The polygon-based method numerically generates the object field of a surface object, whose shape is provided by a set of vertex data of polygonal facets, while the silhouette method makes it possible to reconstruct the occluded scene. A novel technique using the segmented frame buffer is presented for handling and propagating large wave fields even in the case where the whole wave field cannot be stored in memory. We demonstrate that the full-parallax CGH, calculated by the proposed method and fabricated by a laser lithography system, reconstructs a fine 3D image accompanied by a strong sensation of depth.

Reliability-guided digital image correlation for image deformation measurement

Abstract: A universally applicable reliability-guided digital image correlation (DIC) method is proposed for reliable image deformation measurement. The zero-mean normalized cross correlation (ZNCC) coefficient is used to identify the reliability of the point computed. The correlation calculation begins with a seed point and is then guided by the ZNCC coefficient. That means the neighbors of the point with the highest ZNCC coefficient in a queue for computed points will be processed first. Thus the calculation path is always along the most reliable direction, and possible error propagation of the conventional DIC method can be avoided. The proposed novel DIC method is universally applicable to the images with shadows, discontinuous areas, and deformation discontinuity. Two image pairs were used to evaluate the performance of the proposed technique, and the successful results clearly demonstrate its robustness and effectiveness.

Measurement method for photoluminescent quantum yields of fluorescent organic dyes in polymethyl methacrylate for luminescent solar concentrators

Abstract: A method for measuring the photoluminescent quantum yields (PLQY) of luminescent organic dyes is presented. The self-absorption probability calculated at different dye concentrations is used to determine the absolute quantum yield from the observed values. The results for a range of commercially available dyes show high quantum yields, even at high concentrations, and an absence of quenching. The PLQY of several dye mixtures are also presented. The results indicate an absence of any reduction of PLQY in a dye mixture as compared with the individual PLQY of the dyes.

Freeform surface lens for LED uniform illumination.

Abstract: A method using a freeform surface lens for LED secondary optic design is proposed in this paper. By Snell's Law, the differential equations are given to build the relationship between the normal direction of a freeform surface and its input/output ray vectors. Runge-Kutta formulas are used to calculate the differential equations to design the freeform surface. Moreover, the optical model for uniform illumination is simulated and optical performance is analyzed. A practical freeform surface lens for LED uniform illumination is fabricated using an injection molding method. By the process, our system demonstrates a uniform illumination with a divergence half-angle of 6° and an efficiency of 78.6%.

Elimination of a zero-order beam induced by a pixelated spatial light modulator for holographic projection.

Abstract: A technique is proposed theoretically and verified experimentally to eliminate a zero-order beam caused by a pixelated phase-only spatial light modulator (SLM) for holographic projection. The formulas for determination of the optical field in the Fourier plane are deduced, and the influence of the pixelated structure of a SLM on the intensity of the zero-order beam is numerically investigated. Two currently existing techniques are studied and a new one is presented. These three techniques are used separately to eliminate the zero-order interruption, and the optical performances of the reconstructed patterns are compared. The new technique results in higher reconstruction quality and diffraction efficiency. A short animated movie is illuminated for holographic projection display. The experimental results show that the zero-order beam can be efficiently eliminated by the new technique. It is believed that this technique can be used in various optical systems that are based on pixelated phase-only SLMs, such as holographic optical tweezers and optical testing systems.

Optimal dark hole generation via two deformable mirrors with stroke minimization

Abstract: The past decade has seen a significant growth in research targeted at space-based observatories for imaging exosolar planets. The challenge is in designing an imaging system for high contrast. Even with a perfect coronagraph that modifies the point spread function to achieve high contrast, wavefront sensing and control is needed to correct the errors in the optics and generate a “dark hole.” The high-contrast imaging laboratory at Princeton University is equipped with two Boston Micromachines Kilo-DMs. We review here an algorithm designed to achieve high contrast on both sides of the image plane while minimizing the stroke necessary from each deformable mirror (DM). This algorithm uses the first DM to correct for amplitude aberrations and uses the second DM to create a flat wavefront in the pupil plane. We then show the first results obtained at Princeton with this correction algorithm, and we demonstrate a symmetric dark hole in monochromatic light.

