Performance of a range gated system is strongly affected by the laser, sensor, target, and atmospheric parameters. This paper performs a theoretical analysis to investigate the influence of multiple factors on range gated reconstruction. The effects of several factors are discussed based on the operating principle of range gated reconstruction, fundamental of radiant energy, signal to noise ratio (SNR), and bidirectional reflection distribution function (BRDF) models. The presented findings establish a comprehensive understanding of the influence factors in range gated reconstruction which are of interest to various applications and future improvement works to perform accurate range correction and compensation.
Recently, multi-layer surface profiling and inspection has been considered an emerging topic that can be used to solve various manufacturing inspection problems, such as graded index lenses, TSV (Thru-Silicon Via), and optical coating. In our study, we proposed a gated wavefront sensing approach to estimate the multi-layer surface profile. In this paper, we set up an experimental platform to validate our theoretical models and methods. Our test bed consists of pulse laser, collimator, prism, well-defined focusing lens, testing specimen, and gated wavefront sensing assembly (e.g., lenslet and gated camera). Typical wavefront measurement steps are carried out for the gated system, except the reflectance is timed against its time of flight as well as its intensity profile. By synchronizing the laser pulses to the camera gate time, it is possible to discriminate a multi-layer wavefront from its neighbouring discrete layer reflections.
General precursors and growth model of Laser Induced Damage (LID) have been the focus of research in fused silica material, such as polishing residues, fractures, and contaminations. Assuming the absorption due to trapped material and mechanical strength is the same across the surfaces, various studies have shown that the LID could be minimized by reducing the light field intensification of the layers upon the laser strikes. By revisiting the definition of non-ionising radiation damage, this paper presents the modelling work and simulation of light intensification of laser induced damage condition. Our contribution is to predict the LID growth that take into various factors, specifically on the light intensification problem. The light intensification problem is a function of the inter-layer or intra-layer micro-optical properties, such as transmittance and absorption coefficient of the material at micro- or sub-micro-meter range. The proposed model will first estimate the light propagation that convoluted with the multiply scattering light and subsequently the field intensification within the nodule dimension. This will allow us to evaluate the geometrical factor of the nodule effect over the intensification. The result show that the light intensification is higher whenever the backscattering and multiple scattering components are higher due to its interference with the incoming wave within its coherency.
Range gated imaging is a remote sensing acquisition which involves the emission of a laser pulse and an intensified
camera to gate the reflected laser pulse. Range accuracy has always been an issue especially when a highly accurate
reconstructed model is expected as the final outcome. The reflected pulse profile and pulse instability are among the
issues that affect the range accuracy when a general solution such as constant offset is not applicable. In this paper, a
study to estimate a more accurate model for the reflected pulse profile has been investigated through experiments. T
Location-Scale model has been proposed to replace the Gaussian model as the general assumption for range-gated image
formation model. The improvement on range accuracy which is around 0.3% has been verified through simulation based
on the acquired samples. The series of range-gated images can be reconstructed into a three-dimensional depth map
through range calculation. This can be used in the subsequent range reconstruction works.
Proc. SPIE. 8845, Ultrafast Imaging and Spectroscopy
KEYWORDS: Visible radiation, Digital signal processing, Light emitting diodes, Data transmission, Multimedia, Data communications, System integration, Computer architecture, Systems modeling, Standards development
Visible light communication (VLC) technology has attained its attention in both academic and industry lately. It is determined by the development of light emitting diode (LED) technology for solid-state lighting (SSL).It has great potential to gradually replace radio frequency (RF) wireless technology because it offers unregulated and unlicensed bandwidth to withstand future demand of indoor wireless access to real–time bandwidth-demanding applications. However, it was found to provide intrusive uplink channel that give rise to unpleasant irradiance from the user device which could interfere with the downlink channel of VLC and hence limit mobility to users as a result of small coverage (field of view of VLC).To address this potential problem, a Hybrid VLC system which integrates VLC (for downlink) and RF (for uplink) technology is proposed. It offers a non-intrusive RF back channel that provides high throughput VLC and maintains durability with conventional RF devices. To deploy Hybrid VLC system in the market, it must be energy and cost saving to attain its equivalent economical advantage by comparing to existing architecture that employs fluorescent or LED lights with RF technology. In this paper, performance evaluation on the proposed hybrid system was carried out in terms of device cost and power consumption against data throughput. Based on our simulation, Hybrid VLC system was found to reduce device cost by 3% and power consumption by 68% when compares to fluorescent lights with RF technology. Nevertheless, when it is compared to LED lights with RF technology, our proposed hybrid system is found to achieve device cost saving as high as 47% and reduced power consumption by 49%. Such promising results have demonstrated that Hybrid VLC system is a feasible solution and has paved the way for greater cost saving and energy efficient compares with the current RF architecture even with the increasing requirement of indoor area coverage.
