The performance of linearly chirped gratings is optimized by designing and changing the parameters of apodization functions, aiming at some issues such as the unexpectedly fluctuating group delay ripple (GDR) in chirped fiber Bragg grating (CFBG) which are applied to achieving all-optical dispersion compensation in 40 Gb/s optical communications. This method is proposed by selecting some apodization functions with different roll-off characteristics, using genetic algorithm to optimize the apodizing parameters, and combining with transfer matrix method to gain the linearly chirped gratings with low group delay ripple after 200 generations. The numerical results validate the superiority that the ripple occurring in valid bandwidth of flat reflection spectrum can be restrained by adopting asymmetrically partial apodization. Moreover chirped Bragg grating fibers are fabricated of which reflection spectrum bandwidth of 1.06 nm and its delay ripple no more than 45.60 ps, and can be used in dense wavelength division multiplexing system (DWDM) with high capacity.

Experimental system of high power Yb3+-doped double-clad fiber amplifier pumped by diode laser was set up. Frequency upconversion luminescence in this fiber amplifier has been studied theoretically and experimentally. Analysis shows that the green fluorescence at fiber side and ends is due to cooperative luminescence. The cooperative fluorescence spectrum from pump side and signal side of fiber has been measured and compared with theoretical spectrum. The influence of signal and amplified spontaneous emission on frequency upconversion fluorescence has been discussed. The relational expression between the fluorescence intensity and pump power was described theoretically based on the rate equation and the relational curve was obtained by experiment.

In a system, for the reason of gain saturation of semiconductor optical amplifiers (SOAs) the gain of a channel is influenced by other multiplexing channels, and the interchannel crosstalk limits severely the application of SOA. Theoretical study on channel crosstalk due to gain saturation in SOAs is presented. The system bit-error rate (BER) is quantified as a function of the amplifier input power and the number of channels. It is found that more multiplexing channels induce more severe crosstalk because of the gain saturation characteristic of SOAs. Crosstalk mitigation techniques in SOAs are studied theoretically and experimentally. Simulation shows that continuous wave (CW) light injection can restrain output power fluctuation and decrease BER. With ten multiplexing channels, CW light injection can reduce power loss by 2 dB. Experiment shows in a 40 Gb/s two-channel system, interchannel crosstalk caused by SOA can be restrained by CW light injection.

Based on the two-center model, the steady-state nonvolatile two-step, two-color holographic recording performance with continuous-wave lights in the low intensity for LiNbO3∶Fe∶Mn is studied theoretically. By use of numerical method, through comparing the contributions to the space-charge fields from the different electron transfer processes between the deep-trap centers (Mn2+/Mn3+) and the shallow-trap centers (Fe2+/Fe3+), the direct electron exchange between the Mn2+/Mn3+ and the Fe2+/Fe3+ levels through the tunnelling effect dominates the amplitude of the total space-charge field. This direct electron transfer process plays a key role on the two-step, two-color holography performance also. In addition, by comparing the amplitudes of the space-charge fields of the LiNbO3∶Fe∶Mn with the near-stoichiometric LiNbO3∶Fe, the results show that the holographic recording performance in the LiNbO3∶Fe∶Mn is much better than that in the near-stoichiometric LiNbO3∶Fe at low intensity of gating light and high intensity of recording light.

In digital lensless Fourier transform holography, performing inverse Fourier transform directly to the digital hologram for the numerical reconstruction of the object field requires two conditions. Firstly, the paraxial approximation condition must be satisfied in the recording process of the hologram, otherwise a nonparaxial aberration will be produced in the reconstruction process. Secondly, the distances from the object plane and the reference point source to the recording plane of the hologram must be equal, or else a misfocus aberration will be produced in the reconstruction process. The grayscale distribution of the digital lensless Fourier transform holograms recorded under the condition of nonparaxiality or misfocus is analyzed in theory, and corresponding numerical correction methods for nonparaxial or misfocus aberration are introduced, respectively. According to the actual recording conditions, if using proper correction factors for nonparaxiality or misfocus to multiply the grayscale distribution matrix of the recorded digital hologram, the influence of the nonparaxialty or misfocus can be removed. Accurate digital reconstructed image will be obtained by performing inverse Fourier transform directly to the corrected hologram grayscale matrix. The experimental results show that the introduced numerical reconstruction methods can well correct the nonparaxial or misfocus aberration and thus improve the quality of the digital reconstructed image in digital lensless Fourier transform holography.

