Seungtaek Kim

Mixed-color Illumination and Quick Optimum Search for Machine Vision

Quick search for optimal illumination using mixed color is proposed, and the possibility for use in industrial vision systems is considered. Multidimensional formulation of the optimum search methods is reviewed to use multiple inputs for various color sources and one output of image sharpness. The voltage inputs are used to adjust the intensity of each color source and the optimal illumination is found by the maximum of the sharpness. The optimum search methods reveal that the number of iterations for the optimal condition in mixed color illumination was under 0.1% the number required by equal section search. The searching paths and convergence were considered in both of the input coordinate and the color coordinate. The results show that the optimum search methods make it possible to find the optimal lighting conditions quickly and accurately.

A review of light intensity control and quick optimum search in machine vision

Optimum searching methods were applied to find optimal illumination of a single color to enhance the quality of a monochrome image and reduce searching time. The problem was defined as 1D searching optimization between light intensity and image sharpness. The image sharpness is the degree of distinctness and is a non-linear function of the input voltage, which can be used to adjust the light intensity. We considered conventional optimum search methods, such as steepest descent, conjugate gradient, Newtons method, bisection, and golden section. These derivative methods and direct searches were tested for a sample pattern using single color lights under coaxial illumination. The iteration for the optimal condition was 6.7% of full scanning in average. The test results show that the steepest descent and golden search are recommended. We checked the possibility of applying these search methods to automatic lighting in machine vision.

Effects of Current Modulation Conditions on the Chromaticity of Phosphor Converted (PC) White LEDs

For two well-known modulation methods, stepwise current modulation (SCM) and pulse width modulation (PWM), the effects of driving current modulation conditions on chromaticity were experimentally investigated in a white LED lighting system. For the experimental implementation of both SCM and PWM, a white LED lighting was fabricated using phosphor converted (PC) white light emitting diodes (LEDs) and a driving circuit module was developed. By using them, the variations of illuminance, color coordinates, and spectrum were evaluated under various forward current conditions. Through the analysis in color coordinates, yellow shift in SCM and blue shift in PWM were observed on chromaticity diagrams with increasing average driving current. In addition, in order to analyze color deviation quantitatively, color distance before and after current increase, and the correlated color temperature (CCT) were calculated. As a result, for the white LED lighting in both modulation conditions, the maximum difference in the calculated CCT was obtained close to 1000 K. It means that careful consideration is required to be taken in the design of illumination systems to avoid serious problems such industrial accidents.

Properties of Defective Regions Observed by Photoluminescence Imaging for GaN-Based Light-Emitting Diode Epi-Wafers

A photoluminescence (PL) imaging method using a vision camera was employed to inspect InGaN/GaN quantum-well light-emitting diode (LED) epi-wafers. The PL image revealed dark spot defective regions (DSDRs) as well as a spatial map of integrated PL intensity of the epi-wafer. The Shockley-Read-Hall (SRH) nonradiative recombination coefficient increased with the size of the DSDRs. The high nonradiative recombination rates of the DSDRs resulted in degradation of the optical properties of the LED chips fabricated at the defective regions. Abnormal current-voltage characteristics with large forward leakages were also observed for LED chips with DSDRs, which could be due to parallel resistances bypassing the junction and/or tunneling through defects in the active region. It was found that the SRH nonradiative recombination process was dominant in the voltage range where the forward leakage by tunneling was observed. The results indicated that the DSDRs observed by PL imaging of LED epi-wafers were high density SRH nonradiative recombination centers which could affect the optical and electrical properties of the LED chips, and PL imaging can be an inspection method for evaluation of the epi-wafers and estimation of properties of the LED chips before fabrication.

Optimal RGB Light-Mixing for Image Acquisition Using Random Search and Robust Parameter Design

Obtaining a fine image is one of the major issues in industrial vision, and light mixing techniques are one of the alternatives. Auto-lighting using a multiple color mixer requires iterative actions. Random search shows high efficiency in finding the optimal illumination. However, random search is one of the numerical algorithms to find local minimum, so the algorithm parameters affect the performance of auto-lighting. The relation between the light mixing and the image fineness is mathematically nonlinear, and it is difficult to tune the parameters reliably. This study proposes a method to determine reliable parameters in random search for optimal illumination in image inspection using a color mixer. The Taguchi method was applied to maximize the image fineness and minimize iterations. The parameters selected for Taguchi analysis were the initial voltage, initial variance, and convergence constant. An L 25 55 orthogonal array was constructed in consideration of the 5 parameters and 5 levels. The determined parameters were applied to retests, which showed fewer iterations and the acquired image was close to the best case.

Color mixing and random search for optimal illumination in machine vision

This study proposed how to find optimal illumination for industrial vision in short time using random search algorithm and multiple color light sources. The fineness of an image captured by a monochrome camera is varied by illumination and can be evaluated by image sharpness. The relation between the sharpness and the illumination is non-linear, so direct optimum methods are applicable to mix the multiple sources. Random search is one of the direct optimum methods and were derived from the sharpness as input and N driving voltages for N light sources. The random search was tested in an RGB mixer and reduced the number of iteration for optimal illumination compared with conventional equal step search.

Temperature Dependent Thermal Properties of a GaN‐based Laser Diode Analyzed by an Electrical Method

We report thermal properties of a high power GaN‐based laser diode depending on the ambient temperature, which are analyzed using forward voltage changes due to heat generation during a current injection. The increasing rate of junction temperature and the thermal resistance decrease with the ambient temperature rise. Decrease of resistivities of p‐doped layers at an elevated temperature and a thermoelectric effect at superlattice structures are considered as possible reasons for the temperature dependence.

The new insertion method of liquid powders using the electrode array for e-paper applications

In this paper, the new particle insertion method was proposed and experimentally demonstrated. In the proposed method, the electrode array was adopted to insert the liquid powders into each cell of the liquid powder display panel. The each electrode pattern size was 140 times 140 mum and the size of the electrode pattern array was fabricated on the glass wafer. The each square cell size of the base panel was 220 times 220 mum and the width of the rib on the panel was 20 mum. The voltage range applied to the electrode array was about from 0 to 600 V DC. To monitor the operation characteristics of the liquid powder under the various applied voltage conditions, the computer vision system was also used. The experimental results showed the strong capability of the noble method to manipulate the charged tiny particles for the mass production applications of the e-paper.

Photoluminescence and electroluminescence properties of GaN-based LED chips with defective regions at low excitation levels

Defective regions of LED epi-wafers and chip-wafers could be observed by a photoluminescence (PL) imaging method. As the defective regions contain high density nonradiative recombination centers, the LED chips with defective regions showed different optical properties from those of LEDs with no defective regions found by PL imaging. At low optical excitation power levels, the PL properties were influenced by nonradiative recombination dominantly. Similar phenomena were observed from electroluminescence (EL) properties at low electrical excitation levels. Research results on the similarity and difference between PL and EL properties of LED chips are presented.

Investigations on correlation between photoluminescence images of an LED epi-wafer and characteristics of LED chips

Photoluminescence (PL) imaging is employed in order to inspect InGaN/GaN LED epi-wafers. The image shows a map of integrated PL intensity over the wafer and dark spots with degraded luminescence properties. Dark spots with various sizes indicate areas with nonradiative defects showing that the nonradiative recombination coefficient increases with the size. The PL images are compared with data obtained from LED chips on the wafer after fabrication process. The characterization results for LED chips show that most of the chips fabricated on the dark spots have degraded properties. The result indicates that PL imaging of epi-wafers could be an inspection tool to predict properties of LED chips.