Depth sensitivity and image reconstruction analysis of dense imaging arrays for mapping brain function with diffuse optical tomography

Abstract: The development of diffuse optical tomography (DOT) instrumentation for neuroimaging of humans is challenging due to the large size and the geometry of the head and the desire to distinguish signals at different depths. One approach to this problem is to use dense imaging arrays that incorporate measurements at different source-detector distances. We previously developed a high-density DOT system that is able to obtain retinotopic measurements in agreement with functional magnetic resonance imaging and positron emission tomography. Further extension of high-density DOT neuroimaging necessitates a thorough study of the measurement and imaging sensitivity that incorporates the complex geometry of the head--including the head curvature and layered tissue structure. We present numerical simulations using a finite element model of the adult head to study the sensitivity of the measured signal as a function of the imaging array and data sampling strategy. Specifically, we quantify the imaging sensitivity available within the brain (including depths beyond superficial cortical gyri) as a function of increasing the maximum source-detector separation included in the data. Through the use of depth related sensitivity analysis, it is shown that for a rectangular grid [with 1.3 cm first nearest neighbor (NN) spacing], second NN measurements are sufficient to record absorption changes along the surface of the brain's cortical gyri (brain tissue depth 15 mm).

Suppression of the zero-order term in off-axis digital holography through nonlinear filtering

Abstract: We present experimental validation of a new reconstruction method for off-axis digital holographic microscopy (DHM). This method effectively suppresses the object autocorrelation, namely, the zero-order term, from holographic data, thereby improving the reconstruction bandwidth of complex wavefronts. The algorithm is based on nonlinear filtering and can be applied to standard DHM setups with realistic recording conditions. We study the robustness of the technique under different experimental configurations, and quantitatively demonstrate its enhancement capabilities on phase signals.

Retrievals of atmospheric CO2 from simulated space-borne measurements of backscattered near-infrared sunlight: accounting for aerosol effects.

Abstract: Retrievals of atmospheric carbon dioxide (CO2) from space-borne measurements of backscattered near-infrared sunlight are hampered by aerosol and cirrus cloud scattering effects. We propose a retrieval approach that allows for the retrieval of a few effective aerosol parameters simultaneously with the CO2 total column by parameterizing particle amount, height distribution, and microphysical properties. Two implementations of the proposed method covering different spectral bands are tested for an ensemble of simulated nadir observations for aerosol (and cirrus) loaded scenes over low- and mid-latitudinal land surfaces. The residual aerosol-induced CO2 errors are mostly below 1% up to aerosol optical thickness 0.5. The proposed methods also perform convincing for scenes where cirrus clouds of optical thickness 0.1 overlay the aerosol.

Three-dimensional noninvasive imaging of the vasculature in the mouse brain using a high resolution photoacoustic scanner

Abstract: The application of a novel photoacoustic imaging instrument based on a Fabry-Perot polymer film sensing interferometer to imaging the small animal brain is described. This approach provides a convenient backward mode sensing configuration that offers the prospect of overcoming the limitations of existing piezoelectric based detection schemes for small animal brain imaging. Noninvasive images of the vasculature in the mouse brain were obtained at different wavelengths between 590 and 889 nm, showing that the cerebral vascular anatomy can be visualized with high contrast and spatial resolution to depths up to 3.7 mm. It is considered that the instrument has a role to play in characterizing small animal models of human disease and injury processes such as stroke, epilepsy, and traumatic brain injury.

Real-time phase-only color holographic video display system using LED illumination.