According to the analyses of the interferential multispectral images, a new compression algorithm based on distributed source coding is proposed. There are apparent shift features between the image series, the relative shift between two images is detected by the match algorithm at the encoder. Our algorithm estimates the rate of each bitplane with the estimated side information frame. Then our algorithm adopts a regions of interest (ROI) coding algorithm, in which the rate-distortion lifting procedure is carried out to allocate coding rate in reason according to the difference significance of image regions. The experimental results show that the proposed algorithm achieves improved spectral performance over the conventional algorithms. With different compression ratios, the algorithm satisfies the requirements of interference multi-spectral image compression system, and efficiently reduces the cost of computation and memory usage.

The disadvantages of wavelet transform and contourlet transform are studied. Color image digital watermarking is implemented based on the multi-scale, multi-direction, and shift-invariance of the nonsubsampled transform. The coefficients in different scales and directions are obtained by image decomposition using the nonsubsampled contourlet transform. Embedding intensities of the low and high frequency coefficients are different invisibility and robustness of watermark requirements for according to the color perception characteristics of human visual system, watermark embedded in blue channel is more than the others, which only choose the local of maximal energy to embed. Compared with the algorithms of stationary wavelet transform and contourlet transform under the same embedding rule, this algorithm is more robust, with higher similarity which can be improved by 0.5 at most.

Denoising is a key problem in phase extraction for electronic speckle pattern interferometry (ESPI) fringe. The heat conduction equation (HCE) is applied to denoise for ESPI fringe patterns and its filtering performance is analyzed qualitatively and quantitatively. Based on the HCE filtered result, MBO algorithm and HCE are applied further to binarization and phase map smoothing respectively. And the phase map of the fringe is extracted successfully from a single fringe pattern. The research shows that HCE can efficiently reduce the noise of speckle fringe pattern, improve the edge of binary fringe pattern, and perform phase recovery rather accurately.

A novel anomaly detection algorithm based on the theory of extended mathematical morphology and spectral similarity measurement is proposed for hyperspectral imagery. The spatial and spectral information has been used to locate and detect targets under the condition of none prior knowledge of targets and background. The extended mathematical morphological erosion and dilation operations are performed respectively to extract the targets features. The orthogonal projection divergence is used to calculate the cumulative distance in the erosion and dilation operations to determine the ordering relation. And the orthogonal projection divergence is also performed to measure the spectral similarity to fuse the results of feature extraction. The synthesized hyperspectral images collected by object modularization imaging spectrometer (OMIS) is applied to evaluate the proposed algorithm, the proposed algorithm is compared with RX algorithm by a specifically designed experiment, andit is applied to distinguish the targets with similar spectral characteristics. From the results of experiments, it is illuminated that the proposed algorithm can detect anomalous targets with low false alarm rate and its performance is better than that of RX algorithm under the same condition. It is also illuminated that the proposed algorithm can differentiate targets with similar spectral characteristics well with low false alarm rate.

Extraction and expression of features are critical to improve the recognition rate of ear image recognition. Scale invariant feature transform (SIFT) is a local point features extraction method. It can find those feature vectors in different scale spaces which are invariant for scale changes and rotations and flexible for illumination variations and affine transformations. SIFT is used to extract structural feature points of ear images and get stable feature descriptors. In order to overcome a defect of local descriptor that an image may have multiple similar regions, an auricle geometric feature is fused. Ear recognition based on these fusion vectors is carried out by using Euclid distance as similarity measurement. Experimental results show that the proposed method can effectively extract ear feature points and obtain high recognition ratio by using few feature points. It is robust to rigid transformation of ear image.

A method for accurate measurement of the size of objects from X-ray diffraction enhanced imaging (DEI) and computer tomography was proposed. X-ray DEI is an imaging method based on phase contrast. A simple model of DEI was set up, by which the relationships among crystal angle, valley positions in image, and system blurring were studied. The deviation of projection valley points caused by the blurring effect was discussed, and a new method to measure diameter of a round sample from projective DEI profile was proposed. DEI data from both simulation and Beijing Synchrotron Radiation Faculty ware processed to demonstrate the validity and accuracy of the method. This method presents an accurate measurement method of the size for objects, especially for the inner structure of small biological tissues or samples.

