LEDs for modern top quality informational systems: problems, theory and reality
The article is the result of long research in photometric laboratory of LEDs from various manufacturers throughout the world. Since LED selection for such critical application as video screens is one of the main factors that determine commercial success of the project, the article has great theoretical as well as practical merits.
Full version of this article is available only in Russian version of the magazine.
Summary
The technology for production of optoelectronic devices based on semiconductor light-emitting chips has been developing intensively. As a result, these devices are widely used in informational and light indication systems. LEDs boast a wide color range, combine powerful irradiation with any shape of spatial distribution, may provide any color gradation in a wide dynamic range of intensity that make them ideal light source for numerous applications.
Such wide variety of LED parameters was made possible after solving several technical problems in LED structure. This article provides analysis of LED and chip structure, discusses test results and future tendencies in LED production.
Semiconductor light sources
Humanity was always concerned with a problem of searching for fire and light sources. This article deals with the most sophisticated method of extracting light from stone. All modern semiconductor light sources are based p-n transition that radiate light quantums. Here we will focus only on those ways of creating p-n transition that can emit quantum of electromagnetic radiation when electrical current flows through them.
These are hetero structures with wide areas p-n transition where the width of restricted zone exceeds 1.9 eV. Already structures were created that can irradiate in the whole of visible range, close IR and UV ranges. A wide color range combined with powerful irradiation and any shape of spatial distribution make LEDs an extremely attractive light source with many potential applications.
LEDs
LED is a semiconductor based on radiating crystal that transforms electrical energy into light. LED radiates within a relatively narrow spectrum (up to 25-30 nm) on the scale of spectral distribution of luminous density and therefore can be characterized as a quasi monochrome radiation.
A tremendous variety of LEDs were created on the basis of these semiconductor crystals with p-n transitions. The LED structure determines direction, spatial distribution, luminous intensity, electrical, thermal, energetic and other parameters of radiation from a semiconductor crystal. Naturally, all these parameters are interconnected.
Detailed laboratory study of LEDs of different shapes and purposes from various manufacturers allowed making some important conclusions about quality and areas of LED applications.
Lately, LEDs are progressively used in light fixtures, artistic illumination, critically essential light indicators. This has become possible due to a fast increase in energetic parameters, reliability and service life of these quasi monochrome light sources. The factors that make LED irreplaceable for use in full color screens, signs and other informational devices include low energy consumption, low cost of optics that shapes light distribution diagram and easy control. However the essential factor is the visual perception of LED light by human eye. All these factors will be discussed in great detail in this article.
Theory: Light, technical and electrical characteristics of modern LEDs from various manufacturers
The most common unit that characterizes LED energy parameters is the axis light emission. However this value has no meaning unless the radiation angle is specified relative to a certain level from axis light emission maximum.
Typically, the radiation angle is correlated to a half of the maximum light emission. These two parameters: radiation angle and axial light emission give us approximate idea in what direction and how strong shall be light emission at different viewing angles. To more accurately determine luminous intensity at any observation angle, a double-coordinate ratio is used. It is also frequently called index ellipsoid or indicator function (see Fig. 1).
Fig. 1 Index ellipsoid for LED with oval lens in polar coordinates.
The vertical (smaller angle) and horizontal (larger angle) radiation planes are shown.
The luminous intensity is an important energy characteristic of LED radiation. It is determined as an integral of all energy under the special radiation indicator function. It is this parameter that is most commonly shown by LED manufacturers in their datasheets. This is especially true about powerful devices with wide radiation angle and uniform spatial distribution close to the Lambert distribution. However, even in this case it is impossible to reliably evaluate distribution of light flow inside the diagram and, consequently, determine the LED luminous intensity accurately.
Most simple mathematical formulae used by LED users are absolutely incorrect and lead to serious mistakes in evaluating energy characteristics in LED-based appliances. This is evident in calculating nonsymmetrical radiation distribution diagrams (e.g. LEDs with oval lenses) and indicator function of narrow angle LEDs. Therefore, it is worth looking into the matter closely and consider some methods for determining light emission and its relation with other photometric units because only direct measurement of this value may result in its greater accuracy.
The classical experiment in measuring full light flow using spherical integrator calls for locating light sources at the center of the sphere. But even in this case the relation between the standard lumen is doubtful due to errors caused by non-uniform spectral and zonal characteristics of internal surface of the sphere. Therefore, the most accurate and informative data can be collected using a method of spatial luminous flux scanning, or goniophotometric method.
The instruments needed for this are goniophotometer with sufficient resolution and photometric head with a measured transformation coefficient. The method is based on a gradual rotation of LED and measuring its luminous intensity at various preset angles. The smaller the pitch of the rotation against photometer, the lower is the measurement error and the more accurate is the angular distribution. Modern goniophotometric devices usually have a pitch of 3 to 10 minutes. Simultaneously the axial luminous intensity and its spatial distribution are measured. This is the way to calculate the flux flow.
LEDs are described in both energy and colorimetric parameters. To know these parameters is essential for proper color rendering in any electronic informational system, light and indicator devices, architectural lights, etc.
The CIE (International Committee on Illumination) 1931 XYZ color space, developed in 1931, is still used as a standard reference for defining colors, and as a reference for other color spaces (Fig. 3). As was already said, LEDs have rather narrow band (quasi-monochrome) of radiation with half width spectrum within only 15-30 nm. The CIE chromaticity diagram, derived from this color space, is shown below. The diagram is a two-dimensional plot of colors of constant intensity based on the visual response of the CIE-1931 standard observer, which was determined by physiological measurements of human color vision.
Since the human eye has three types of color sensor that respond to different ranges of wavelengths, a full plot of all visible colors is a three-dimensional figure. This is inconvenient to draw on a two-dimensional sheet of paper, so for convenience the CIE transformed the three-dimensional color space into two artificial dimensions of color (collectively called chromaticity) and one of intensity, and then took a two-dimensional slice through this space at the level of maximum intensity. This slice became the chromaticity diagram.
It should be noted that all characteristics of LED described above are in the direct interdependence, therefore, as a rule, only their totality makes it possible to judge correctly various parameters of LED. However, to determine most accurately the quality, durability and correspondence of LED parameters declared by producer is possible only after conducting the complex of measurements and calculations of its characteristics.
