Photographs of the lights seen from outer space at night are a valuable source of freely available online information of city light distribution. Here we take advantage of recently available imagery from the International Space Station (ISS), which provides night-time high resolution images of most cities at Earth. We perform a multifractal analysis of night lights patterns of some of the largest and most populous cities in the World, searching for valuable information of city light spatial distribution. We calculate the fractal dimension, multifractal spectral width, and multifractal spectral range. Then we correlate these fractal parameters with three city characteristics: light pollution, crime index, and quality of life index. And we find a clear relationship between multifractality and the index of quality of life.
To date, many studies on the optimization of the spectral power distributions (SPDs) have been conducted to maximize the energy performance and color quality of white light-emitting diodes (LEDs). Here, not only for LEDs but for any light source, we numerically calculate the SPDs which maximize luminous efficacy of radiation (LER) in function of the color rendering index and of color temperature. As previously reported in the literature, we obtain optimal SPDs that are discrete spectra rather than continuous, but with considerably higher LER values. Such optimal SPDs may be used to design various lighting technologies: incandescent, fluorescent, high-intensity discharge, laser, and solid-state lighting.
A compact encapsulating lens is proposed and analyzed, which simultaneously collimates and mixes the tunable light from red, green and blue (RGB) light-emitting diode (LED) chips. Colored LED chips are embedded within a spherical lens, and a section of the sphere that surrounds chips is mirrored with a diffuse reflector. Color light is mixed by multiple total internal reflections (TIRs), the scattering mirror breaks TIR, and light escapes only in a narrow beam in the forward direction. The color distribution, the beam pattern, and efficiency are analyzed by Monte Carlo ray tracing calculations.
A novel light luminaire is proposed and experimentally analyzed, which efficiently mixes and projects the tunable light from red, green and blue (RGB) light-emitting diodes (LEDs). Simultaneous light collimation and color mixing is a challenging task because most collimators separate colors, and most color mixers spread the light beam. We performed an experimental study to find a balance between optical efficiency and color uniformity by changing light recycling and color mixing.
Solar energy systems use concentrating optics with photovoltaic cells for optimizing the performance. Advanced
concentrators are designed to maximize both the light collection and the spatial uniformity of radiation. This is important
because irradiance uniformity is critical for all types of photovoltaic cells. This is difficult to achieve with traditional
concentrators, which are built with polished optical surfaces. In this work we propose a new concept of solar
concentrator which uses small diffuser segments in key points to increase the irradiation uniformity. We experimentally
demonstrate this new concept by analyzing the effects on both efficiency and irradiance uniformity due to the
incorporation of scattering ribbons in a compound parabolic concentrator.
There is an enormous range of possible color distributions that may be created with a light cone when the primary source
is an array of multicolor light-emitting diodes (LEDs). If one looks through a lightpipe toward an LED array, multiple
images of the color LEDs can be observed as in a kaleidoscope. A tapered lightpipe behaves as a three-dimensional
kaleidoscope, and then, by changing the position and orientation of the red-green-blue LEDs can produce a plenty of
amazing illumination patterns. We analytically calculate this color spatial distribution of the illumination pattern
produced by a tapered lightpipe. Moreover, we simulate these color illumination patterns, and analyze their structure and
The output power of a light-emitting diode (LED) not only is affected by aging but also by dirt buildup. Environment
and surroundings are typically characterized by the presence of substances, dust, liquids or vapors that may stick to the
LED, reducing its light output. Knowing the effect of dirt on light output, manufacturers and users can efficiently design
a cleaning or maintenance program. In this work, both 5-mm LEDs and high-power LEDs were subjected to output
power tests for different degrees and types of dirt. In particular, I measure the light flux changes due to deposition of
dust (sand), drops of water, coal dust, oil drops, fat (soldering paste), and fingerprints.
An array of spatially distributed LEDs can produce a desired illumination pattern by individually modulating each LED.
A target image can be the desired lighting pattern so that the software could find the best solution to match it. Given a
desired illuminance distribution on a target surface, the luminous flux of each single LED that most closely matches the
desired distribution must be determined. We review a constrained least squares method for this problem. We show how
the quality of the rendering depends on the number of LEDs, array-target distance, and the size of the illuminated area.
In particular, as we observed, there is an optimum illumination distance, which is proportional to the square root of the
target size and varies inversely with a power of the number of LEDs.
Any cluster of light-emitting diodes (LEDs) can be modeled as a directional point source if the far-zone condition is met.
