A new switched-capacitor frequency modulated driver for light emitting diodes

A new type of drivers for light emitting diodes (LEDs) is introduced based on the switched-capacitor frequency modulation. In contrast to conventional constant dc current drivers, the current pulse is provided by this new switched-capacitor LED driver. In the present driver, the charging capacitor is charged and discharged through a LED and the current flow direction is controlled by a metal oxide semiconductor switch. The input current (and thus the LED brightness) is proportional to the switch clock frequency at relatively low frequencies and becomes saturated at relatively high frequencies. This new driver circuit is simple and robust and maintains high efficiency for a wide range of input powers. In addition, the dimming control is easily realized by modulating clock frequency. Finally, this LED driver consumes no dc current and thus provides inherent protection to LED in standby mode.     

Dynamic junction temperature measurement for high power light emitting diodes

Junction temperature of high power light emitting diodes (LEDs), which is crucial for the thermal management of solid-state lighting, needs to be measured accurately. In this paper, a dynamic junction temperature measurement system for LEDs was proposed and the calibration including instrument calibration and factor K calibration were presented. The influence of the fast switch time in dynamic junction temperature test was analyzed and measurement errors caused by sampling delay were quantified. To prove the accuracy of the present system, comparison experiment was conducted. It shows a good agreement between the experimental data and reference value. Experiments also show that the measurement accuracy of the instrument can be up to 0.1?°C, and the standard error of temperature measurement can be controlled within 1%.     

A computer vision system for the inspection of light emitting diodes

This paper outlines the development of a computer vision system to enhance classification criteria for the inspection of infrared light emitting diodes (LEDs). Infrared LEDs used for fibre optics and optical switches are presently classified solely on the basis of power output. This test is performed by focusing the light beam from the LED through a circular aperture onto a solar cell. The output current from the solar cell is translated into a power output reading for the LED. The approach is limited in that it provides no information about other characteristics such as the misalignment of the beam from the mechanical centre or the intensity distribution of the beam. Improved classification criteria and testing methods based on emitted light intensity distribution can be utilised in engineering revision, setting specifications, monitoring manufacturing problems and classification for different applications.     

Calibration of a wide-field frequency-domain fluorescence lifetime microscopy system using light emitting diodes as light sources

High brightness light emitting diodes are an inexpensive and versatile light source for wide‐field frequency‐domain fluorescence lifetime imaging microscopy. In this paper a full calibration of an LED based fluorescence lifetime imaging microscopy system is presented for the first time. A radio‐frequency generator was used for simultaneous modulation of light emitting diode (LED) intensity and the gain of an intensified charge coupled device (CCD) camera. A homodyne detection scheme was employed to measure the demodulation and phase shift of the emitted fluorescence, from which phase and modulation lifetimes were determined at each image pixel. The system was characterized both in terms of its sensitivity to measure short lifetimes (500 ps to 4 ns), and its capability to distinguish image features with small lifetime differences. Calibration measurements were performed in quenched solutions containing Rhodamine 6G dye and the results compared to several independent measurements performed with other measurement methodologies, including time correlated single photon counting, time gated detection, and acousto optical modulator (AOM) based modulation of excitation sources. Results are presented from measurements and simulations. The effects of limited signal‐to‐noise ratios, baseline drifts and calibration errors are discussed in detail. The implications of limited modulation bandwidth of high brightness, large area LED devices (～40 MHz for devices used here) are presented. The results show that phase lifetime measurements are robust down to sub ns levels, whereas modulation lifetimes are prone to errors even at large signal‐to‐noise ratios. Strategies for optimizing measurement fidelity are discussed. Application of the fluorescence lifetime imaging microscopy system is illustrated with examples from studies of molecular mixing in microfluidic devices and targeted drug delivery research.     

