Cupola
The transparent part of light guide which collects sunlight and skylight is generally designed as cupola. The other shapes of the cover of the exterior tube can be used such flat glass, non-spherical shape or cutted cylinder. The cover with shape different to cupola transmits less light.
Shape
Required properties of the dome which collect and transmit light predetermine its shape. The cupola can be may be low, very convex, rounded or stepped, tapered, circular ground plan, elliptical, square, rectangular or diamond-shaped cut. The efficiency of the light guide depends also on the height, shape and curvature of the cupola. I is generally valid that cupola with flatter shape resulted lower efficiency (up to 35%) compared to cupola with conventional hemispherical shape, especially during low sun situations (early morning, late evening or in winter).
Section
- a) clear, spherical, b) cone c) mushroom, d) unilaterally tapered, e) glass shaped, f) partially prismatic glass, g) cupola Lightway.
Plan
- a) circular, b) rectangular, c) segment d) square.
Material
Cupolas are made from hard plastic PC, PMMA or glass. The material of cupolas must ensure long-term durability and required optical properties, the maximum transparency, mechanical resistance, as well as material stability. Extraordinary demands are placed on the ability to resist external climate influences such sun radiation, high temperatures, frost and hail.
The material of cupola has to be also abrasion resistant, waterproof and fireproof. These requirements and high light transmission are important in the light-guide design.
The dome is usually fitted around the perimeter by seal which prevents dust and moisture into the tube.
The cupola surface is smooth, in some solutions around the perimeter it is regularly wrinkled, thus larger area of the cupola surface is created and incident rays can be redirected into the tube. There are covers of prism shapes – diamond which have special grind located on the north wall. This grind acts as a reflector - directs the sun's rays in the desired direction into the tube.
Optical properties such light reflection, transmission and absorption by cupola are expressed by following parameters:
- Reflectance - ro,
- Transmittance - tau,
- Absorptance - alfa.
Table Technical properties of different cupola materials (Plch, 2005)
Type of material | Thickness of material | Transmittance | Reflectance | Absorptance |
mm | tau | ro | alfa | |
Clear glass | 2-4 | 0,90-0,92 | 0,06-0,08 | 0,02-0,04 |
Acrylate - clear | 3 | 0,92 | 0,08 | 0,00 |
Acrylate opal | 3 | 0,55-0,78 | 0,17-0,41 | 0,04-0,05 |