A general condition is derived for the distance beyond which the
far-zone approximation can be used in measuring or
modeling propagation of light from an LED array. A simple equation gives the far-field condition in function of
parameters of influence, such as LED radiation pattern, array geometry, and number of LEDs. We calculate the nearzone
extension of clusters with LED radiation patterns of practical interest; for example Lambertian-type, batwing, and
side emitting. The far-field condition is shown to be considerable shorter for high packaging density LED arrays.
Moreover, the far-field dramatically changes in function of the beam divergence of the LED radiation pattern. For
example, the near-zone of a square LED array with highly directional LEDs (small half-intensity viewing angle) can
extend to more than 70 times the cluster size. This value is far from the classical rule of thumb (5 times the source size).
Since its beginnings, light-emitting diode (LED) has progressed toward greater performance. Today, LEDs are
everywhere, in many shapes, and with a wide range of radiation patterns. We propose a general analytic representation
for the angular intensity distribution of the light emitted from an LED. The radiation pattern equation is determined by
adding a Gaussian or a power cosine expression for contributions from the emitting surfaces (chip, chip arrays, or for
some cases a phosphor surface), and the light redirected by the reflecting cup and the encapsulating lens. Mathematically,
the pattern is described as a sum of Gaussian or of cosine-power functions. The resulting equation is widely applicable
for any kind of LED of practical interest. We successfully model the radiation patterns from several manufacturer
A direct or bottom LED backlight is a key concept in large area LCD displays because it does not use a light guide, is
flat, and is easy to assemble. In this paper, a method of luminance management for a bottom LED backlight is proposed
and demonstrated. We analytically calculate both the power consumption and brightness uniformity in function of: screen
brightness, screen size, backlight thickness, transmittance of the LCD panel, reflective cavity efficiency, gain and cone
angle of enhancement films, LED array configuration, and the average luminous flux and radiation pattern of a single LED.
Moreover, a 42-inch LCD television with this backlight design approach is made and demonstrated. The bottom backlight
incorporates an array of RGGB 4-in-1 multi-chip LEDs within a highly reflective box behind a diffuser and a dual
brightness enhancement film. We predict with an accuracy of 94% the brightness uniformity and with 96% the luminance
Light-emitting diodes (LEDs) can be chosen to emit light in a wide variety of highly saturated colors. As a consequence,
a hybrid lamp assembled with colored LEDs and with one incandescent or fluorescent source easily allows the user to
dynamically select the desired color point without additional filters, with high color rendering index, and at a low cost.
We measure some properties of a color tunable lamp that uses both colored LEDs and an incandescent or a fluorescent
source. For the LED-incandescent type, we assemble an array of blue LEDs with a typical incandescent bulb source, and
to assemble a LED-fluorescent type we used an array of red LEDs with a commercially available compact fluorescent
lamp. Incandescent and fluorescent sources have a fixed intensity, while LED intensities are adjusted to tune color. For
LED-incandescent lamps, our experimental data show that the correlated color temperature (CCT) can be linearly tuned
with the electric current of the LED array. The LED-fluorescent lamp exhibits a CCT that exponentially varies with the
drive current of red LEDs.
An array of light-emitting diodes (LEDs) assembled upon a spherical surface can produce a wider angle distribution of light than a typical array (i.e., an array assembled by mounting LEDs into a flat surface). Arranging each single LED into an optimal placement, the uniformity of the illumination of a target can be improved. We derive approximate formulas and equations for the optimum LED-to-LED angular spacing of several spherical arrangements for uniform far-field irradiance. These design conditions are compact and simple tools that incorporate an explicit dependence on the half-intensity viewing angle (half width half maximum angle) of LEDs.
The remarkable properties of light-emitting diodes (LEDs) make them ideal sources for many applications ranging from indicator lights, optical communication systems, and displays to solid-state lighting. In this paper a radiometric approach to realistically model the intensity spatial distribution of encapsulated LEDs is presented. We provide an analytical relationship between the radiated pattern and the main LED parameters (chip, encapsulant, and reflector).
Light-Emitting Diodes (LEDs) have matured to the point that they can be considered to replace the inefficient and short life incandescent lamps in many lighting applications. Though modern high power LEDs produce up 120 lumens per device, several individual LEDs must be mounted on panels to obtain practical powers. In this paper we analyze, by considering each single LED as an imperfect Lambertian emitter, the first order design of a lamp consisting of several LEDs assembled upon a spherical surface to uniformly illuminate far targets. Practical formulas are derived for the optimum LED-to-LED spacing, i.e., the optimum packaging density, of ring array configurations to achieve uniform far-field irradiance.