Recently, semiconductor manufacturers have been striving for high speed, large scale multi-layer wafer surface measurement. In this paper, we propose a novel technique in multi-layer wave-front sensing. The measurement uses a gated camera in pico second shutter that can be synchronized to a pico second laser pulse, up to μm accuracy. Subsequently, we propose a compensation technique using time-of-flight wave-front sensing to reconstruct the multilayer surfaces using our proposed gated imaging technique.
Range-gated imaging can improve the signal to backscattering noise ratio (SBR) in turbid media. This is achieved by synchronizing a short duration, high intensity pulse with precise camera gating. It is well known that shorter pulse length and shorter camera gate duration can enhance the SBR. However, there is no analytical model of the backscattering noise (as a function of the pulse length and gate timing) that can be used to minimize backscattering noise within the camera-captured signal. We propose a formulation (a modification of Falk's lidar equation) that models the backscattering noise as a convolution with a fixed upper limit. This formulation predicts a variation of backscattering noise within the returning signal. In particular, the model predicts higher SBR toward the tail region of the target-reflected irradiance. It confirms the experimental results reported by other authors. Additionally, the model explains experimentally observed SBR improvement for shorter pulses and shorter gating intervals (if adequately positioned within the returning pulse).
In order to improve underwater visibility, general considerations are planned in two stages. There is hardware upgrading followed by system optimization stage. For the former, we choose to improve the underwater visibility with advanced techniques: range gated imaging system, and the optimization in terms of image processing techniques. Four selected image enhancement technique has been tested, namely Contrast Stretching, CLAHE, Illumination-reflectance Model and Homomorphic Filtering. Quantitative image quality measures are used to evaluate the enhanced imaging techniques. Three image assessment techniques are used to quantify image quality of the imaging system in increased turbidity condition, namely Modified Fidelity (MF), Modified Strehl Ratio (MSR2), and Contrast-to-Noise Ratio (CNR). In the first stage, the quantitative measures have shown at least 40% improvement from non-gated to gated images in increased turbidity. Finally, the enhancement techniques further improve the gated images with limited noise amplification issues.
Images associated with Underwater Imaging Systems are frequently degraded due to absorption and scattering effects from its underwater environment. The absorption effect reduces the signal strength, and the latter effect reduces both signal strength and image resolution. The optimization of underwater imaging system parameters predominantly focuses on maximizing signal strength and minimizes scattering effects. In the domain of underwater images, the assessment of image quality is highly subjective and lacks the availability of a "standard" objective criteria, which allows a comparison of different imaging techniques and their effectiveness to be performed in a more objective manner. This paper focuses on an experimental performance evaluation of underwater imaging system through objective image quality measurement. The technique is based on 2 dimensional grayscale image of USAF (United State Air Force) target. These targets have been used extensively in underwater imaging system development. It has resolution bars in various frequencies and arrangement, which enable spatial frequencies and signal strength analysis. 3 different image evaluation techniques are used to quantify image quality of an Underwater Imaging System in increased turbidity condition. Peak Signal to Noise Ratio (PSNR) measures the output signal over its Mean Square Error (MSE). Fidelity, F, represents the absolute difference of amplitude values and the Correlation coefficient, r, reflects only changes of signal shape. The metrics scales are considered as a measure of similarity between the idealized and the derived image. In these techniques, the indices are applied to corresponding pixels of the derived and ideal images. The fidelity, F and correlation coefficient, r are applied to thermal imaging evaluation by Volodymyr Borovytsky and Valery Fesenko in 2000. The underwater images have similar characteristics to that from thermal imaging system. There is a significant component of random noise effects (from background or environments) degrading the original signal to be captured by the camera. Test conditions and processes may also induce unwanted errors or distortions into the measurement of the image quality. These include camera movement, light intensity and lens zoom. The sensitivity of some of these effects on the indexes are also presented. Some modifications of the evaluation techniques are proposed. The Modified Fidelity, MF is linearly correlated with the image quality (scattering effects) of the testing images compared to the other methods. Hence, MF is a suitable measure to quantify the scattering effect of underwater images.