LRT Digest 2 Tubular daylight guidance systems
Authors: Carter David 
Published: July 2014

Executive summary

This Digest concerns passive tubular daylight guidance systems (TDGS), the most commercially successful method of daylight guidance. TDGS were introduced some 20 years ago and thus now may be considered a mature technology. They are now manufactured in large numbers worldwide and installed in a wide range of building types. This Digest summarises the research effort on the devices themselves, design methods for, and economics of, the systems, and their relationship with the buildings they light. TDGS comprise a device for collecting light, a transport section lined with a highly efficient optical material and a means of distributing the light within the interior. Collectors consist of a clear polycarbonate or acrylic dome located at roof level which captures of the order of 80% of unobstructed global illuminance (the combined total of sunlight and skylight) when mounted horizontally. These work efficiently in all latitudes. For locations above 35° latitude where sunny conditions predominate, collectors may be enhanced by the addition of devices which intercept low elevation direct sun. Rigid aluminium hollow mirrored guides transport light using multiple specular reflection at the inner wall. If the light paths are long compared with the axial length of the guide, the number of reflections is large and thus overall light transmission depends on surface reflectance of the mirror and the proportions of the tube in terms of length to diameter. Guide performance is thus highly sensitive to the reflectance of the mirror, and commercially available guides use materials having a specular reflectance of the order of 99.6%. Guides having a specular reflectance less than 99.6%, or non-rigid guides, are unlikely to prove satisfactory in use. Guides may include bends and will reduce overall guide transmittance depending on the severity of the bend and the surface reflectance of the mirror. To ameliorate their effects, designers should route guides so as to minimise bends and specify guides lined with 99.6% specular reflectance material. Commercially available output devices are either opal or prismatic polycarbonate circular flush or domed diffusers or, alternatively, a diffusing or lensed square device which fits into standard suspended ceiling systems. TDGS pose unique photometric problems due to their large size, lack of specialist photometers and light sources, and their modular nature. Research has established the flux transmission efficiency of the TDGS modular ‘kit of parts’ by experimentation, but many of the published results are specific to the components tested. Other work has established flux transmission efficiency for bends and output devices. Luminous intensity distributions from commercially available output devices have also been published. The CIE ‘performance indices’ method is the only standard photometry system published to date and permits the flux transmission efficiency of the TDGS components to be determined based on a standard source. Manufacturers' published guidance concerned with design or specification of TDGS are notionally derived from photometric data but, with few exceptions, manufacturers' literature does not state the photometry system used. TDGS are a more expensive method of delivering daylight to building interiors than windows and conventional roof-lights but have the capacity to deliver daylight into areas of deep-plan buildings which the latter cannot. TDGS are of the order of 50% more expensive than electric systems which deliver similar levels of illuminance. The largest market sector for TDGS is user-owned domestic buildings, and there is anecdotal evidence of high user satisfaction in this sector. TDGS are now installed in many working environments such as offices, light industry and health care buildings. Research has shown that the light delivered via guides to working environments is recognised by users as daylight, and there is evidence that user satisfaction improves with increased quantities of daylight delivered. A daylight penetration factor (DPF) approaching 2% is suggested as providing a ‘well day-lit space’ which favourably influences user well-being and productivity. The use of small conventional vertical windows to supplement TDGS enhances user satisfaction due to external view which permits detection of weather and diurnal illuminance variation. There are a number of methods of illuminance distribution prediction within rooms in a building lit by a TDGS. These vary in sophistication depending on the available photometric data and the accuracy required. They include ‘first principles’ illuminating engineering calculation processes; ‘rules of thumb’; methods based on tabulated transmission tube efficiencies and a utilisation factor calculation; and spacing criteria for regular arrays of guides to give a uniform distribution of daylight. Two pieces of specialist software which permit the analysis of daylight delivered to interiors by TDGS of various configurations are publicly available. There has been research on the thermal properties of TDGS. Heat loss depends on length, diameter and insulation properties of the guides, and steady-state heat loss to the exterior of a building via TDGS may be estimated using published U values. Similarly, published solar gain factor data may be used to estimate heat gain via TDGS. Generally, the thermal performance of a building equipped with TDGS compares well with that of a similar building lit using conventional windows or roof-lights. TDGS potentially offer paths for spread of fire within buildings. A number of design techniques and devices, such as fire stopping and fireproof ducts, may be used to prevent spread of fire between building compartments via TDGS. Buildings equipped with TDGS present no greater risk of surface spread of flame or propagation of fire between buildings than buildings equipped with conventional lighting systems.