4.5 Breakdown of patent showing the ‘Environmental contradiction’ and solutions
hierarchy
From the patent it was clear that the company had made a strategic commitment to try to
develop a lamp that would pass the TCLP test without cheating whilst still producing a lamp
that would be competitive. In real terms this meant that, to pass this test they would have to
reduce the mercury content of standard fluorescent tubes by at least 75% whilst achieving
an energy efficient, 20.000 hour lamp life.
From the company’s strategic point of view the ‘Environmental contradiction’ was between
remaining competitive in the lighting market and complying with environmental legislation
without cheating. Figure 7 shows the ‘Environmental contradiction’ that the company was
trying to solve between lamp performance characteristics and harmful materials in lamps.
Figure 7 breakdown of patent US5898265.
The lamp’s life-time is affected by mercury absorption in the glass envelope over time,
electrode failure and tube blackening from spitting electrodes. The lamp’s energy
consumption is affected by the efficiency of the phosphors to convert the UV radiation into
visible light. Figure 7 shows the combined approach described in the patent which
addresses all these performance factors (see columns 1,2 and 5).
The use of the best tri-chromatic phosphors that efficiently convert the UV radiation into
three main bandwidths of visible light, namely, red, green and blue.
The small metal shields around the cathodes inside the tube catch the spitting from the
cathodes that otherwise cause the tube to blacken and thereby shorten its life.
Over time the amount of mercury vapour inside the bulb slowly decreases due to its
absorption in the phosphor layers and the glass envelope. Special ‘barrier’ coatings help to
reduce this effect.
The most innovative part of the patent is shown in column 3 and 4 of figure 7. Traditionally
lamps have always been overdosed with mercury. This was done because the actual
mercury absorption rates in the tube were unknown and manufacturing techniques were
inaccurate. This patent describes the method for calculating the minimum mercury dosage
required for competitive lamp life and a novel manufacturing method that accurately inserts
that minimum dose in the tube (see columns 3 and 4).
The extremely low dose of mercury is accurately inserted in the tube by containing it within
a small glass capsule, which is mounted on one of the end guards in the tube. There is a
metal wire encircling this glass capsule. After the production of lamp is complete, the
sealed glass capsule is heated inductively by a high frequency electromagnetic field which
causes the wire to cut the glass capsule and release the mercury into the tube.