Abstract: A real-time full-color phase-only holographic display system generates holograms of 3D objects. The system includes a 3D object formed by voxels, an internet-based transmission capability that transmits the object information to the server, a real-time hologram generation unit, and a holographic display unit with incoherent illumination. The server calculates three phase holograms for RGB components using multiple GPUs. The resultant phase holograms are saved into an RGB bitmap image and loaded to the phase-only spatial light modulators (SLMs). SLMs are illuminated uniformly by LEDs, and reconstructed waves are aligned and overlapped by using high precision optics and stages. Experimental results are satisfactory.

Angular shape of the oceanic particulate volume scattering function in the backward direction

Abstract: Analysis of several million particulate volume scattering functions (VSFs) from different field sites around the world's oceans and coastlines revealed that the shape of the VSF in the backward direction was remarkably consistent (5% or less variability at angles between 90° and 170°). In agreement with theoretical models and past field measurements, the variability of the VSF shape (the VSF normalized to the backscattering coefficient) was found to be lowest between 110° and 120°. This study concludes that under most oceanic conditions, estimates of the particulate backscattering coefficient, using single angle scattering measurements near 110° to 120° and suitable conversion factors, are justified and should have a maximum uncertainty of less than a few percent once instrument noise is accounted for.

Measurement of Gaussian laser beam radius using the knife-edge technique: improvement on data analysis

Abstract: We revisited the well known Khosrofian and Garetz inversion algorithm [Appl. Opt.22, 3406-3410 (1983)APOPAI0003-6935] that was developed to analyze data obtained by the application of the traveling knife-edge technique. We have analyzed the approximated fitting function that was used for adjusting their experimental data and have found that it is not optimized to work with a full range of the experimentally-measured data. We have numerically calculated a new set of coefficients, which makes the approximated function suitable for a full experimental range, considerably improving the accuracy of the measurement of a radius of a focused Gaussian laser beam.

Three-dimensional volumetric measurement of red blood cell motion using digital holographic microscopy

Abstract: Measurement of blood flow with high spatial and temporal resolutions in a three-dimensional (3D) volume is a challenge in biomedical research fields. In this study, digital holographic microscopy is used to measure the 3D motion of human red blood cells (RBCs) in a microscale volume. The cinematographic holography technique, which uses a high-speed camera, enabled the continuous tracking of individual RBCs in a microtube flow. Several autofocus functions that quantify the sharpness of reconstructed RBC images are evaluated to locate the accurate depthwise position of RBCs. In this study, the squared Laplacian function yields the smallest depth of focus and locates the depthwise positions of RBCs with a root mean square error of 2.3 microm. By applying this method, we demonstrate the measurement of four-dimensional (space and time) trajectories as well as 3D velocity profiles of RBCs. The measurement uncertainties of the present method are also discussed.

Digital simulation of scalar optical diffraction: revisiting chirp function sampling criteria and consequences.

Abstract: Accurate simulation of scalar optical diffraction requires consideration of the sampling requirement for the phase chirp function that appears in the Fresnel diffraction expression. We describe three sampling regimes for FFT-based propagation approaches: ideally sampled, oversampled, and undersampled. Ideal sampling, where the chirp and its FFT both have values that match analytic chirp expressions, usually provides the most accurate results but can be difficult to realize in practical simulations. Under- or oversampling leads to a reduction in the available source plane support size, the available source bandwidth, or the available observation support size, depending on the approach and simulation scenario. We discuss three Fresnel propagation approaches: the impulse response/transfer function (angular spectrum) method, the single FFT (direct) method, and the two-step method. With illustrations and simulation examples we show the form of the sampled chirp functions and their discrete transforms, common relationships between the three methods under ideal sampling conditions, and define conditions and consequences to be considered when using nonideal sampling. The analysis is extended to describe the sampling limitations for the more exact Rayleigh-Sommerfeld diffraction solution.