The detection ratio of dim target with low signal-noise-ratio (SNR) in cloud background especially using low frame frequency detector decreased notably. Optical flow histogram (OPH), its characters is proposed, and its character is presented. Then the reason of low target detection radio from infrared image using low frame frequency detector is analyzed. And based on this analysis, an algorithm of infrared dim target detection based on background motion compensation is advanced. This algorithm uses OPH to compute the background motion vector, and make background compensation. By distinguishing movement difference between target and background, the comparison result is gained. In order to reduce the false alarm, this result is filtered by Robinson filter to reduce the residues edges. The experimental results have proved that this algorithm can improvegreatly the probability of infrared point target detection, and detect dim targets with SNR of 1.

To deal with the interactive separation of region of interests (ROI) in the process of three-dimensional (3D) visualization, a half-automatic adjusting method based on the optical model for volume rendering is introduced. According to the optical transfer-function model and the intensity distribution of 3D data, the interactive model is described by adjusting the RGB color and transparency values respectively with the help of the 3D gradient field. In order to make the method more effective, the interactive model and the texture feature of modern graphics are both applied to accelerate the process. The experimental results show that an optimal separating effect is achieved by adjusting the corresponding parameters in the transfer function model in real time. And the frame rate is up to 30 f/s in the interactive process. With this method, the optimal parameters are obtained in the interactive volume visualization.

The parameters of seawater, such as temperature can be detected based on the detection of Brillouin scattering signal. Because Brillouin frequency shift is a function of two variables, temperature and salinity, the precise measurement about the two independent variables by Brillouin frequency shift is impossible. It is found power of Brillouin scattering signal is also a function of those two variables, so the method to simultaneously measure temperature and salinity by measuring the frequency shift and power is proposed and the synchronous measurement about temperature and salinity is realized.

The reflection spectrum shape of the Bragg grating sensor will be distorted even with multi-peaks when there exists great strain gradient in the axis direction of the FBG, so it's meaningful for developing strain profile reconstruction method in structure health monitoring field. With the problems that noised reflection spectrum will decline the precision of strain reconstruction, a novel method based on the support vector regressin (SVR) preprocessing of noised reflection spectrum is proposed for the non-uniformly distributed strain profile reconstruction along the axis of Bragg grating. This method combines fitness sorting improved genetic algorithm with transform-matrix reflection spectrum construction. The reflection spectrum is viewed as a time serial and SVR is developed to constrain the noise for effective regression of reflection spectrum due to its global optimization and well generalization. Subsequently, the strain distribution is treated as a piecewise constant function, the spectral response is calculated using the transform-matrix approach, and then improved genetic algorithm is employed for reconstruction simultaneously. The simulations for strain profile reconstruction are conducted in various kinds of strain distributions. Results show that the SVR reflection spectrum regression is very effective and the proposed algorithm can improve precision of non-uniform strain distribution reconstruction.

In order to achieve all-round angle detection in laser warning with a certain accuracy, we proposed a method of measuring laser direction using geometrical projection. By calculating the shading area when laser projects on the board with given parameters, we got the current ratio from the different quadrants of the measurement, and the direction of the incident laser can be judged by the corresponding relation between projection area ratio and angle using the program for angle detection. For a minimum resolution of angle detection, the limit angles are calculated as (0°,90°), (33°, 21°), (33°,69°), (45°,90°) in Matlab, and the minimum voltage 0.05 V in experiment. The simulation calculation and the experimental data show that the design can achieve the detection requirements of the minimum identification of 1°.

For the large aperture telescope with segmented primary mirrors, a low-coherence-source interferometric detection system based on the Michelson interferometric principle was put forward to detect the phase error between the segmented mirrors, correct the misaligned segments and achieve the mirror coplanar. The concrete structure of the interferometric detection system with the low-coherence source was presented, and the detection principle of the interferometric detection system was narrated. Double-center-wavelength combination low-coherence-source light sources were applied to detecting the phase error of the segmented mirrors and the minimum signal-to-noise ratio was analyzed. The result shows that the application of the double-central-wavelength combination low-coherence source light sources system largely raises the ability of signal distinguishing of the center fringe of the interferograms, improves the measurement accuracy, and makes the low-coherence-source interferometric detection system extract the phase error with high precision.

