Letters
Specular reflectors
Retrofit (specular) reflectors have been mentioned several times in your columns as a means of saving energy (See, for example, Specular Reflectors Great Savers, IAEEL 2/92). One typical claim manufacturers make is: "A simple but effective solution is to remove one fluorescent lamp from a double- tube fitting and replace it with a reflector. 50% of the light from a fluorescent tube shines upwards; by reflecting the otherwise lost light downwards, one tube will do the work of two." If this proposition is put to your readers, alarm bells should start to ring and they should call in an independent lighting engineer.The main difference between white enamel surfaces and mirror surfaces is the way in which light is reflected: From enamel surfaces it is mainly diffused and from mirrored it is mainly specular, and it is possible to control and direct the light more accurately. When coupled with properly designed luminaires, specular reflectors will produce efficient glare-free lighting. Your readers may find that, if improperly applied, reflectors will concentrate light immediately below the luminaire, that the maintained illuminance over the whole area will be considerably reduced, and that the uniformity will be undermined.
Ernest Magog
Director
Lighting Industry Federation Ltd., London, UK
Reply: The reader correctly points out that removing half the lamps from a fluorescent fixture (or actually disconnecting one of the ballasts) and inserting a specular reflector will not produce the same amount of light as the original fixture. The specular reflector will increase the fixture efficiency but not by the factor of two implied by the dubious sales pitch quoted by Mr. Magog.
Research indicates that the effect of removing half the lamps from a clean fixture and inserting a specular reflector will generally increase the fixture efficiency by 10-25%. Thus if the efficiency of the original fixture was 70%, then the specular reflector will boost the efficiency to 77-87%. If the internal surfaces of the original fixture were seriously degraded with age, then the specular reflector may cause a larger relative increase in fixture efficiency but only because the fixture had a lower efficiency to begin with.
This leads to two more subtle points aboout how specular reflectors should be applied: First, the fixture interior and lens are usually cleaned during the retrofit process. The cleaning action alone may cause a significant increase in fixture efficiency independent of the installation of the specular reflector. However, the increase due to cleaning is only temporary since with time, the fixture output will gradually drop until the fixture is once again cleaned.
Second, because of thermal effects, the power drawn by a two-lamp fixture with specular reflector will be somewhat more than half the power drawn by the four-lamp fixture. This is because the decreased ambient temperature in a two-lamp fixture (with reflector) causes each lamp to draw slightly more power and produce proportionally more light than in a hotter four-lamp fixture (also see article on p. 6).
However, even taking this thermal effect into account, the end-user should expect that a clean two-lamp fixture with specular reflector will produce 65-70% of the light from a clean four-lamp fixture without specular reflector. As a result, the use of specular reflectors as an energy-conservation measure is best applied in spaces that are currently overlit.
Finally, Mr. Magog points out that a fixture with a specular reflector may produce more light directly below the fixture but throw out much less light to the sides. This change in distribution may cause significant problems of non-uniformity if the original fixtures were placed at the limit of their spacing criteria. Also, the more downwardly directed light distribution from the specular reflector fixtures may cause objectionable scalloping on walls, especially if the perimeter row of fixtures is mounted more than 0.6-1 m from the wall.
Editors