We analyze the effects on color uniformity of the near-field light distribution due to different cluster configurations (at optimum packaging density for uniform irradiance) of light sources using mixed red, green and blue (RGB) light emitting diodes (LEDs). A photometric analysis and experimental results that show the near-field performance that can be achieved with several cluster configurations of multicolor LEDs is presented. Contour maps for the color variation (in reference to illuminant D<sub>65</sub>) in function of spatial coordinates of light distribution are given.
It has recently been proposed a new application of optical thin films as one-dimensional filters for spatial frequencies (Opt. Lett. 30, p. 914). Some possible applications include detection of extrasolar planets, high-sensitivity angle sensors, and beam-splitter cubes for special purposes. In this paper, the performance of thin-film spatial filters toward changes in wavelength is studied to characterize their behavior in optical systems that involve polychromatic light. We show how the performance of these devices under white-light illumination is influenced by the selection of design parameters.
The unique feature of color variability in light emitting diode (LED) sources made of red, green, and blue LEDs (RGB-LEDs) allows the user to select the desired color point of the lamp. The highest color uniformity is obtained using LED clusters with high-density packaging. However, packaging density of LED arrays is limited by cost, available space, and particularly by thermal problems. This paper presents an investigation of the effects on color uniformity of illumination due to different cluster configurations and packaging density of RGB-LEDs. We present a photometric analysis and experimental results that show the performance that can be achieved with a number of different cluster configurations of LEDs.
Thin-film optical filters selectively absorb, transmit and reflect certain parts of the electromagnetic spectrum. We propose and analyze a new concept of thin-film filter that directly and selectively transmits and reflects certain parts of the spectrum of spatial frequencies. The transmittance and reflectance are short-pass functions or long-pass functions of the angle of incidence. We discuss optical filters designed with dielectric thin films between two right angle prisms to selectively cancel a reflected or transmitted plane wave front for different angles of incidence. A detailed analysis of these optical filters with respect to the index of refraction of the films and prisms, width of films, and polarization of light is presented. Applications on extrasolar planet detection are briefly discussed.
Solid state lighting is becoming the preferred choice for many lighting applications that require homogeneous illumination distribution. Light-emitting-diode (LED) sources for lighting are usually composed of several individual LEDs. Nevertheless, packaging density of LED arrays is limited by cost, available space, and particularly by thermal problems. This paper presents an investigation of the effects on illumination uniformity of light sources consisting of multiple LEDs due to different array configurations and packaging density of LEDs. The performance of various array configurations is analyzed using a radiometric analysis and experimental results.
Light sources consisting of multiple Light-Emitting Diodes (LEDs) are becoming the preferred choice for many lighting applications that require uniform illumination distribution. However, packaging density of LED arrays is limited by cost, available space, and particularly by thermal problems. Therefore low density or diluted arrays of LEDs are becoming the option for many applications. This paper presents an investigation of the effects on illumination uniformity due to dilution on light sources consisting of multiple LEDs. The characteristics of illumination uniformity are obtained for several diluted-array configurations and degrees of dilution. This analysis is performed using a radiometric analysis, and considering each LED as an imperfect Lambertian emitter. Analytical expressions are derived for the maximum degree of dilution for different configurations of LED arrays.
A compact, aberration-selective, reversal interferometer with variable rotational shearing of wave front is described. It is constructed with only two beam-splitter cubes, each one with one face aluminized, cemented together, and mounted in a rotary holder. This interferometer is quite insensitive to vibrations, because the wave fronts pass only through the cemented cubes to generate the interference pattern. All the rotationally symmetric aberrations are removed and the asymmetrical ones can be selectively isolated with an appropriate rotation of the cubes. The variable shearing angle is shown to control the sensitivity for detection of rotationally asymmetric aberrations like coma and tilt.
We analyze the polarization changes introduced by a rotated Dove prism on the linearly polarized light, using the Jones calculus. The state of polarization changes from the linear to a mildly elliptical one when a plane wave front passes through a rotated Dove prism: its semi-mayor axis is nearly parallel to the input plane of polarization, for any angle of rotation. The interferogram contrast remains high for all shearing angles in spite of polarization changes when the Dove prism is incorporated into a rotational shearing interferometer. These results are confirmed experimentally.
An analytical expression is derived for the tilt introduced into a wave front by a Dove prism with manufacturing errors: error in the base angles and in the pyramidal angle. We found that the tilt decreases when the base angles are increased above the values of traditional design. The increase in the length-aperture ratio of a prism is detrimental to its performance. However, a Dove prism with a widened aperture increases throughput and keeps prism weight manageable for implementation in the rotational shearing interferometer. Thus, we propose a Dove prism designed with a widened aperture to increase throughput in the rotational shearing interferometer and with larger base angles to minimize the wave-front tilt introduced due to manufacturing errors.