Based on simulation of the impact of gamma nonlinearity of the projector on phase error, an algorithm is proposed to build a look-up table (LUT) for phase error compensation by analyzing the projector grating directly. In this algorithm, a set of grating images of a uniform flat surface board are captured and the relation of grating phase and phase error in established. Then the relation is analyzed and stored in a LUT for phase error compensation. Experimental results show this algorithm reduces greatly the phase error caused by the gamma nonlinearity of the projector. The measurement precision of the system reaches 0.043 mm, 5.6 times higher than that before error compensation.

A magneto-optical modulation method for optical polarization measurement is proposed based on rectangular wave signal. By means of computer simulation, Lissajou figure of the rectangular wave magneto-optical modulation method was investigated, and the principle and precision of this method were studied. The study indicates that, the Lissajou figure of the rectangular wave magneto-optical modulation method is made up of seceral discrete points. By observing whether the discrete points are superposed each other, it can be judged whether the analyzer is at extinction position, and then polarizaiton analysis can be made. In additon, if monitor whether the output signal wave is direct current with electronic instrument, automatical polarizaiton analysis can be performed. Compared with the frequency doubling method based on sine wave magneto-optical modulation, the rectangular wave magneto-optical modulation method possesses higher precision, and automatical measurement is easier to carry out.

A new spectral beam combination scheme based on cascaded volume Bragg gratings (VBG) was presented. The number of beams can be multipled with the number of cascaded gratings. A system as example based on two cacaded VBGs was given. All the beams were firstly divided into two groups according to their wavelength. Each group was then combined by a VBG with corresponding central wavelength. Because the angular selectivity of the two VBGs are not overlapped, all the beams can be steered to propagate in the direction of the Bragg angle of the second VBG. The power of all beams can then be added at the near and far fields. In order to assure the consistency of the propagating direction of all beams, a formula to accurately determine the incident angle of all beams was derived using rigorous coupled-wave theory. Numerical results show that the average diffraction efficiencies of all beams are higher than 80%, and the deviation of the propagating direction of all diffracted beams are less than 4×10-7 rad when the incident angle of all beams deviates from their theoretical value less than ±5′.

A laser diode (LD) side-pumped two rods quasi-continuous-wave (QCW) Nd∶YAG green laser has been demonstrated. The influence of thermal birefringence effect is analyzed. Considering the compensation, a heat-stablized cavity is designed. In the experiment, two sequent Nd∶YAG rods side-pumped by 30×20 W laser diodes array and type-Ⅱ critical phase-matched HATR-KTP crystal are used. The spot sizes on rod B as a function of pumping current before and after compensation are simulated. The green laser output power of 164 W has been measured at single laser diode pumping current 21.6 A, with the repetition rate of 27.2 kHz, pulse width of 130 ns, optical-to-optical conversion efficiency of 13.7%, beam quality factors of M2x=9.52, M2y=9.86, and output power fluctuation of 2.3%. The results show that the system can work at wide stable range after the compensation of the thermal birefringence effect.

In the process of the laser-matter interaction, when the density of the free electron reaches the damage threshold, the dielectrics will be broken down. Then, the subsequent part of the pulse energy will be strongly absorbed by the laser plasma, the light will be suspended, and this moment is defined as the cut-off time. The cut-off time makes a tremendous impact on the transmission of laser pulse energy. The effects of the input pulse energy on the cut-off time and the cut-off time on the pulse energy transmsstivity are discussed. The corresponding quantitative analysis and simulation are carried out, the simulation results and experimental results fit well. The pulse energy transmissivity is determined by the location of cut-off time. When the distribution of cut-off time at the pulse front edge, on the peak value and at the pulse back edge, the laser pulse energy transmittance will be less than 50%, equal to 50% and greater than 50% respectively. The on-line measurement of the laser pulse energy transmissivity also can be used for online inspection of damage of optical elements.

To reduce the negative influence of noise on the correction of camera lens distortion, a nonmetric calibration method for camera lens distortion is proposed. In this method, all curves distorted from straight lines are found out and closed by connecting their endpoints with straight-line segment, and the sum of squares of area of those closed curves is computed as distortion measurement whose minimum is determined with the aid of genetic optimistic algorithm, and thus the distortion parameters are gotten. Computation indicates that this distortion measurement offers preferable ability of anti-noise, and that genetic algorithm is able to avoid convergence to local minima. So root-mean-square (RMS) errors of rectification images are decreased. Experiment shows that RMS errors between the rectified images obtained by calibrating with each image and those obtained by calibrating with all images are small. Both computation and experiment demonstrate that this calibrating method is robust.

