**9.2 Cold mirror coatings**

Spectral splitting coatings can be used to divide solar spectrum into various broad band regions. By this, various regions of the solar spectrum can be separated for use for different purposes such as photovoltaic or photo thermal devices (Lambert, 1985).

A "cold mirror" device has opposite spectral response to that of the "heat mirror". That is, cold mirror films reflect highly (low transmittance) in the VIS region of solar spectrum and reflect poorly, but transmits highly in the IR region, thus splitting the spectrum into short wavelengths and long wavelengths. The high energy waves i.e. the short wavelengths are used for photovoltaic generation while the low energy waves, the long wavelengths (IR) are used for photo thermal heating. This device can be used in "green house" with special arrangements of baffles on the roofs. The device will reflect the photosynthetic active radiation (PAR), 0.35 ≤ λ ≤ 0.75 µm waves into the green house while transmitting the IR into the air channels which can be redeployed to maintain a suitable warm temperature in the green house. ZnS/ MgFs and Ti O2/ Si O2 have been used to achieve these coatings.

## **9.3 Solar control coating**

Solar control coating is a design intended to reduce the incoming heat radiation through windows of a building by reflecting off the heat radiation (IR). To achieve comfortable indoor temperatures, that is, to achieve cooling in a building, solar control coating surfaces that are transparent at 0.4 ≤ λ ≤0.7 µm and reflecting at 0.7 ≤ λ ≤3 µm may be used for the material of the windows in the building. By this, the infrared part of the solar radiation is reflected back, which is possible through the use of solar control windows. A 50% reduction in the internal heating of a building without noticeable reduction in the lightning of the interior of the building had been achieved. The use of such windows may achieve the same objective of a controversial air conditioner in a building. Solar control coating are particularly applicable in hot climate countries such as Nigeria.

In solar control and energy conserving windows, low transmittance windows are employed. If the medium is generally opaque to the passage of radiation but selectively transmits a particular small range of radiation, it is said to operate as a window in that range. A low thermal transmittance window reduces the heat radiation through the window. To achieve low thermal transmittance window therefore, surface coatings that transmits at 0.3 ≤ λ ≤ 3 µm and reflects at 3 ≤ λ≤ 100 µm may be used. This means that maximum use is made of the solar energy in the short wavelengths range while the transmittance of thermal radiation is minimized.
