10.07.2015   BACKGROUND

Calculation of the view-through index

The potential to view an object in the surrounding environment through a window can be hindered by:

• low luminous transmittance of the window;

• significant amount of diffuse transmittance component;

• back reflection from the window;

• obstructing elements in the window.

1view—thr

— f (?

prim—reg ’ ^scat—diff , Pback )

Since these individual influences are combined into a single complete equation, the total equation for the view through index looks complicated [4]. The complexity of the equation, however, has no negative effect on the simplicity of the input and output, and the total equation can be summarised as follows:

lum

The following parameters are involved:

Tprim-гед the regular portion of transmittance;

Tscat-diif the scatter or diffuse portions of transmission;

pretro the back reflectance (from indoor to indoor) of the daylight product;

Bobstr the disturbance factor for the blockage;

clum the correction factor for the accommodation to the level of luminance from

the light through/from the daylight product due to transmission and back reflection.

The disturbance factor, B0bstr, is calculated according to:

f por obstr

X ref

Bobstr Є

In which:

fpor is the porosity factor of the obstruction (i. e. the transparent part of the product’s

surface);

Xobstr is the characteristic length of the obstruction in the daylight product;

Xref is the reference length for obstructions in the daylight product.

The view through index value varies between 0 and 1, corresponding to vision denied and perfect view, respectively.

The view through index calculation requires goniophotometric measurements, therefore the index was calculated only for samples 12 and 13.

Table I Calculation of view through index

Parameter

Sample 12

Sample 13

Xostr (mm)

1.05

0.77

fpor

0.669

0.738

Bostr

0.933

0.960

Pretro

0.049

0.112

clum

0.623

0.767

^vrim-reg

0.542

0.676

A@prim^scat-diff

0°

0°

@width-scat-dif

2.6°

2.5°

scat-dif

0.079

0.088

fiew-thr

0.827

0.862

The next table I shows the value of the relevant parameters and the final view through index for sample 12 and sample 13.

The results show that the presence of the decor on the samples tested has a small influence on the capability of view through the window (index values not far from unity), and that sample 13 should allow a better vision of the landscape than sample 12 (index values respectively of 0.862 and 0.827).