Charles W. Pearson

Biography
Charles W. Pearson was a lamp design engineer at GE's NELA Park laboratories in Cleveland OH, USA. His most famous contribution to the world of light source technology was his 1955 invention of the axial-filament general service incandescent lamp, which will be described in detail below.

Development of the Axial Filament Incandescent Lamp
Until the mid 1930s tungsten lamps used filaments that had been coiled once, and were therefore called single coils. Around 1934 the British lamp manufacturers perfected a concept that had been patented much earlier by Bernie Lee Benbow in 1917, concerning a coiled filament that had itself been coiled again, to achieve a coiled-coil. Philips benefitted from a British factory at the time and introduced its own coiled-coil lamps in the same year, and the technology was adopted by General Electric and introduced to the Americas soon afterwards, around 1936.

In the high voltage European countries the coiled-coil lamps were at first manufactured only in ratings of 40W, 60W, 75W and 100W, and in the low voltage Americas only in 50W, 60W and 100W ratings. The reason is because the gain in efficacy with the second coiling becomes more significant at lower power ratings. Above 100W there was little to be gained by applying the more expensive coiled-coil filaments - and below 40W there were technical difficulties associated with forming the delicate coiled-coil structures at the finer wire diameters required for such low power filaments.

For several decades following the introduction of the coiled-coil lamps, it was considered that incandescent lamps had by then reached the pinnacle of their performance and that no further improvements were possible. However in the early 1950s, Charlie Pearson noted an interesting phenomenon. Considering the popular American 100-watt 120-volt lamps, their coiled-coil filaments were mounted perpendicular to the lamp axis, in the so-called CC-6 configuration. Since most lamps are operated vertically cap-up or cap-down, this means that the filament is horizontal in operation. Pearson discovered that if the filament was operated vertically, the heat loss to the gas filling was reduced - this caused the lamp filament to operate at higher temperature, and emit more light. This effect is not unusual - however it was surprising to note that the lifetime also increased. In all other cases a hotter filament would lead to a reduced lifetime.

Starting around 1951, Pearson therefore took on the task of orienting the coiled-coil differently to take advantage of the reduced gas loss and increased life. Far from being simple, this required not only a redesign of the filament to optimise it for operating under different conditions, but also considerable changes in the lamp construction as well as new manufacturing techniques and machinery. Changes to the alloy of the leadwires were even developed, to result in a mechanically robust mount assembly. The result was what has since become well-known as the typical American axial filament incandescent lamp, the first model of which is illustrated below.

Pearson applied for a patent on October 14 1955, which was granted as U.S. Patent 2,877,375 on Mar 10 1959. It can be immediately appreciated that such an improvement could be realised in lamps of various power ratings, in particular to bring a benefit to the higher wattage lamps for which a coiled-coil transverse filament on its own does not bring any notable improvement beyond a single-coil filament. Quoting from the patent:

"By changing the filament from a horizontal to a vertical position the lamp current is decreased and the light output increased at rated voltage. In addition to increased efficiency, the life of the lamp is also increased. The decrease in current signifies an increase in filament resistance which, in turn, signifies an increase in filament temperature. It is thus apparent that the filling gas in the lamp has less cooling effect on a vertically operated filament than on a horizontally operated one. The reason for the increased life is not clear. However, it is possible that equilibrium tungsten (filament) vapor pressure in the immediate vicinity of a vertically operated filament may be maintained with a lower rate of vaporization from the filament than in the case of a horizontally operated filament because of the presence of vapors which originate from a lower portion of the filament."

GE wasted no time with its commercial introduction of the new concept. Just three days after filing for a patent, a press release was issued on October 17 1955. It is evident that such lamps were not ready for sale at that time, and the news release was merely indicating how GE's future incandescent lamps would look. However the importance of the invention is underscored with a statement that this represented the most important development of incandescent lamps for the past 42 years, since Langmuir's invention of the gasfilled single-coil lamp in 1913.

Commercialisation of the new design was swift. By December 1955 advertisements were placed in the press to introduce the new lamps - at first only in ratings of 750W and 1000W, with a note that other ratings from 50 to 500W would follow. These high wattages perhaps had the most to gain from the new design, because bulb blackening is a significant cause of lumen depreciation in high wattage lamps, whose filaments operate hotter than in lower power lamps. Here another advantage of the axial filament was found that it reduces the rate of bulb blackening, by concentrating the deposited tungsten in a smaller area of the bulb and therefore reducing the impact on light output. The new versions claimed an increase of 15% in average luminous flux during the life of the lamps - sufficient 'free' extra light to entirely offset the purchase price of the lamp itself! GE translated this into a bonus for consumers of over $ 100 million US dollars per year.

