The Efficiency of a Recent Hollow Light Guides Installation
The Efficiency of a Recent Hollow Light Guides Installation
It is not easy to quantify the optical efficiency of hollow-light guides, HLGs, since they are too large to fit into the measurement devices normally used to establish luminaire efficiency. However, it is possible to determine the efficiency of a HLG installation (the so-called utilization factor). Since this factor depends on the type of luminaire and light source, lamp reflector, reflectivity of ceiling and walls, etc., comparisons must be made with great care.
A recent installation can be found in the post office terminal in Sundsvall, Sweden. In this installation, 24 sulfur-plasma lamps have been used together with HLGs to illuminate an area of ~3 300 m^2.
No officially approved independent evaluation of this Nutek-sponsored project has yet been carried out, but some interesting preliminary conclusions can be drawn. Shortly after the installation was completed, the post office measured the lighting levels to be ~390 lux, on average. The Swedish supplier of the lamps and light guides also took measurements and found the levels to be higher, slightly above 500 lux, on average. However, until these numbers are verified they must be taken with a grain of salt: In the report submitted by the supplier to Nutek, the numbers appear to be unreliable. For example, the total light output emitted from one tube is said to be equal to the amount of light produced by the two lamps feeding the tube. This is theoretically impossible since, even if the tube itself delivered light with 100 % efficiency-which is unlikely-losses of 75-80 % would be incurred at the lamp reflector. Moreover, no description of how the measurements were made was provided.
In this article, values will be assumed to be 400 lux.
Taking the post office's own measurements as the starting point, the system's efficiency can be calculated based on the following data:
- Total floor surface: 3 300 m^2.
- Light levels: ~400 lux/m^2.
- 24 light sources @ 120 000 lumen/source
- Total installed power:
24 x 1 425W = 34 200W
Conclusions:
- The total flow from the light sources should be 2 880 000 lumen (lm), which gives an average of 872 lm/m^2.
- Since 1 lux = 1 lm/m^2, the optical efficiency of the installation can easily be calculated by dividing actual values by the total flux per m^2 emitted from the lamps.
- 400 lm/m^2 divided by 872 lm/m2 gives a utilization factor of ~45%. If actual values were as high as 500 lux, the efficiency would exceed 57 %, which is doubtful given that this is above what is normally considered as being theoretically possible for this type of installation. Note that the efficiency of the HLG installation is independent of the efficiency of the light sources.
But is 45 % a good value? Another installation in Sweden with 4 similar lamps that distribute light over a large area by means of secondary reflectors in the ceiling reached an efficiency well above 60%. (IAEEL Newsletter 4/94 described an earlier version of this installation). The difference in efficiency can be explained mainly by the fact that the HLG system relies on both the reflection and transmission of light, and thus sustains greater losses than a system that relies solely on a limited number of reflections to distribute the light via highly reflective materials such as Miro aluminum or the 3M Silverlux film. Both these materials reflect more than 95% of the light impinging on the surface.
A useful comparison can be made with conventional fluorescent and HID luminaires: The post office installation requires about 2.5W/100lux/ m^2. With the present generation of more efficient switched power-supply sulfur lamps, the installed power would still be some 2-2.2W/ 100lux/m^2. This can be compared with Nutek's program requirements that, for industrial premises, allow a maximum of 2.2W/100lux/m^2 for fluorescent lighting and "average demanding visual tasks" and only 1.5W/ 100lux/m2 for HID lamps.
Another comparison can be made with fluorescent lighting. In a recent Nutek-sponsored supermarket project where the conditions were rather similar-high vertical illumination levels while luminance (glare) had to be kept low-a newly developed fluorescent luminaire achieved about 700 lux at an installed power of less than 1.7W/ 100lux/m^2.
Since capital and energy costs of the HLG installation probably would be relatively high even in future mass-produced systems, it remains to be determined whether the maintenance and possible visual quality benefits are worth paying extra for.
Nils Borg