Ivan Moreno

Modeling the radiation pattern of LEDs.

Light-emitting diodes (LEDs) come in many varieties and with a wide range of radiation patterns. We propose a general, simple but accurate analytic representation for the radiation pattern of the light emitted from an LED. To accurately render both the angular intensity distribution and the irradiance spatial pattern, a simple phenomenological model takes into account the emitting surfaces (chip, chip array, or phosphor surface), and the light redirected by both the reflecting cup and the encapsulating lens. Mathematically, the pattern is described as the sum of a maximum of two or three Gaussian or cosine-power functions. The resulting equation is widely applicable for any kind of LED of practical interest. We accurately model a wide variety of radiation patterns from several world-class manufacturers.

Designing light-emitting diode arrays for uniform near-field irradiance

We analyze the first-order design of light sources consisting of multiple light-emitting diodes (LEDs) to uniformly illuminate a near target plane by considering each single LED as an imperfect Lambertian emitter. Simple approximate equations and formulas are derived for the optimum LED-to-LED spacing, i.e., the optimum packaging density, of several array configurations to achieve uniform near-field irradiance.

High-performance LED street lighting using microlens arrays

An efficient LED lamp that illuminates the street with high quality is presented. The luminaire shows high optical efficiency, high optical utilization factor, low glare, and illuminates the street with high uniformity. The concept is simple but effective: a cluster of LEDs with TIR lenses are put inside a reflective box, which is covered with a microlens sheet; the reflective cavity improves efficiency by light recycling; each TIR lens collimates the LED light for the microlens array; and the microlens sheet uniformly distributes light only into the street. We verify its feasibility by Monte Carlo ray-tracing for the main types of road lighting arrangements: central, zigzag, and single-side pole positions.

Color distribution from multicolor LED arrays

We describe a fully-analytical, simple yet sufficiently accurate method to compute the color pattern of the light emitted from multicolor light-emitting diode (LED) assemblies. Spatial distributions for both color variation and correlated color temperature (CCT) as a function of typical parameters of influence, such as LED spectrum, spatial distribution of LED radiation, target distance, LED-to-LED spacing, and number of LEDs, are shown. To illustrate the method, we simulate and analyze the color patterns of linear, ring, and square RGB (red, green, and blue) arrays for Lambertian-type, batwing, and side emitting LEDs. Our theory may be useful to choose the optimal value for both the array configuration and the array-diffuser distance for lighting systems with color mixing devices.

Analysis of the far-field region of LEDs.

Versatility in the design of optical systems is one of the key features of light-emitting diodes (LEDs) that has attracted considerable attention. In the analysis of systems using LEDs, it is useful to know if the distance is far enough from the LED to allow the radiation pattern to be simulated by the point source approach. We propose three far-zone conditions for LED light modeling: the far-field distance, and for practical purposes the quasi far-field and minimum far-field distances. Different types of LEDs have different far-field ranges. We analyze these differences by modulating key parameters like geometrical structure of encapsulating lens, chip size, chip shape, chip position, and package errors. We find that far-field region considerably depends more on the shape of both lens and chip than all other parameters.

Calculating model of light transmission efficiency of diffusers attached to a lighting cavity

A lighting cavity is a reflecting box with light sources inside. Its exit side is covered with a diffuser plate to mix and distribute light, which addresses a key issue of luminaires, display backlights, and other illumination systems. We derive a simple but precise formula for the optical efficiency of diffuser plates attached to a light cavity. We overcome the complexity of the scattering theory and the difficulty of the multiple calculations involved, by carrying out the calculation with a single ray of light that statistically represents all the scattered rays. We constructed and tested several optical cavities using light-emitting diodes, bulk-scattering diffusers, white scatter sheets, and silver coatings. All measurements are in good agreement with predictions from our optical model.

Far-field condition for light-emitting diode arrays

In practice, any cluster of light-emitting diodes (LEDs) can be modeled or measured as a directional point source if the detector is far enough away from the cluster. We propose a far-zone condition for measuring or modeling propagation of light from an LED array. An equation gives the far-field distance as a function of the LED radiation pattern, array geometry, and number of LEDs. The far field is shorter for high packaging density clusters, and the far field considerably increases with increasing beam directionality of LEDs. In contrast with the classical rule of thumb (5 times the source size), the near zone of an array with highly directional LEDs can extend to more than 60 times the array size. We also analyze the effect of introducing random variations of light flux among LEDs of the array, which shows that far-field variability is low in high packaging density arrays.

Collimating lamp with well color mixing of red/green/blue LEDs

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. Our method is simple and compact; it only uses a short light pipe, a thin diffuser, and a total internal reflection lens. We performed an experimental study to find a balance between optical efficiency and color uniformity by changing light recycling and color mixing.

Illumination uniformity assessment based on human vision

Assessing how uniform the light distribution is throughout an illuminated target is important in many applications, but traditional methods do not quantify the variability of illuminance as the human visual system (HVS) does. Considering that most light patterns are intended for humans, I propose a simple metric that assesses the uniformity in a similar way as humans do. This uniformity indicator is based on the fact that the HVS is highly sensitive to spatial frequencies and then uses the Fourier transform and the contrast sensitivity function of the HVS in a practical way.

Uniform illumination of distant targets using a spherical light-emitting diode array

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 sur- face. Arranging each single LED into an optimal placement, the unifor- mity of the illumination of a target can be improved. We derive approxi- mate formulas and equations for the optimum LED-to-LED angular spacing of several spherical arrangements for uniform far-field irradi- ance. These design conditions are compact and simple tools that incor- porate an explicit dependence on the half-intensity viewing angle half width half maximum angle of LEDs.