Another advertisement appeared in the General Electric Review¹ that compared a standard crosswise filament (CC-6) with the new stand-up filament (CC-8) as well as the wall blackening difference between the two lamps. The pictorial difference is shown here. The advertisement read:

   "General Electric announces the most important development in light bulb filaments in 42 years - a filament that stands on end. It gives up to 15% more light..."
   "The 750 and 1000 watt Bonus Line G-E Lamps are available now... Other sizes from 60 to 500 watts are being redesigned to use the new stand-up filament.
   "Four years of intensive research were required to solve the technical problems involved in changing from the usual crosswise position to a vertical position.
   "The light gain achieved by the new stand-up positioning is a minimum of 6%. The new positioning makes it practical for the first time also to use a specially designed coiled-coil filament in bulbs of 300 watts and over. In these larger bulbs, the two improvements combine to bring the light increase to 15% during the life of the bulb...
   "The new stand-up filament increases light output in two ways.
   "First, the hotter a filament burns the more light it gives, and the vertical position allows the filament to burn hotter without burning out any sooner.
   "Second, the effect of the bulb blackening on light output is cut to a minimum. Blackening is concentrated in the stem of the bulb when it burns base up, or in a small spot at the opposite end when burned base down.
   "G-E incandescent bulbs using the new filament are called the G-E Bonus Line."

The GE 1956 Large Lamp Catalog lists only the original 750W and 1000W ratings. It is noted that the beam distribution from some floodlights might not be satisfactory with the new compact filament, and for such applications GE would continue to offer the former lamps with C-7A filament configuration.

The GE 1960 Large Lamp Catalog shows that the range had only been extended to one other rating of 100W and this only with inside frosted bulb. The previous version with the former CC-6 transverse coil was also listed, which allows the performance of the two lamps to be compared. The axial coil version claims a flux of 1700lm vs the 1640lm of the previous design, an increase of about 3.5%. Incidentally the new axial filament marked the first use of the smaller A-19 size bulb for the 100W rating in the USA - formerly the A-21 bulb had been standard for this rating. It is also interesting to note that the new filament configuration was only offered with inside frosted bulbs - the clear version remained with the former CC-6 transverse coil.

Moving forward now to a GE 1972 Large Lamp Catalog the majority of the 100W range had migrated to the CC-8 coil and the flux had been increased again to 1750lm. This improvement however does not stem from the vertical coil, because the few types still being made in the former A-21 envelope with transverse coil had also improved from 1640 to 1690lm - the change presumably being due to a design optimisation of the 100W coil. The CC-8 filament had also been adopted in the 150W rating whose bulb size was reduced from A-23 to A-21, and to a new 200W lamp in A-23 bulb to replace the former PS-30. Similarly a new compact 300W PS-25 with medium base joined the fomer 300W PS-30 medium and mogul based lamps, and a 500W mogul PS-35 replaced the PS-40. The vertical CC-8 filament was also adopted in 3-Way lamps containing a 100W filament, and in one PAR-38 reflector spot lamp. Perhaps a little deceptively at this time, GE stopped listing the luminous flux values of the lamps having the former filament construction - presumably in an effort to move customers towards the newer design which would surely have been cheaper to produce.

The expected roll-out of the axial filament lamp to other general service lamps did not happen. Indeed GE did not even upgrade its 40W and 25W general service incandescent lamps to a coiled-coil filament until the 1980 and 1987 catalogues respectively - and then only with the old-style transverse CC-6 coil. This is believed to have been due to metallurgical difficulties associated with the stability of such fine coiled-coil filaments, which could be controlled with a horizontal filament but were not overcome for a vertical-burning filament.

Since GE participated in a full patent cross-licensing agreement with most of the world's principal lamp manfuacturers until the early 2000s, other companies had access to this technology in return for sharing their lamp patents with GE. As such in the Westinghouse 1975 and Sylvania 1973 catalogues, a full line of 60, 75, 100, 150, 200, 300, 500, 750 and 1000W lamps are listed with axial filament. Interestingly Westinghouse employed two 60V 500W filaments connected in series beside each other for its 120V 1000W design - perhaps for reasons of increased robustness.

In the high voltage European countries there were also efforts to copy the GE technology, but this was not possible owing to the reduced mechanical strength of the necessary 220-250V filaments³. High voltage GLS filaments typically require two filament supports, whereas in America the more robust 120-130V filaments required only one support, or for some ratings none at all. It was not possible to develop an economical filament mount assembly for high voltage axial coiled-coil lamps that was compatible with the very high speed automatic production machinery for these types. Moreover, whereas the benefit of coiled-coil filaments benefits primarily the lower current types, the vertical coiled-coil has most impact on the high current lamps and the gain for low power high voltage (hence low current) European GLS lamps was limited. The extra expenditure to make an axial filament high voltage lamp would have offset any performance improvement from this design, hence such lamps were not developed.

Development of the 2-Way Lamp
Charlie Pearson also patented a lamp in which a silicon rectifier diode was mounted in the base (U.S. 3,148,303, dated Sep 8 1964) Drawings from that patent are shown to the left. The purpose of the diode is to allow a single filament to operate at more than one level of illumination. In a normal lamp without a diode the base has two metallic contacts on the base. In the case of a lamp with a diode there are three contact areas on the base.

The lamp can be used with a multiple contact switch socket or it can be used in normal 3-way sockets. In the case of a 100-watt coiled-coil operated in series with the recifier a current of about 0.42 ampere is drawn and the light output is about one-third the output at the normal current of 0.833 ampere.