This work concerns measurements of the radiant intensity emitted by LEDs. The influence of selected factors and parameters on the final measurement result are discussed. The research was conducted using two type of detectors: light meter and CCD camera, to compare the degree of influence of these parameters depending on the measurement instrument used.
In this paper we describe our own construction of a tuneable light source based on a set of light emitting diodes covering the visible spectrum using a homogenizing rod instead commonly used low energy-efficient integrating spheres. The expected prime application of the source is a medical endoscopic system, however it is possible to use it also for other purposes requiring both multispectral operation and a tuneable white light source. We describe the construction of the source and include precise characterization of the output white light – distribution of CCT, Duv, Δu′ v ′ and colour rendering indexes (Ra, R9, Rf , Rg) of light in several planes located at various distances. The obtained results prove that our source is characterized by very good colour rendition according to the Ra and Rf method for various correlated colour temperatures (2700–6500) K. As an example of application images of the Macbeth colour chart registered with an RGB camera included in the laboratory measurement stand are presented. The obtained results prove that, after whole system calibration, this source can be used in many applications, where evaluation of objects requires precise analysis of their colour and multispectral procedures.
Thanks to a very high luminous efficacy of LED lamps (over 160 lm/W) they are the most preferred light sources in lighting applications today. The useful lifetime of LED modules exceeds 50,000 hours. Chromatic parameters of lamps making use of SSL (Solid State Lighting) have already equalled classic solutions, although they were noticeably worse not so long ago. High values of the Colour Rendering Index (CRI) and ease of control over the luminous flux cause that lamps with LEDs have become very attractive solutions. Today, the most important problem concerns LED drivers supplied from the 230 VAC mains. The lifetime of switched-mode converters, including electrolytic capacitors, is considerably shorter than that of LEDs. This paper discusses the features of alternative drivers for LED modules which are supplied directly from the 230 VAC mains and do not contain any electrolytic capacitors. In particular, power factor and efficiency of lamps with one or two LED strings are analysed and some hints concerning optimal design of such lamps are given. A unique feature of this work is a detailed analysis of harmonics contents in the supply current of such drivers, proving their conformity with the relevant standard. Finally, some problems associated with flicker resulting from the considered type of supply are mentioned.
The development of technology and design of light management systems remains dynamic. Among all the benefits offered by these systems, the most valuable might definitely be the possibility of saving energy consumption. Knowing the value of energy savings is the key factor that users need to know before deciding to use a lighting management system (the type of light management system). For this purpose, it is useful to simulate the operation of the lighting control system, for example in the DIALux program. Such simulation helps evaluate potential savings in electricity consumption using the proposed lighting control system. In the DIALux program, it is possible to change the luminous flux value of luminaires. In such a case, it becomes possible to semi-simulate the light management system’s operation as we don’t receive actual information on reducing installed power of the lighting system during reduction of the luminous flux value of luminaires. This article shows what type of technical data are important to use for the DIALux program to properly and accurately simulate light management systems and to receive accurate data on energy saving. It also presents the results of photometrical and electric parameter measurements (Φ – luminous flux, P – power, PF – power factor, THDi – total harmonic distortion of current). The article discusses the power control characteristics obtained on the basis of these measurements and explores the source of differences between simulation of energy saving calculations and real measured energy savings. An existing lighting control system installed in an office reception area was used to compare calculations with the real value of energy consumption reduction. The impact of electronic power and control systems on electrical network parameters is also an important problem mentioned in this article. It also explores the effect of power regulation of LED luminaires and LED modules on the value of the power factor and total harmonic distortion (current) value (THDi).
Designing, optimizing and analyzing optical systems as part of the implementation process into production of modern luminaires require using advanced simulation and computational methods. The progressive miniaturization of LED (light emitting diode) chips and growth in maximum luminance values, achieving up to 108 cd/m2, require constructing very accurate geometries of reflector and lens systems producing complex luminous intensity distributions while reducing discomfort glare levels. Currently, the design process cannot function without advanced simulation methods. Today’s simulation methods in the lighting technology offer very good results as far as relatively large conventional light sources such as halogen lamps, metal halide lamps and high pressure sodium lamps are concerned. Unfortunately, they often fail in the case of chip-on-board LED light sources whose luminous surface dimensions are increasingly often contained inside a cube of the side length below 1mm. With the high sensitivity of such small chips and lenses with dimensions ranging from a just a few to between 10 and 20 mm, which is presented in this paper, modern luminance distribution measurement methods, luminance modelling and ray tracing methods should be used to minimize any errors arising from incorrectly projecting the design in the final physical model. Also, very importantly, focus should be directed towards reducing a chance of making a mistake while collimating the position of the light source inside the optical system. The paper presents a novel simulation calculation method enriched with an analysis of optical system sensitivity to a light source position. The results of simulation calculations are compared with the results of laboratory measurements for corresponding systems.
