Flat Prism Daylighting System

by the CADDET Japanese National Team


Several types of solar-tracking light collection systems for daylighting exist; some concentrate sunlight using lenses, some collect light with parabolic mirrors, others reflect light using flat mirrors and still others refract light with prisms. All these systems have to adjust the action of their lenses, mirrors or prisms to the movement of the sun with three-dimensional control, so they need large, complex control mechanisms.


A flat prism daylighting system mounted on the roof of an engineering works

In contrast, the SOLIGHT, developed by Sanyo Electric Co Ltd, is a new daylighting system which automatically tracks the sun with a simple mechanism consisting of just two flat prism discs. Each disc rotates horizontally and together they direct sunlight into a room by refracting the light four times. More than 500 units have already put to practical use in Japan.


Figure 1 shows the principle of solar tracking using a combination of two flat prisms (Fresnel prisms). When the sun is at a low altitude, the two prism discs rotate in the same direction. In this case both upper and lower prism discs refract solar rays towards the same direction (Figure 1a). When the sun is high, the prism discs rotate in opposite directions refracting the solar rays in different directions (Figure 1b). In this way, the system can always direct solar rays down into a room. SOLIGHT can control the direction of solar rays coming from elevation angles of 10 84.


Figure 1.  Principle of flat prism solar tracking mechanism

In order to collect solar light efficiently for daylighting, it is important to let as much light as possible pass through the prisms. When the sun is at a relatively low altitude, some of solar rays refracted by a prism in a flat prism disc have their paths blocked by the unrefracting face of its adjacent prism. Sanyo has studied this effect and Table 1 shows the optimum specifications for a flat prism developed by this research.

Table 1: Specifications of the flat prism

2nd prism

1st prism


1st face

2nd face

1st face

2nd face

Apical angle of prism





Angle of unrefracting face





Pitch of prism

1 mm


0.5 mm




Diameter of flat prism

720 mm


2 mm

Controllable range

Solar altitude: 10-84o


55.4% +/- 13%


The amount of visible light outdoors under a clear sky is greater than one imagines, and the system can adequately illuminate a room with 1/10 of the outdoor illuminance. For example, at noon on the spring and autumn equinoxes, when the sun is over the equator, outdoor illuminance is 100,000 lx. At the same moment, a SOLIGHT unit provides luminous flux of 13,000 lm for a room, equal to the visible light provided by four 40 W fluorescent lamps (power consumption: 150 W), nine 100 W incandescent lamps (power consumption: 870 W), or one 400 W mercury lamp (power consumption: 240 W).

The power consumption of a SOLIGHT is less than 0.9 W. But, equipped with a photovoltaic panel, it does not use any power from utility grids.

A microprocessor in the unit automatically controls solar tracking using preprogrammed data on the movement of the sun. In addition, the amount of light passing through the prism discs can be regulated by remote control. This dimming mode operation makes it possible to keep a room comfortably illuminated, even under strong direct sunlight in summer. The light introduced by the flat prism set of a SOLIGHT unit is scattered throughout the room by a light distribution plate shielding the unit. Two types of plate, a diffuser type and a spotlight type, are available for use according to the application and location.

This daylighting system can direct light to a given place even with the sun at low altitude early in the morning, in the late afternoon or during the winter, and deliver relatively stable amounts of light all year round. Even under cloudy skies, it can bring the same level of light into a room as an opal skylight dome. The plastics used for the components transmit only visible light, screening out harmful ultra violet rays and infrared rays which would add a heat load to the room. In this manner, the system suppresses glare and does not raise room temperatures excessively in summer.

A mechanical engineering company introduced this daylighting system to its newly built works as part of an energy-saving strategy. One hundred SOLIGHT units mounted on the building at intervals of 7 m deliver enough light to the floor from the 7.5 m high ceiling to make artificial lighting in the works during the daytime almost unnecessary. People working here feel that the natural daylighting increases their comfort.

Table 2: Specifications of the SOLIGHT system

Outside dimension width, length, height

1.14 m wide x 1.14 m long x 0.4 m high


32 kg (plastic plate shielding)
42 kg (wired sheet glass shielding)

PV panel output

two 1.67 W modules (AM 1.5, 1 kW/m2, 25o C)

Power Consumption

< 50 MW (average)


35 dB (maximum)

Light collecting area

aperture area: 0.7 m2 (840 mm x 840 mm)


effective area: 0.4 m2 (720 mm prism diameter)

Amount of light collected

direct solar radiation: 13,000 lumen (a)

Light collection range

Solar altitude 10-84o

Light collection time

08:00 16:00 hours (approximately) (b)

Service environment

Outside temperature

 -20 C +45o C


Indoor humidity

 < 95%


Installation condition

within 1 of horizontal


Assuming that the daylighting system can be used for 200 days a year, one SOLIGHT unit saves 43 l/year oil equivalent and 141 kg/year of carbon dioxide emission compared with fluorescent lighting (four 40 W lamps consuming 150 W). Compared with a 400 W mercury lamp consuming 240 W, the SOLIGHT unit saves 68 l/year of oil and 225 kg/year of carbon dioxide.

The use of this daylighting system can bring a large saving in power costs for lighting. At present, however, the system is priced at a rather high JPY 500,000/unit (where JPY is the Japanese yen) and only one size of unit is available (with an aperture 840 mm x 840 mm). More work is therefore needed to develop a wide range of models and sizes suitable for home use, and to reinforce the system's capabilities by adding functions such as ventilation.
Considerable effort is also required to reduce the price.

For more information contact the CADDET Japanese National Team in Tokyo.

The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.

The articles published in the Newsletter reflect the opinions of the authors. They do not necessarily reflect the official view of CADDET.

Enquiries concerning the Newsletter should be addressed to
Pauline Toole, Editor, CADDET Centre for Renewable Energy, ETSU, Harwell, Oxfordshire OX11 0RA, United Kingdom. Tel: +44 1235 432968, Fax: +44 1235 433595.