Shades of Grey

By one of those coincidences that seem uncanny (until you think about how many non-coincidences you have every day) I happened to be singing Lily the Pink in my head when someone asked me the difference between efficacy and efficiency. Now ten years ago I could have trotted out an answer straight away (as lamps are measured by the former, rather than the latter) but this is what Wikipedia has to say:

1. In lighting design, “efficacy” refers to the amount of light (luminous flux) produced by a lamp (a light bulb or other light source), usually measured in lumens, as a ratio of the amount of energy consumed to produce it, usually measured in watts. This is not to be confused with efficiency which is always a dimensionless ratio of output divided by input which for lighting relates to the watts of visible energy as a ratio of the energy consumed in watts. The visible energy can be approximated by the area under the Plancks curve between 300 nm and 700 nm for a blackbody at the temperature of the filament as a ratio of the total energy under the blackbody curve. Efficiency values for light from a heat source are typically less than two percent.

Anyway, Ken, a bright spark at CCT Lighting, told me how they actually measured luminous flux in a laboratrory- they put the lamp in the centre of a sphere, the inner surface of which has a number of photo cells with known characteristics. This way, all of the light that escapes from the envelope is measured, the only nuisance being where the light shines on the construction elements such as the cap.

I got back into stage lighting via the architectural route (theatre techniques in business environments) and In my architectural lighting 101, our wonder lamp was a compact source metal halide one, 150 Watts of power consumption but it emitted 12,000 lumens. At eighty lumens per watt, this compared very favourably with 500 Watt tungsten lamps that typically yielded only 20 Lumens/Watt (at the cost of low lamp life). Being a discharge lamp, it was useless for dimming but it had a 12,000 hour life which meant that it only had to be relamped every two years or so in normal use. This made it ideal for highlighting architectural features in large buildings so consequently I had some interesting trips along very high access walkways in order to focus the lanterns (or projectors, as the architects preferred to call them).

One one memorable visit to a site in Stockley Park with several huge atria, the only access was via an enormous cherry picker called a spider. Because it was working at the extent of its range, we sometimes had to manouvre to the required position from different angles as the balance sensors would inhibit further movement when we pushed the stability envelope. It is a little unnerving when every movement makes the working platform bounce several inches.

The Contractor also totally overlooked that it wasn’t possible to accurately focus lanterns on a sunny day in a glazed atrium. Fortunately, we had allowed for this but they had to pay their Spider driver double time that evening.

So, to return to a familiar theme, are low energy lighting products the future? Yes, but not in their present form. They are effectively miniature fluorescent lamps with a lot of associated electronics to make them strike an arc and as they convert ultra violet light to visible light by the use of phosphor coatings, they are complex to make and tricky to dispose of. Meanwhile, the light emitting diode broke the 100 lumens/Watt barrier last year and looks likely to reach 145 Lumens/Watt early next year. LEDs can be dimmed, they last up to 60,000 hours and their colour performance is getting better with each iteration of design. They are widely used in decorative lighting but they still have a long way to go to replace halogen downlighters. There are lots of other engineering and aesthetic problems along the way but the future looks bright.

60,000 hours is a lot- that is switched on continuously for nearly seven years and if cycled to match normal domestic use, they could outlast offspring in the family home. (Assuming the children can actually afford to move out when they grow up, that is).