Lighting Research Center Lighting Research Center

Spectral Response, continued

Daylight spectra in an office

The relative spectral content of daylight is different inside a room than it is outside. The transmittance properties of the window and the reflectance properties of the room surfaces influence the interior daylight spectrum. To investigate the magnitude of these spectral changes, daylight spectra were measured inside a south-facing office located at the Lighting Research Center in Watervliet, NY. Thin, high clouds blocked most of the direct sunlight and provided a bright white sky, but faint shadows could still be seen. The glazing was a single pane of clear glass. The wall and ceiling were painted white and the floor is off-white vinyl tile with tan speckles. A dark rug covered about a third of the floor near the window.

The figure below shows the incident spectrum on the room ceiling, back wall and the sky radiance viewed through the window glazing for comparison.

Clear glass window glazing affects the daylight spectrum very little for wavelengths longer than 400 nm. Once inside the room, the attenuation of the shorter wavelengths can be attributed to absorption by the pigments of the paint used on the walls and ceiling. Titanium dioxide is a common constituent of most light-colored paints and it strongly absorbs radiation below about 420 nm. The dramatic increase in the spectral content for wavelengths longer than 680 nm is much more significant. The measurements show that, as a whole, the extreme long wavelength visible and near IR radiation is absorbed less than the rest of the visible spectrum. The reason for this is that most organic colorants, that is, the organic dyes and pigments found in most natural and synthetic materials, do not absorb radiation of wavelength longer than about 700 nm. Therefore, organic objects such as fabrics, rugs, plants, plastics, and wood products are highly reflective or transparent for wavelengths longer than 700 nm. This phenomenon makes proper filtering of the IR extremely important to the proper operation of photosensors. In addition, not only must the IR radiation be blocked; the long wavelength visible light, from 650 to 750 nm, should be attenuated because luminous efficacy (the photopic human eye response) is very low throughout this spectral region.

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