Environmental Engineering Reference
In-Depth Information
Presently, at least three commercial fiber optic daylighting systems are being
marketed around the world, Himawari (Japan), Parans (Sweden), and Sunlight
Direct (United States) [4]. Of these, the only one for which the author has
reliable cost and performance information is the Sunlight Direct ''Hybrid
Solar Lighting'' (HSL) system. The Himawari system (Fig. 5.2) is believed to
be the first commercial fiber optic daylighting system and dates back to at least
the early 1970s. That system, which uses a tracking concentrator with multiple
Fresnel lenses, transmits the light through quartz fibers, which have very low
loss but are very expensive compared to plastic optical fibers. Anecdotal infor-
mation from a source familiar with a Himawari system sold in the United States
several years ago, suggests that the Himawari system cost was (at that time)
between $1/lumen and $5/lumen depending on the system type. By contrast, the
HSL system being marketed by Sunlight Direct (capable of producing around
40,000 lumens) had an installed cost of approximately $0.50/lumen in 2007 but
is being projected at $0.35/lumen for 2008. Of the three known fiber optic
daylighting systems, the Sunlight Direct HSL system is believed to be the only
one that combines natural and artificial light in a hybrid luminaire and is
specifically designed for compatibility with existing ceilings and lighting. The
HSL system has been installed in a significant number of beta test sites, includ-
ing research institutions, universities, and retail outlets [5]. The evolution of the
HSL system will be discussed in more detail, but first we should consider some
of the motivations behind the development of any daylighting system.
5.4 Biological and Cultural Benefits of Daylighting
Much has been written about the potential benefits of natural lighting versus
artificial lighting. There is enough anecdotal information to suggest that the
solar spectrum (which is dramatically different than the spectra emitted by
electric light sources) may be a better option, in terms of overall physical and
mental health, than electric light. By extension, the reasoning suggests that
improved mental and physical health results in positive behavioral changes.
There have been some controlled experiments to validate various claims, but
many of the claims are still rather speculative. Before discussing the claimed
benefits of daylighting, it is worth considering some heuristic discussions
regarding biological mechanisms that could play a role in producing them.
It is widely accepted that the spectral response of the human eye, under
normal levels of illumination (the so called ''photopic'' response), is very similar
to the spectrum of visible sunlight, which is shown in Fig. 5.3. Currently, there is
no single undisputed photopic spectral diagram, but one from the International
Commission on Illumination (CIE) that is frequently used is shown in Fig. 5.4
[6]. For the purpose of this discussion it is not important to quibble over just
how precisely the two spectra are matched. The point to be appreciated is that,
from a systems standpoint, when the spectral response of a receiver (the eye) is