Bootstrap approach for estimating process quality yield with application to light emitting diodes

Process capability indices have been widely used by quality professionals for measuring process performance. Although process yield is the most common criterion used in the manufacturing industry for measuring process performance, a more advanced measurement formula Y_q, called quality yield index, has been proposed as an alternative measure of process performance. Quality yield can be viewed as the classical process yield minus the truncated expected relative process loss, within the specifications, which focuses on customer satisfaction. By taking customer loss into consideration, the advantage of using the quality-yield measure as process performance is that the formula can be applied to processes with arbitrary distributions. Unfortunately, statistical properties of the estimated Y_q are mathematically intractable. Therefore, capability testing cannot be performed. In this paper, a nonparametric but computer intensive method called bootstrap is used to obtain a lower confidence bound on quality yield for capability testing purposes. Simulation studies are conducted to examine the sampling distribution of the estimated Y_q. An application using the index Y_q for the light emitting diode manufacturing process is presented for illustration purposes.     

Traceable atomic force microscope based on monochromatic light interference

Atomic force microscope (AFM) is widely applied to the measurement of the micro-nano structures due to its three-dimensional spatial resolution of sub-nanometer. However, the height measurement traceability in the z-axis is complex to be implemented in conventional AFMs. In this paper, a traceable AFM is developed based on the monochromatic light interference (MLI) principle without probe calibration. The height change of the AFM's probe is directly detected by extracting the phase change of the MLI fringes on the probe tip with the Hilbert transform based phase extraction algorithm, and the three-dimensional surface topography is reconstructed with a surface recovery algorithm. The configuration of tracing to the wavelength of the monochromatic light in real-time further improves the measurement accuracy of the MLI-AFM. A prototype MLI-AFM is established to demonstrate its measurement accuracy enhancement.     

A light emitting diode based photoelectrochemical screener for distributed combinatorial materials discovery

Combinatorial approaches for targeted discovery of new materials require rapid screening systems to evaluate large numbers of new material compositions. High-throughput combinatorial materials discovery is a capital-intensive undertaking requiring sophisticated robotic sample preparation and rapid screening assays. A distributed approach to combinatorial materials discovery can achieve similar goals by increasing the breadth of participation and reducing the size of the capital investment. The discovery of new photoactive materials for solar fuels production demands a screening device to probe materials for electrochemical current production upon irradiation with visible light. We have developed a system that uses an array of pulsed light-emitting diodes (LEDs) synchronized with a two-electrode potentiostat that can measure the photoelectrochemical responses of combinatorial sample arrays deposited on conducting glass plates. Compared to raster scanning methods, this LED system trades spatial resolution for a substantial reduction in scan time.     

Note: design and characterization of an optical light source based on mixture of white and near-ultraviolet light emitting diode spectra

An optical light source based on a solid-state lighting technology is designed. Main components of the light source are a phosphor-converted white and a near-ultraviolet (near-UV) light emitting diodes (LEDs), the spectral power distributions (SPDs) of which are mixed using a fiber optic combiner. The near-UV LED is used for improving insufficient SPDs of the white LED at shorter wavelengths of the visible radiation. Stable direct current power supplies are also designed and used to operate each of the LED separately. Three steps of the driving current can be selected by means of serial resistors altered with a commutator at nominal current values of ～40%, ～50%, and ～69%. The light source can be used for many characteristic measurements within the scope of photometry and colorimetry.     

A stereovision model applied in bio‐micromanipulation system based on stereo light microscope

A bio‐micromanipulation system is designed for manipulating micro‐objects with a length scale of tens or hundreds of microns based on stereo light microscope. The world coordinate reconstruction of points on the surface of micro‐objects is an important goal for the micromanipulation. Traditional pinhole camera model is applied widely in macrocomputer vision. However, this model will output bad data with remarkable error if it is directly used to reconstruct three‐dimensional world coordinates for stereo light microscope. Therefore, a novel and improved pinhole camera model applied in bio‐micromanipulation system is proposed in this article. The new model is composed of binocular‐pinhole model and error‐correction model. The binocular‐pinhole model is used to output the basic world coordinates. The error‐correction model is used to correct the errors from the basic world coordinates and outputs the final high‐precision world coordinates. The results show that the new model achieves a precision of 0.01 mm in the X direction, 0.01 mm in the Y direction, and 0.015 mm in the Z direction within a maximum reconstruction distance of 4.1 mm in the X direction, 2.9 mm in the Y direction, and 2.25 mm in the Z direction, and that traditional pinhole camera model achieves a lower and unsatisfactory precision of about 0.1 mm.     