In recent years, GaN-based light-emitting diode (LED) has been widely used in various applications, such as RGB lighting system, full-colour display and visible-light communication. However, the internal quantum efficiency (IQE) of green LEDs is significantly lower than that of other visible spectrum LED. This phenomenon is called “green gap”. This paper briefly describes the physical mechanism of the low IQE for InGaN/GaN multiple quantum well (MQW) green LED at first. The IQE of green LED is limited by the defects and the internal electric field in MQW. Subsequently, we discuss the recent progress in improving the IQE of green LED in detail. These strategies can be divided into two categories. Some of these methods were proposed to enhance crystal quality of InGaN/GaN MQW with high In composition and low density of defects by modifying the growth conditions. Other methods focused on increasing electron-hole wave function overlap by eliminating the polarization effect.
The article proves that common LEDs may act as photodetectors with limited sensitivity, if they are polarized with an appropriate reverse voltage. The measured responsivities are ranged between 0.002 and 0.156 A/W and they depend on the LED type. The only one exception are white (phosphorescent) LEDs which do not exhibit any photosensitivity. There have also shown that a bandwidth of LEDs, which were examined in a role of photodetectors, is of a few tens of MHz, which is an order of magnitude greater than their modulation bandwidth as transmitters. The reasons of the observed LEDs behaviour are explained theoretically. The obtained results are indicated that some of them may be used as both transmitters and receivers in the VLC links working in a bi-directional half-duplex mode.
In literature, it is known that a Light Emitting Diode (LED) could be used as a light sensor. It is also known that its emitted light spectrum and sensitivity spectrum can be partially overlapped. This work presents how commercial LEDs can be used as light emitters and simultaneously as sensors of the reflected portion of the light emitted by themselves. The realized devices present a unique characteristic: the transmitter and the receiver coincide spatially as they are the same device. This ensures the perfect overlapping between transmission and reception radiation lobes that could provide many benefits in several applications like as distance measurements or image sensors. Some simple electronic configurations that use LEDs as detectors of their own emitted light are presented. It has been also demonstrated how these LEDsTx-Rx can work as image sensors by acquiring an image of a simple test object, and how they can realize distance sensors with respect to other known techniques. Further advantages can be obtained by realizing LEDTx-Rx array in single integrated devices. With the realization of such devices, it will be also possible to experiment new constructive solutions for commonly used applications, without the need of using separate emitter and receiver.
The flat horizontal polymer loop thermosyphon with flexible transport lines is suggested and tested. The thermosyphon envelope consists of a polyamide composite with carbon based high thermal conductive micro-, nanofilaments and nanoparticles to increase its effective thermal conductivity up to 11 W/(m°C). Rectangular capillary mini grooves inside the evaporator and condenser of thermosyphon are used as a mean of heat transfer enhancement. The tested working fluid is R600. Thermosyphon evaporator and condenser are similar in design, have a long service life. In this paper three different methods (transient, quasi-stationary, and stationary) have been used to determine the thermophysical properties of polymer composites used as an envelope of thermosyphon, which make it possible to design a wide range of new heat transfer equipment. The results obtained contribute to establish the viability of using polymer thermosyphons for ground heat sinks (solar energy storage), gas-liquid heat exchanger applications involving seawater and other corrosive fluids, efficient cooling of superconductive magnets impregnated with epoxy/carbon composites to prevent wire movement, enhance stability, and diminish heat generation.
LED light must be cooled to avoid reaching a certain temperature. Two different 3D practical domains of LED light are modelled, (i) square aluminium plate with a cylindrical plate and an LED module (model I), (ii) the same provision of model I with 25 fins (model II). ANSYS 16.0 is used for solving the problem. Temperature distribution, junction temperature (Tj) and heat flux are estimated. Analyses are carried out for various ambient temperatures (Ta) and for different LED power dissipations (Q) to identify the safe operating conditions. In model I, it is found that 38% of working conditions go beyond the critical limit of Tj and it is reduced to 21.4% in model II. In model II, for low Ta of 30 and 40ºC with all Q considered in this analysis are safer. If Ta is between 30 and 80ºC, then Q must be maintained at 0.5 to 1.25 W. Beyond this, conditions are not safe.
We aimed to investigate whether educational activities in the form of guided tours through an exhibition change the appreciation of art when young experts (i.e. first-years students of artistic faculties) view contemporary art in a gallery. Participants viewed and assessed the artworks presented at the gallery twice – before and after taking part in a guided tour led by a gallery educator. The guide-led tour increased both understanding and ratings (the hedonic value) of the artworks, which is consistent with the “effort after meaning” hypothesis and also with the model of aesthetic appreciation and aesthetic judgments. Our results suggest that the reception of works of art by young experts is changed when they are under the influence of extensive contextual information.