According to Maxwell equations and boundary conditions of electromagnetic field, the general dispersion equations are reduced for the guided modes in rectangular dielectric waveguides. The field distributions of the guided modes are numerically simulated for the waveguides with ordinary dielectric materials and the rectangular waveguides with left-handed materials (LHMs), respectively. The simulation results show that field distribution of the Ex22 mode is restricted mainly in the central part of waveguide with LHMs compared with that of Ex00 mode of the waveguide with dielectric materials. Based on Marcatili's method, the dispersion equation of surface guided modes in rectangular waveguides with LHMs is deduced, and the field distribution of lower-order modes is numerically simulated. The results show that energy of field mode is located mainly in the four angles and edges of waveguide with LHMs.

Intense room-temperature midinfrared luminescent emission has been observed in PbTe/CdTe quantum wells. A theoretical model is developed to study the spontaneous emission and optical gain of the new hetero-structure. In the model the discrete energy level is calculated based on the k·p envelope function method and finite-confinement-potential approximation. Strain effects and anisotropic band structure characteristics of PbTe are taken into account. The dependences of spontaneous emission spectra on intraband relaxation and carrier density are investigated. It is shown that the calculation results are in agreement with the experimental data of photoluminescence. For PbTe/CdTe quantum wells (QWs) with well width of 20 nm, two emission peaks are dominant in the spontaneous emission spectra, which blueshift with carrier density increasing while intraband relaxation is considered in calculation. The shift of ground state emission is from 372 meV to 397 meV and that of the first excited state emission is 15 meV, as the carrier density increases from 2×1017 cm-3 to 2.8×1018 cm-3. The shifts of transition energies are attributed to carrier-carrier and carrier-phonon scattering mechanisms. As compared to the PbTe bulk material, the PbTe/CdTe QWs have higher and broader optical gain, which make them a promising material for mid-infrared laser operating in continuous-wave mode at room temperature.

A method of improving image reconstruction quality for time-domain breast diffuse optical tomography is proposed based on panel detection and finite-difference method. A multi-level scheme is presented that uses discrete wavelet transform to reach different resolution levels of wavelet domain decomposition and in some degree to reduce the number of unknown optical parameters at coarse resolution levels and alleviate the ill-posedness of inverse problem. Compared with the traditional full-spatial domain voxel based algorithm, this method can efficiently improve the reconstruction quality and spatial resolution. Numerical simulation results illustrate that the spatial edge-to-edge resolution of the reconstructed image is 4 mm and the quantitativeness is significantly improved.

In order to study the changing law of the starch free radicals, the luminescence dynamics equation about the starch free radicals was set up based on the theory of biophotonics and the dynamics model of chemiluminescence. According to the relationship between luminous intensity and density of the free radicals, the annihilation function of the starch free radicals is obtained. By fitting the luminescence curve at different temperatures, the relative coefficient about the function is fund to be all above 0.98, which can describe the relaxation law about the free radicals accurately.

A new device is developed for real-time monitoring of laser beam quality and adaptive collimating of laser beam direction. Based on lens imaging principle, the device is composed of one CCD camera for two-dimensional imaging of laser spot and two piezoelectrically drived mirrors to correct laser beam shift by the principle of two points exactly defining a line in space. The adaptive control is performed through a home-made computer program using Lab View software. The system can collimate the beam direction in a resolution of 0.5 μrad and 1 Hz adjusting frequency, and correct the laser spot shift of period above 1 s.

In order to realize flat-top transmission property for frequency selective surface (FSS) structure, a novel composite element FSS composed of rectangle metal grid and conventional ring loop element is presented. By applying the spectral-domain approach, the novel free FSS screen is calculated and analyzed theoretically. As a result of the numerical analysis, it is shown that the flat-top passband of 8 GHz and sharper skirts, diplexing closely separated signal bands, for example, X- and Ku-band signals, can be obtained with the novel structures. In addition,the performance of double bands available can maintain stabe for different incidence angles.

