Civil Engineering Reference
In-Depth Information
Fig. 18 Spectral direct a and
diffuse b transmittance of an
SPD. The voltage was varied
between zero and 100 V ac,
as indicated by arrows. From
Barrios et al. (
2013
)
(a)
1.00
0
10
20
30
40
50
60
70
80
90
100
0.80
0.60
100 V
0.40
0.20
0 V
0.00
400
800
1200
1600
2000
2400
Wavelength (nm)
(b)
0.10
0
10
20
30
40
50
60
70
80
90
100
0.08
100 V
0.06
0.04
0.02
0 V
0.00
400
800
1200
1600
2000
2400
Wavelength (nm)
polyhalides and have large optical anisotropy. Herapathite (quinine bisulfate
polyiodide) is the most well-known compound in this class (Kahr et al.
2009
); it was
used extensively in early work on polarizers and other optical devices (Knowles
2009
). The optical anisotropy was discussed in detail in a recent theoretical study by
Liang et al. (
2009
). Various related compounds have been investigated in later SPDs
(Takeuchi et al.
1997
; Fanning et al.
2003
). When a sufficient AC voltage is applied
between the transparent conductors, the particles align and become parallel to the
electric field and the overall transmittance goes up. Decreasing the voltage makes
the particles more randomly oriented and the device gets darker until it attains a
bluish-black color. Figure
18
shows spectral direct and diffuse transmittance for an
SPD that was investigated recently by Barrios et al. (
2013
). It is apparent that T
lum
can be varied when the voltage is changed, whereas T
sol
is not affected to the same
extent. The diffuse scattering lies on the level of several percent, i.e., windows
incorporating SPDs are not free of haze.
Liquid crystals are used in other types of electrochromic devices. The most
common construction is based on polymer-dispersed liquid crystals (PDLCs) and
essentially switches between two states with strongly differing light scattering
(Cupelli et al.
2009
; Gardiner et al.
2009
). Applications are related to privacy
rather than to energy control. Another possibility is offered by some organic
compounds, which can produce optical absorption when a small electrical current
