Journal of Light
&
Visual Environment

Volume 27 Number 3, December 2003

CONTENTS

English Abstracts

Present Status of White LED Lighting Technologies in Japan
T. Taguchi

“The light for the 21st century” Japanese national (Akari) project, which is based on the high-efficient white light-emitting diode (LED) lighting technologies using near ultraviolet (UV) LED and phosphor system, has been started at 1998. The near UV white LED system linked with semiconductor technologies on GaN LED and ZnS phosphors for general lighting applications has for the first time been proposed in the project. The outline and purpose of this project are briefly introduced. In particular, we have demonstrated high-efficient nUV LED having external quantum efficiencies more than 43% around an emission wavelength of 400 nm. Basic illumination properties of the high luminous efficacy (>40 lm/W) and the high general color rendering index (Ra>90) white LED sources are described. The near UV white LED technologies in conjunction with phosphor blends can offer superior color uniformity, high Ra and the excellent light quality for many lighting applications.

KEYWORDS: white LED, LED lighting, near UV LED, luminous efficacy, color rendering, external quantum efficiency

High Output Power Near-Ultraviolet and Violet Light-Emitting Diodes Fabricated on Patterned Sapphire Substrates Using Metalorganic Vapor Phase Epitaxy
K. Tadatomo, H. Okagawa, Y. Ohuchi, T. Tsunekawa, H. Kudo, Y. Sudo, M. Kato and T. Taguchi

Near-Ultraviolet (NUV) and violet light-emitting diodes (LEDs) with an InGaN multi-quantum-well (MQW) structure were fabricated on patterned-sapphire substrate (PSS) using a single growth process of metal-organic vapor phase epitaxy (MOVPE). The PSS with parallel grooves along the <1120>_GaN direction or the <1100 >_GaN direction was fabricated by standard photolithography and subsequent reactive ion etching (RIE). In this study, the PSS with parallel grooves along the < 1120>_GaN direction was used. The GaN layer grown by lateral epitaxy on a patterned substrate (LEPS) had dislocation density of 1.5×10⁸ cm^-2. The LEPS-NUV (or violet)-LED chips were mounted on the Si bases in a flip-chip bonding arrangement. When the LEPS-NUV-LED (the emission peak wavelength λ _p382 nm) was operated at forward-bias current of 20 mA at room temperature, the output power (P_o) and the external quantum efficiency (η_e) were 15.6 mW and 24%, respectively. When the LEPS-violet-LED (λ_p405 nm) was operated at forward-bias current of 20 mA at room temperature, the output power and the external-quantum efficiency were 26.3 mW and 43%, respectively. The PSS is very effective in reducing the dislocation density and increasing the extraction efficiency in the LEDs because of the scattering of the emission light at the patterned GaN/sapphire interface.

KEYWORDS: patterned sapphire substrate, PSS, MOVPE, LED, UV, white LED, solid state lighting

Short Wavelength LED Based on III-V Nitride and its Applications
N. Shibata, T. Uemura, H. Yamaguchi and T. Yasukawa

The short wavelength LED (“TG Purple”) based on III-V nitride semiconductors is established in addition to blue and green ones. This short wavelength LED is realized by adjusting indium chemical content of the well layer. Light output power of “TG Purple” is high and FWHM (full width at half maximum) is narrow. “TG Purple” is applied to light source of the white LED. Combination of “TG Purple” and various phosphors can generate white light with high luminous intensity and good color rendering. “TG Purple” is also applied to light source of air-purifier. This air-purifier is being offered as standard equipment in luxury automotives and several applications of this system are widely utilized in room air conditioners and refrigerators.

KEYWORDS: short wavelength LED, III-V nitride semiconductor, white LED, phosphor, air purifier

On Deviations between Observed and Theoretically Estimated Values on Additivity-Law Failures
Y. Nayatani and H. Sobagaki

The authors have reported in the previous studies that the average observed results are about a half of the corresponding predictions on the experiments with large additivity-law failures. One of the reasons of the deviations is studied and clarified by using the original observed data on additivity-law failures in the Nakano experiment. The conclusion from the observations and their analyses clarified that it was essentially difficult to have a good agreement between the average observed results and the corresponding theoretical predictions in the experiments with large additivity-law failures. This is caused by a kind of unavoidable psychological pressure existing in subjects participated in the experiments. We should be satisfied with the agreement in trend between them.

KEYWORDS: additivity-law failure, brightness/ luminance (B/L) ratio, the VCC method, the VAC method;

Causes of Individual Differences on Brightness/ Luminance (B/L) Ratios
Y. Nayatani and H. Sobagaki

On individual variation of Brightness/Luminance (B/L)-ratio effect, the causes of its individual differences are studied. Based on the present study, the following conclusions are confirmed. 1) The individual differences on B/L-ratio effect are caused by the differences in attitude between subjects on equi-brightness judgement. 2) For the present, we must satisfy with a theoretical prediction of the average experimental B/L ratios derived by observations using a number of subjects.

KEYWORDS:brightness/luminance ratio, inter-observer variation, direct brightness matching, hetero-chromatic brightness matching

Equivalent Luminance Contrast Representing the Relationship between Color Difference and Readability of Chromatic Documents
N. Hara, I. Namba and T. Noguchi

The purpose of this study is to obtain data based on the readability of chromatic documents for use in visual environment design. That data would seem to be the same as the existing data based on the readability of achromatic documents. Experiments are undertaken in order to clarify the influence of color on readability. The results show that the same CIELAB color difference does not have the same influence on readability. In addition, it is thought that the equivalent luminance contrast is invariable irrespective of the size of a letter and is instead determined by the combination of the colors of the letters and the background. It is suggested that the readability of chromatic documents can be estimated by using the equivalent luminance contrast and the existing relationship between the visual factors and readability of achromatic documents.

KEYWORDS: chromatic document, readability, visual factors, color difference, equivalent luminance contrast, visual environment

A Healthy Future for Office Lighting?
A.D. Tenner

For long standards and recommendations for office lighting are based on the horizontal illuminance at the working plane, this to ensure that the visual needs for a reading and writing task are fulfilled. Today, working in a modern office includes much more than just reading and writing. Working on a computer and communication with one or more persons have become the major tasks of an office worker. Thus the lighting requirements have to change too. That daylight is an important factor for a healthy and energy efficient office lighting has become widely accepted.
Also knowledge has been obtained about the influence of light on mood and performance; apart from the visual system also the brain responds directly to light.
Over the last decade research has been performed within Philips Lighting in Eindhoven, to obtain knowledge about the preference and acceptance of office lighting by the workers and about the non-visual effects of light on mood and performance.
This article gives an overview of the work that has been done and the implications for office lighting and office lighting design.

KEYWORDS: lighting, daylight, lighting preference, health and mood, office Lighting, non-visual effects