A light efficient optically sectioning microscope

We describe a simple method by which optically sectioned images may be obtained. The system geometry is similar to that of a tandem scanning microscope but a one-dimensional grid pattern is used rather than an array of pinholes. This produces a composite image consisting of an optically sectioned image superimposed on a conventional image. A blank sector on the disc is used to provide a wide-field image. Image subtraction yields the optically sectioned image in real time.     

Numerical apertures of light microscope objectives

The numerical aperture of light-microscope objectives is measured via the exit angle of the rear lens towards the image space, and the magnification of the objective. The method is reliable because of its simplicity and is independent of special instrumentation such as apertometers. Results from eight commercially available objectives indicate fair agreement of nominal data with measured data for the magnifications, but not for all numerical apertures.     

The tandem scanning reflected light microscope

Reflected light microscopy of biological material has been a very difficult task and many different but hardly successful attempts have been made to get usable images. The main reasons for this state are: Weak reflections from the biologically important structures in the object as against strong reflection at its surface; reflections at optical surfaces in the microscope which cause a deterioration of contrast; and the mixing together of reflections from many levels in the object so that the signal coming from the focussed-on level is lost in noise and d.c. components. We tried, and successfully, to reverse this situation and to make it possible for the signal to overwhelm spurious and scattered light using double, or, as we call it, tandem scanning. The object is illuminated only in small patches lying in one plane and moving across this plane, and only the light reflected from these illuminated patches is allowed to pass into the image plane and participate in image formation. The image consists of points which travel over the image plane and which are geometrical images of the illuminated patches in the focussed-on object plane. To ensure that only the light belonging to these geometrical images be allowed to enter the image, the image plane is covered with an opaque diaphragm having holes in locations exactly corresponding to the locations of the geometrical images of the illuminated object points; this diaphragm travels in concordance with the first scanning and so the complete image of the focussed-on object plane is formed successively. Both scans are performed by a single device, a Nipkow disc carrying in its annular periphery several ten thousands of holes arranged in Archimedean spirals. The disc is 100 mm in diameter and rotates about 100 rpm driven by an electric motor. On one side the disc is illuminated in a circle 18 mm in diameter, and the light transmitted through several hundred holes and reflected in a mirror system passes a microscope objective which forms the images of the disc holes in the object plane. The light reflected here passes through the same objective and mirror system (one mirror being a “limitlessly thin” beam splitter) to pass conjugate aperture holes on the observation side of the Nipkow disc. As the disc lies at the intermediate image plane of the objective lens on both its illuminating and observation sides, only light emanating from reflection or fluorescence in the plane of focus can contribute to the image. High contrast images of very thin focussed-on layers are thus formed. The practical arrangements are such that very large specimens can be examined: The specimens for this microscope need not themselves be microscopic.     

Quantitative characterization of the carbon/carbon composites components based on video of polarized light microscope

The components of carbon/carbon (C/C) composites have significant influence on the thermal and mechanical properties, so a quantitative characterization of component is necessary to study the microstructure of C/C composites, and further to improve the macroscopic properties of C/C composites. Considering the extinction crosses of the pyrocarbon matrix have significant moving features, the polarized light microscope (PLM) video is used to characterize C/C composites quantitatively because it contains sufficiently dynamic and structure information. Then the optical flow method is introduced to compute the optical flow field between the adjacent frames, and segment the components of C/C composites from PLM image by image processing. Meanwhile the matrix with different textures is re‐segmented by the length difference of motion vectors, and then the component fraction of each component and extinction angle of pyrocarbon matrix are calculated directly. Finally, the C/C composites are successfully characterized from three aspects of carbon fiber, pyrocarbon, and pores by a series of image processing operators based on PLM video, and the errors of component fractions are less than 15%.     

