09.06 Directional glazings

Categories: Advanced Glazings

Introduction
Directional glazings work by changing the path of incident radiation, either reflecting the radiation away from the building or redirecting the transmitted radiation to where it is of most use (or least inconvenience) within the room.  Whilst many of these products are sometimes described as transparent insulation materials they are placed in a separate section here because their behaviour under direct light is generally specular - they do not diffuse or disperse incident light.
 

Prismatic panels
A prismatic panel has a surface that is cut into a wedge pattern.  The surfaces of each wedge refract or reflect direct light, depending upon the angle of incidence.  Generally these panels are designed and arranged so that strong direct sunlight is reflected, whilst some of the diffuse light is admitted into the building.  Some typical prismatic panel arrangements are shown in this image.

An innovative use of prismatic panels is demonstrated by the Hüppe Form ‘Daylight Technique’ system, in which small prismatic panels are used to form the slats of a Venetian blind, thereby giving a high level of control over lighting levels.  Light which passes though the prismatic blind is then directed onto a second reflecting blind, which redirects the light onto the ceiling.  By co-ordinating the position of both blinds it is possible to obtain a uniform light level within a room whilst still permitting a limited view out.

Prismatic panels, and their effect on room lighting levels, are discussed by Bartenbach and Klingler [1994].

The angular surface of a prismatic panel is liable to gather dirt and dust, and this reduces the effectiveness of the panel.  The panel is usually either cleaned regularly or protected by being enclosed in a sealed glazing unit.

The nature of the prismatic panel is to prevent direct light transmission - by the same mechanism any view out is prevented.  The double-blind system used by Hüppe Form does allow a limited view out, in the form of horizontal strips of view, and this is preferable to no view.

Glazings which work on a refractive principal also tend to cause some separation of the light into a coloured spectrum - prisms are traditionally used to demonstrate the polychromatic nature of ‘white’ light.  However, where a large number of small parallel prisms are used this problem tends to disappear as the separation of the transmitted light is only visible at the edges of the transmitted light beam.
 

Louvre panels
Siemens market a daylight system which comprises a rigid plastic louvre system mounted between two panes of glass.  A cross-section is shown in this image.

The curved plastic louvre is coated with a highly reflective aluminium layer, and the louvres are sloped so that light is reflected back from the louvre at some incident angles but reflected through the louvre at other angles.  The louvre system is usually arranged so that the direct path through the louvre is protected from direct solar radiation (facing north in the northern hemisphere).  This type of system can be used to give more uniform overhead lighting.

This is a simple passive technology, and is subject to the same issues as for an ordinary double glazing unit.  Durability of the plastic louvre may need to be considered.
 

Laser-cut panels
An alternative to prismatic and louvred glazings comprises a transparent plastic sheet which is cut through with a laser to give a series of parallel reflecting surfaces, as shown in this image.  Light striking the cut surfaces is reflected onto a suitably positioned surface, and by installing this type of panel in a moveable frame it can be adjusted to follow the sun.

As with any plastic-based products there are concerns about durability.  However, this type of device should be easy to produce and simple to use.  The only other limitation is that the angle of the cut is fixed, and the transmitted light will change position as the sun moves.  Fitting this type of system into a tiltable frame will allow the sun to be tracked, giving a more even light distribution within the room.
 

Angular selective glazings
All surfaces reflect, absorb and transmit solar radiation by differing amounts for different angles of incidence.  It is already known that tilting a glazing forward means that less solar energy is transmitted when the sun is higher in the sky.  However, it is possible to accentuate this effect by developing coatings with a non-symmetrical internal structure.  This image (taken from Lampert and Ma [1992] pp70) gives cross-sections through coatings in which a columnar structure has been deposited at an angle to the substrate.  Radiation striking the surface at an angle b can pass straight through, whilst at other angles the radiation is diffusely scattered by the columns.

A system like this can be used to block sun at some solar altitudes whilst allowing solar energy through at others.

The biggest problem with this type of coating is likely to be in installing the glass the right way around.  Information on the technical aspects of these types of coating is difficult to obtain, and the short review by Lampert and Ma [1992] is the only significant source of references.  It is considered however that a possible development of this technology is to develop angular selective liquid crystal coatings.

It is also important to know whether angular selective coatings can be applied to adhesive films, and if so what happens if the film is stretched during installation?