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Upload organization:The University of Tokyo Graduate School of Agricultural and Life Sciences, Department of Biological and Environmental Engineering, Bio-Environmental Engineering Lab.  Upload date:2011/11/10

Controllable Spectrum Artificial Sunlight Source System using LEDs

Professor Kazuhiro Fujiwara
Department of Biological and Environmental Engineering,
Graduate School of Agricultural and Life Studies,
The University of Tokyo

URL: http://www.kankyo.en.a.u-tokyo.ac.jp/member/fujiwara/index.html


Light-emitting diodes (LEDs), as used in fairy lights and traffic lights, are an integral part of our daily lives. At the same time, they are vital tools in research that looks at the impact of light on living organisms, known as photobiology. Today, new and unique LED light emitting systems are being developed to be used in photobiological research. 

Current LED Use in Photobiological Research
By selecting a certain type of LED, it is possible to prepare various variations of monochromatic light. The monochromatic light or composite light (created by combining several monochromatic lights) prepared is then used to irradiate the living organism, plant body, organ, or cell which is the subject of the research, for a certain term (number of hours) at a set strength and a set spectral distribution, and observations are taken on the response of the research subject. This type of research project is being carried out all over the world today. But what if there was a light source that allowed the researcher to control the spectral distribution of the irradiating light freely and dynamically? Such a system would make enable the examination of diverse light environments, the investigation of which has previously been impossible. This in turn would doubtless generate much significant scientific knowledge and discovery. 

Creating Artificial Sunlight for Research Manipulation
This research is focused on the development of a lighting system with a spectral distribution approximate to that of ground level sunlight. It aims to produce this not just as a reference light, but also to arbitrarily modify this using a range of wavelengths in to produce light with diverse spectral distributions. Importantly, the system allows the continuous irradiation of those lights, in any order and with total flexibility. The official name of the system is the ‘Controllable Spectrum Artificial Sunlight Source System using LEDs’. Artificial sunlight is an appropriate choice for the reference light because of the significance of natural sunlight for organisms living above ground. In order to produce light with multiple different spectral distributions, the system’s light source uses 32 different LEDs, each with a different peak wavelength. Spectral distribution is then controlled by adjusting the voltage applied (the supply current) to the LEDs with differing peak wavelengths. 

The system is now in its second generation. The surface area of the light outlet is 7 cm2, and the spectral irradiation (light intensity) produced reaches up to one fifth of that of ground level sunlight on a clear day at solar noon in the middle of summer (111 W m-2 to wavelength range of 380–940 nm). The third generation system is near to completion: the surface area of its light outlet will be ten times that of the second generation model, enabling spectral irradiation (SI) that is 2.5 times stronger than the current maximum intensity. 


photo1 
Photograph 1.
Set-up of Artificial Sunlight Source System using LEDs. The temperature around the light source unit must be maintained at 15°C and is therefore installed within the growth chamber. The spectrometer is only fitted when measuring spectral irradiance (SI) from the light radiance port. 


photo2 
Photograph 2.
Appearance of LED module when electric voltage has been applied to produce spectral irradiance (estimated best approximated SI) estimated as the best approximation of one sixth of the spectral irradiance of ground level sunlight at 13:00 on a clear day (May 12, 2007) in Bunkyo-ku, Tokyo, Japan. 
LEDs with a peak wavelength of more than 810 nm appear not to be illuminated in the picture. 


photo3
Photograph 3.
Appearance of light source irradiation when electric voltage has been applied to produce spectral irradiance (estimated best approximated SI) estimated as the best approximation of one sixth of the spectral irradiance of ground level sunlight at 13:00 on a clear day (May 12 2007) in Bunkyo-ku, Tokyo, Japan. 


fig.1 
Figure 1. 
Spectral irradiance and light outlet when electric voltage has been applied to produce the estimated best approximate SI, and the spectral irradiance (estimated best approximate SI) estimated as most similar to target SI for the spectral irradiance of one sixth of that of ground level sunlight (target SI, 1/6th of sunlight SI) at 09:00, 11:00, 13:00, and 15:00 on a clear day (May 12, 2007) in Bunkyo-ku, Tokyo, Japan, and the light source irradiance of the same.
Spectral irradiance here was continually applied at 2-second intervals. Results indicate that this artificial sunlight source system can dynamically control the spectral distribution of the irradiating light.

References:
Fujiwara, K. and A. Yano (2011) Controllable spectrum artificial sunlight source system using LEDs with 32 different peak wavelengths of 385–910 nm. Bioelectromagnetics 32(3): 243-252.
Fujiwara, K., T. Sawada, S. Goda, Y. Ando and A. Yano (2007) An LED-artificial sunlight source system available for light effects research in flower science. Acta Horticulturae 755: 373-380.
Fujiwara, K. and T. Sawada (2006) Design and development of an LED-artificial sunlight source system prototype capable of controlling relative spectral power distribution. Journal of Light & Visual Environment 30(3): 170-176.