A switched supply tunable red-green-blue light emitting diode driver

This paper presents a new switched supply tunable red-green-blue (RGB) light emitting diode (LED) driver. The RGB LEDs act not only as light emitting devices but also as rectifying diodes in the presented driver circuit. The RGB LED color control is realized by controlling the switched supply voltage amplitude, frequency, and duty cycle. The driver efficiency is high since the only loss in the driver circuit is the switch and can be further reduced by direct alternate current supply.     

Photogrammetric calibration of a stereo light microscope

Quantitative stereo light microscopy has been used in very rare cases to obtain geometric representations of microscopic objects. Although the instrument itself has limitations, most of them efficiently overcome by other 3D microscopes, it bears considerable advantages in observing and measuring dynamic scenes with multiple objects. The biggest asset of the stereo method is that the full 3D work space is imaged in one shot - a property which distinguishes stereo from all the scanning techniques in 3D microscopy. With the work presented in this paper, we contribute to making the stereo technique applicable to the numerous mensuration tasks in the microscopic domain, where its potential would be invaluable.We report the photogrammetric calibration of a common main objective lens type stereo light microscope. Such a calibration is the initial step in ensuring accurate measurements with this instrument. First, we derive a mathematical formulation of the imaging function and discuss the estimation of the parameters involved. Then, three main problems of the practical implementation of the framework are addressed: the finding of a calibration standard, the automatic measuring of the many image coordinates required, and the stabilization of the parameter estimation. Finally, results of various calibration runs are presented and analysed under different aspects. Among these, the most important is the accuracy of the calibrated instrument in measuring 3D positions and positional relationships. With a Zeiss Stemi 11, Achromat 1.6 x we achieve accuracies of 1 per thousand laterally and 1-2% axially relative to the volume of the work space. On the highest magnification level this corresponds to 700 nm and 1.8 &mgr;m, respectively.     

Junction temperature measurement of light emitting diode by electroluminescence

Junction temperature (JT) is a key parameter of the performance and lifetime of light emitting diodes (LEDs). In this paper, a mobile instrument system has been developed for the non-contact measurement of JTs of LED under LabVIEW control. The electroluminescence (EL) peak shift of the LED is explored to measure the JT. Commercially available high power blue LEDs are measured. A linear relation between emission peak shift and JT is found. The accuracy of the JT is about 1 °C determined by the precision of the emission peak shift, ±0.03 nm, at 3σ standard deviation for blue LED. Using this system, on-line temperature rise curves of LED lamps are determined.     

Quantitative characterization of carbon/carbon composites matrix texture based on image analysis using polarized light microscope

A quantitative characteristic method was proposed for characterizing the matrix texture of carbon/carbon(C/C) composites, which determined the mechanical and physical properties of C/C composites. Based on the cloud theory that was commonly used for uncertain reasoning and the transformation between quantitative and qualitative characterization, so the relationship between the extinction angle and texture types was built by the cloud models for describing the texture of microstructure, moreover, linguistic controllers were established to analyze the matrix texture in accordance with the features of the polarized light microscope (PLM) image. On this basis, the extinction angle could be calculated from the PLM image of the C/C composites. In contrast to the results of measurement, the errors between calculative values and measured values were maintained 1–2° in basically. Meanwhile, the PLM image of C/C composites was segmented by the component, in particular, the matrix with mixed textures was further segmented by the difference of texture. It means that the quantitative characterization of C/C composites matrix based on single PLM image has been realized. Microsc. Res. Tech. 78:908–917, 2015. © 2015 Wiley Periodicals, Inc.