The light transmission of a doubling glazing system is about 0.82 for good quality clear glass. This is the diffuse transmission. Where the angle of incidence of beam radiation is greater than 60 degrees there is a rapid fall off in transmission. The angle of beam radiation is determine by the azimuth and altitude of the sun or the vertical shadow angle and horizontal shadow angle.

As the transmission depend on the azimuth as well as the altitude during the early morning and late afternoon much of the incident radiation is reflected. Hence an equivalent diffuse sky (in terms of vertical illuminance) will provide greater light transmission into the room.

In real test cells (and real rooms) the reveals can play a role in restricting beam transmission into the room ( cf egg crate brise soleil). The smaller the opening for a given reveal depth the greater this effect. In our case the window finally installed was smaller than that originally envisaged and thus the effect more pronounced that expected. To minimise the reveal effect it was decided to mirror the reveals. A typical sunny day with the window removed before mirroring the reveals is shown below. When the window was installed the effect is reduced becuse of the frame and its smaller thickness.

The result of these two effects is clearly shown in the graphs below. The blinds tend to diffuse the beam radiation but not completely. The amount of diffusion depends on the weave and colour which affect both diffraction and interreflection effects.

While both vertical blinds and venetian blinds (if adjusted correctly) can excude all direct beam radiation this is not the case with the awning type. For the south facing room some solar penetraion occurs in the early morning and late afternoon. More importantly ground reflection significantly enhances the amonut of daylight entering the room so that the effective light transmission is much higher than that suggested by the transmission of the material alone. There may be a strong arguement for use spectraselective paving material outsiide such blinding systems to increase daylight while minimsing solar gain.

Data for typical Glazing
Product Light transmittance Light reflectance
Single clear class 0.9 0.08
Double clear glass 0.82 0.15
Triple clear glass 0.75 0.2
Double clear glass
with low E coating
0.75 0.17
Typical data for blinds
white pastel dark black
Transmittance Reflectance
Opaque 0.0 0.7 0.5 0.3 0.1
Medium trenslucent 0.2 0.6 0.4 0.2 0.1
High translucent 0.4 0.4 0.3 0.2 0.1

CEN TC 89 Thermal performance of buildings and building components

First Stage Testing (Transmission)

Date Configuration
23 May 2000 Measurements start
June 2000 Reveals painted white
15-19 June 2000 No glazing
28 June - 7 July 2000 High reflectance reveals
7 July 2000 Mirrored reveals
7-10 July 2000 Vertical Blind
10-14 July 2000 Awning
14-17 July 2000 Louver Blind (horizontal)
18-19 July 2000 Vertical Blind
19-20 July 2000 No Blind
20 July 2000 > Awning
Type Ref Colour Position
Awning 525 Satrinee Grey/white Horizontal to sill
Vertical Blind 92/200046 . Fully closed
Louver Blind . Grey Horizontal
inside the cell

Testing began with a period of trials to find the most sutible place to site the internal photocell. This resulted in a position with the light cell facing the cieling. During this period we found it necessary to increase the dynamic range of the photocell.