Spectral modulation for full linear polarimetry

Abstract: Linear (spectro) polarimetry is usually performed using separate photon flux measurements after spatial or temporal polarization modulation. Such classical polarimeters are limited in sensitivity and accuracy by systematic effects and noise. We describe a spectral modulation principle that is based on encoding the full linear polarization properties of light in its spectrum. Such spectral modulation is obtained with an optical train of an achromatic quarter-wave retarder, an athermal multiple-order retarder, and a polarizer. The emergent spectral modulation is sinusoidal with its amplitude scaling with the degree of linear polarization and its phase scaling with the angle of linear polarization. The large advantage of this passive setup is that all polarization information is, in principle, contained in a single spectral measurement, thereby eliminating all differential effects that potentially create spurious polarization signals. Since the polarization properties are obtained through curve fitting, the susceptibility to noise is relatively low. We provide general design options for a spectral modulator and describe the design of a prototype modulator. Currently, the setup in combination with a dedicated retrieval algorithm can be used to measure linear polarization signals with a relative accuracy of 5%.

Fast computation of hologram patterns of a 3D object using run-length encoding and novel look-up table methods

Abstract: In this paper we propose a new approach for fast generation of computer-generated holograms (CGHs) of a 3D object by using the run-length encoding (RLE) and the novel look-up table (N-LUT) methods. With the RLE method, spatially redundant data of a 3D object are extracted and regrouped into the N-point redundancy map according to the number of the adjacent object points having the same 3D value. Based on this redundancy map, N-point principle fringe patterns (PFPs) are newly calculated by using the 1-point PFP of the N-LUT, and the CGH pattern for the 3D object is generated with these N-point PFPs. In this approach, object points to be involved in calculation of the CGH pattern can be dramatically reduced and, as a result, an increase of computational speed can be obtained. Some experiments with a test 3D object are carried out and the results are compared to those of the conventional methods.

Review of three-dimensional holographic imaging by multiple-viewpoint-projection based methods

Abstract: Methods of generating multiple viewpoint projection holograms of three-dimensional (3-D) realistic objects illuminated by incoherent white light are reviewed in this paper. Using these methods, it is possible to obtain holograms with a simple digital camera, operating in regular light conditions. Thus, most disadvantages characterizing conventional digital holography, namely the need for a powerful, highly coherent laser and extreme stability of the optical system, are avoided. The proposed holographic processes are composed of two stages. In the first stage, regular intensity-based images of the 3-D scene are captured from multiple points of view by a simple digital camera. In the second stage, the acquired projections are digitally processed to yield the complex digital hologram of the 3-D scene, where no interference is involved in the process. For highly reflecting 3-D objects, the resulting hologram is equivalent to an optical hologram of the objects recorded from the central point of view. We first review various methods to acquire the multiple viewpoint projections. These include the use of a microlens array and a macrolens array, as well as digitally generated projections that are not acquired optically. Next, we show how to digitally process the acquired projections to Fourier, Fresnel, and image holograms. Additionally, to obtain certain advantages over the known types of holograms, the proposed hybrid optical-digital process can yield novel types of holograms such as the modified Fresnel hologram and the protected correlation hologram. The prospective goal of these methods is to facilitate the design of a simple and portable digital holographic camera that can be useful for a variety of practical applications, including 3-D video acquisition and various types of biomedical imaging. We review several of these applications to signify the advantages of multiple viewpoint projection holography.

Scintillations of partially coherent multiple Gaussian beams in turbulence.

Abstract: For an incidence composed of partially coherent multiple Gaussian beams, Huygens-Fresnel principle-based on-axis scintillation index is formulated in a weakly turbulent homogeneous horizontal atmospheric path. Our general formulation is applied to two examples of partially coherent annular and partially coherent flat-topped Gaussian beams. Compared to partially coherent single Gaussian beam scintillations, annular beam scintillations seem to possess higher values for all partial coherence levels, whereas flat-topped Gaussian beam intensity fluctuations are slightly larger, especially at lower coherence levels and at larger source sizes. At the same source partial coherence, annular beams exhibit smaller scintillations for larger ring sizes. For flat-topped Gaussian beams, except for very small and very large source sizes, as the number of Gaussian beams forming the flatness increases, intensity fluctuations also increase, a trend applicable for different degrees of coherence. A trend valid for both single and multiple Gaussian incidence, except for certain annular beams of large primary beam sizes, is that the scintillations decrease as the source becomes less coherent. Being applicable for all degrees of source coherences, for both beams examined, scintillations increase steadily as the Rytov plane wave scintillation index increases.

