USING LASER CUT PANEL IN WINDOWS. (SIMPLE, ANGLED, CHANNEL and HOW MUCH)
BASIC FUNCTION OF LCP PANELS IN WINDOWS
Laser cut panel (LCP) contains many tiny mirrors used to redirect daylight to the ceiling rather than the floor.
Daylight on the floor is wasted as heat, gives little illumination and lots of reflected glare. Daylight on the ceiling is reflected down onto work surfaces giving useful diffuse illumination.
LCP combines well with conventional window shading devices such as Venetian blinds, external awnings or roller blinds to give both good daylight AND good shade.
TYPES OF LCP.
USING SIMPLE LCP
In Simple LCP the cuts are made normal to the surface of the panel and part way or all the way through. As the internal mirrors are horizontal sunlight is reflected up at the same angle it comes in. To illustrate how sunlight is distributed in the room below the scale of the panel is increased horizontally to make the optics of the redirection more obvious. The actual horizontal thickness of the panel is just 6 mm.
Note that at higher sun angles most sunlight is redirected upward. It is only low elevation sunlight that is redirected to the back of the room.
BENEFITS: Simple LCP acts as a very thin internal light shelf redirecting a large fraction of sunlight onto the ceiling. It is useful in high clerestory windows to illuminate relatively shallow rooms. Simple LCP is especially useful in buildings with high ceilings e.g museums, art galleries and gable roofed churches.
PIT FALLS: As about one half of lower elevation sunlight enters without redirection this can give rise to a direct sunlight glare source on work surfaces. Thus simple LCP is mainly useful on Northern facades or with an external or internal louvres (Venetian blinds) to eliminate the low elevation sunlight if used on other facades.
USING ANGLED LCP.
In Angled LCP the cuts are made at an angle 5o – 15o to the normal to the surface of the panel and part way or all the way through. As the internal mirrors are sloping at an angle q sunlight is reflected up at about 2q lower than the angle it comes in. In the illustration below the scale of the panel is increased horizontally to make the optics of the redirection more obvious. The actual horizontal thickness of the panel is just 6 mm.
Note that at higher sun angles most sunlight is redirected upward and deeply into the room. There is a range of input angles at which all light is redirected upward. Note that any sunlight not redirected passes through the panel at the same angle it came in.
BENEFITS: Angled LCP acts as a very thin internal light shelf redirecting a large fraction of sunlight onto the ceiling very deep in the room. It is useful in the top part of windows to illuminate relatively deep rooms. Angle LCP is especially useful in deep plan buildings with low ceilings e.g office buildings. It should only be used in the top part of Northern facade windows in combination with Venetian blinds for reasons explained below.
PIT FALLS: Most lower elevation sunlight enters without redirection and this can give rise to a direct sunlight glare source on work surfaces. Thus angled LCP should be used only on Northern facades or with an external or internal louvres (Venetian blinds) to eliminate the low elevation sunlight if used on other facades. Note that at lower elevation angles some sunlight is redirected at low angles below horizontal into the room. THIS IS POTENTIALLY A SEVERE GLARE PROBLEM - for occupants looking towards the window it is like looking at the sun. Thus angled LCP should always be used with internal louvers (Venetian blinds) that can be adjusted to block below horizontal redirected sunlight when it occurs.
USING CHANNEL LCP.
Channel panel was designed to eliminate the problem encountered with Angled LCP. In channel LCP the cuts are made at an angle of 12o to the normal to the surface on both sides of the panel meeting or nearly meeting in the middle. The Channel LCP is typically 12 mm thick rather than 6 mm thick and is typically about twice as expensive as Simple and Angled LCP. However, it has considerable advantages for the input of sunlight to buildings for natural lighting. In the illustration below the scale of the panel is increased horizontally to make the optics of the redirection more obvious. The actual horizontal thickness of the panel is just 12 mm.
Note that at higher sun angles most sunlight is redirected upward and deeply into the room. There is a range of input angles at which all light is redirected upward. Note that any sunlight not redirected upward passes through the panel at the same angle it came in.
BENEFITS: Channel LCP acts as a very thin internal light shelf redirecting a large fraction of sunlight onto the ceiling very deep in the room. It is useful in the upper part of windows to illuminate relatively deep rooms. Channel LCP is especially useful in deep plan buildings with low ceilings e.g office buildings. It can be used in the top part of windows by itself as it is only at low to medium angles that any light passes downwards. All light passing downward does so at the same angle it came in so Channel LCP is very effective when used in combination with Venetian blinds blocking sunlight through the lower window.
PIT FALLS: There are no specific pitfalls with Channel panel. It is designed to eliminate the possibility of sunlight being redirected into occupants view so there are no glare issues.
HOW MUCH LCP TO USE
Sunlight is a very powerful source of illumination. Direct sunlight passing normally through a one square metre area carries 100,000 lumens of visible light and 1 kW of radiant energy. Also, Australian cities have, on average, 8 hours of direct sunlight each day.
Consider an angled LCP or Channel panel 1 metre wide and 0.5 metre high in the upper part of window. If direct sunlight at 60o elevation is incident directly on the window this half square metre panel receives about 45,000 lumens.
At 60o sun elevation all sunlight is redirected into the room at 20o elevation and over the ceiling by Channel and by Angled. Taking into account reflection loss in the panel itself, 40,000 lumens is distributed over the ceiling. If the ceiling is matt white (80% reflective) 32,000 lumens is diffusely reflected down to illuminate work surfaces in the room. If the room is 5 m deep, 4 m wide and 3 m high the surface area illuminated (floor plus walls) is 74 square metres. Thus, if no other light enters the room the average illuminance on wall and floor surfaces is 32,000/74 = 430 lux. This exceeds the standard level of 320 lux for office buildings in Australia. The external horizontal illuminance is 100,000*sin60 = 87,000 lux. So 430 lux average is 0.5% of the external horizontal illuminance.
Thus, in sunlight, relatively small areas of LCP panel (typically 2.5 square metres of panel per 100 square metres of floor area) can provide all illumination requirements in sunlight. In overcast conditions and “cloud over sun” conditions the level falls to about 1/5 to 1/10 of the level found above. At these times electrical lighting must be added. As occupants are reluctant to control electrical lighting it is evident that, to save energy and electricity costs, LCP daylighting systems should be combined with automatic electric light dimming systems in office buildings.
ILLUSTRATING CHANNEL PANEL COMBINED WITH FIXED VENETIAN BLIND
In the diagrams below the horizontal (thickness) dimension of Channel panel is expanded wrt the room for clarity. The actual thickness is 12 mm.
Nearly all high elevation sunlight is redirected at a low elevation over the ceiling of the room – providing useful diffuse illumination to the interior.
Most mid elevation sunlight is redirected at low elevation over the ceiling of the room. - providing useful diffuse illumination. Any sunlight passing downward passes the Channel panel without change of direction and is blocked by the Venetian blind.
Most low elevation sunlight is redirected sharply upward and is blocked by the Venetian blind. Reflection from the underside of the Venetian blind provides diffuse illumination to the interior. Any sunlight passing downward passes the panel without change of direction and is blocked by the Venetian blind.
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