Three kinds of artificial composite materials are involved: left-handed material (LHM), negative dielectric permittivity material (NDPM) and negative magnetic permeability material (NMPM). For the p- and s-polarizations, the properties of surface plasmon polaritons (SPPs) including the existence regions, dispersion relations and excitation of SPPs are investigated in detail in two three-layered asymmetric structures, i.e., dielectric/LHM/NDPM and dielectric/LHM/NMPM. It is found that the properties of SPPs in different frequency regions are strongly dependent on the parameters of composite materials, such as, the slab thickness and plasma frequency for NDPM or LHM. The possibility of exciting and observing SPPs is discussed by attenuated total reflection (ATR) technique. The ATR spectrum is also calculated for p- and s-polarizations cases at these interfaces.

By means of time-evolution operator, we have studied the emission spectrum of a Λ-type three-level atom interacting resonantly with two-mode entangled coherent-states cavity fields. The physical spectrum expression of radiation emitted by the atom is given. We have discussed the structure of the emission spectrum for an entangled coherent-states input. The results indicate that whether two lower levels are degeneracy or not, the emission spectra of Λ-type three-level atom show Rabi splitting when the average photon number of two-mode entangled coherent states cavity fields is very small. And intensity increases with entanglement of two-mode entangled coherent-states cavity fields. When two lower levels are degeneracy, the structure of the resonant spectra of the atomic system has many symmetric peaks if the average photon number of two-mode entangled coherent-states cavity fields is very small, and the structure of the resonant spectra of the atomic system has five symmetric peaks if the average photon number of two-mode entangled coherent-states cavity fields is very large. When two lower levels are not degeneracy, the structure of the resonant spectra of the atomic system has many symmetric peaks if the average photon number of two-mode entangled coherent-states cavity fields is very small, and the structure of the resonant spectra of the atomic system has ten symmetric peaks if the average photon number of two-mode entangled coherent-states cavity fields is very large. Two substantial differences exist between emission spectra of the atom system with two-mode entangled coherent states cavity fields and that with non-entangled coherent field. One is that, the stronger the entanglement of two-mode entangled coherent-states cavity fields is, the higher the intensity of Rabi peaks are, the other is that entangled coherent character induces the structure of symmetric multi-peak of the resonant spectra of the atomic system.

Visible and near infrared reflection spectroscopy (Vis-NIRS) was studied as a fast and promising techniques for weed detection. Two kinds of weeds [threeseeded mercury (Acalypha australis L.) and quatrefoil duckweed (Marsilea quardrifolia L.)] and two kinds of crops [soybean (Glycine mas) and corn (Zea mays)] were investigated. A total 120 samples (30 for each species) of the plant leaves were prepared. The spectra were acquired by an ASD Fieldspec handhel spectrometer. After data pre-processing and principle component analysis on spectra from 400 to 1000 nm, one influential outliner was detected. After full-cross calibration of the 79 samples using partial least squares regression, the remaining 40 samples were predicted by the established model. Correlation coefficient of prediction set was 0.987. 100% recognition rate was obtained based on artificial thresholds. So, Vis-NIR spectroscopy is an available alternative for recognizing crops from weeds.

Two-photon Dicke-narrowing selective reflection (TDNSR) spectroscopy is studied in a system of Λ-type three-level atoms confined between two dielectric walls. TDNSR lines are investigated for the cases of different thicknesses of the vapor film and different strengths of the pump field. Because of de-excited behavior induced by atom-wall collision, effect of slow atoms in a thin atomic vapor film and Doppler-free configuration, sub-Doppler lines can be obtained in many cases. It is also found that TDNSR lines change periodically with the ratio of L/λ (L is the thickness of the atomic film and λ is the wavelength of the probe field). TDNSR disappears for L=2mλ/4 (m is an integer) and revives a typical dispersion shape for 2mλ/4<L<(2m+2)λ/4 with an enhanced symmetry when L approaches to (2m+1)λ/4. As the pump field is exactly tuned to the transition 2〉-3〉, the fast variation of TDNSR near Δd=0 (Δd is the detuning of the probe field) indicates a dramatically group-velocity reduction of the probe field.

The performances of modern colour-difference formulae were investigated to evaluate very small colour difference at human eye’s threshold level based on object surface colours. A panel of 21 observers performed the visual assessment to 100 sample pairs using perceptibility method. The validity of the visual results was verified via the observer variation estimation and the comparison with the BFD's and Witt's published data. The detailed analysis indicates that CIEDE2000 recommended by CIE performs best and CIELAB the worst among the six modern colour-difference formulae, namely CIELAB, CMC, BFD, CIE94, LCD and CIEDE2000, for predicting very small colour difference at the level of human eye's threshold.