Propagation length of surface plasmons in a metal film with roughness.

Abstract: The propagation of laser-excited surface plasmons along a gold film with surface roughness is directly observed via scattered light. The attenuation length of surface plasmons in a broad wavelength interval is calculated for smooth gold and silver films. The surface roughness, which was characterized with an AFM, introduces corrections to the attenuation length, angular dependence of the surface plasmon resonance, and the effective dielectric constant of the metal film. These corrections are also taken into account and discussed.

Atomic layer deposition of Al 2 O 3 and TiO 2 multilayers for applications as bandpass filters and antireflection coatings

Abstract: Al2O3 and TiO2 thin films have been deposited on Si wafers, quartz, BK7 glass, and polycarbonate substrates by atomic layer deposition (ALD). The refractive indices and growth rates of the materials have been determined by spectroscopic ellipsometry and transmission electron microscopy. The influence of substrate temperature and precursor on the refractive indices has been investigated. The refractive index of TiO2 significantly increases with temperature, whereas the Al2O3 films are temperature insensitive. The films deposited using H2O2 as oxygen source show a slightly higher refractive index than the films prepared with H2O. Multilayer narrow-bandpass filters and broadband antireflective coatings have been designed and produced by ALD.

Experimental and theoretical inspection of the phase-to-height relation in Fourier transform profilometry

Abstract: The measurement of an object's shape using projected fringe patterns needs a relation between the measured phase and the object's height. Among various methods, the Fourier transform profilometry proposed by Takeda and Mutoh [Appl. Opt.22, 3977-3982 (1983)]APOPAI0003-6935 is widely used in the literature. Rajoub et al. have shown that the reference relation given by Takeda is erroneous [J. Opt. A. Pure Appl. Opt.9, 66-75 (2007)JOAOF81464-425810.1088/1464-4258/9/6/S10]. This paper follows from Rajoub's study. Our results for the phase agree with Rajoub's results for both parallel- and crossed-optical-axes geometries and for either collimated or noncollimated projection. Our two main results are: (i) we show experimental evidence of the error in Takeda's formula and (ii) we explain the error in Takeda's derivation and we show that Rajoub's argument concerning Takeda's error is not correct.

Hologram generation by horizontal scanning of a high-speed spatial light modulator

Abstract: In order to increase the image size and the viewing zone angle of a hologram, a high-speed spatial light modulator (SLM) is imaged as a vertically long image by an anamorphic imaging system, and this image is scanned horizontally by a galvano scanner. The reduction in horizontal pixel pitch of the SLM provides a wide viewing zone angle. The increased image height and horizontal scanning increased the image size. We demonstrated the generation of a hologram having a 15° horizontal viewing zone angle and an image size of 3.4 inches with a frame rate of 60 Hz using a digital micromirror device with a frame rate of 13.333 kHz as a high-speed SLM.

Simple and efficient method for specularity removal in an image.

Abstract: For dielectric inhomogeneous objects, the perceived reflections are the linear combinations of diffuse and specular reflection components. Specular reflection plays an important role in the fields of image analysis, pattern recognition, and scene synthesis. Several methods for the separation of the diffuse and the specular reflection components have been presented based on image segmentation or local interaction of neighboring pixels. We propose a simple and effective method for specularity removal in a single image on the level of each individual pixel. The chromaticity of diffuse reflection is approximately estimated by employing the concept of modified specular-free image, and the specular component is adjusted according to the criterion of smooth color transition along the boundary of diffuse and specular regions. Experimental results indicate that the proposed method is promising when compared with other state-of-the-art techniques, in both separation accuracy and running speed.