This invention relates in general to low-pressure arc discharge lamps and more particularly to compact fluorescent lamps having an increased voltage drop thereacross.
It is often desirable to substantially increase the voltage drop in a low-pressure mercury discharge lamp without decreasing the efficiency of the discharge in producing UV radiation. This is especially true when it is desirable to keep the arc length of the lamp as short as possible and consequently minimize the overall physical dimensions of the lamp while maintaining the same light output. This is especially important in the case of compact fluorescent lamps in which effort is made to obtain the maximum possible light output from the smallest possible volume.
The positive column of a typical low-pressure discharge, such as the mercury argon discharge that forms the basis for the fluorescent lamp, is characterized by the fact that the properties of the plasma are independent of Z, the dimension along the arc length axis. The electron density, the electric field, and the electron energy distribution are all constant and independent of Z. Thus, if positive column length were to be increased at constant discharge current, the only properties of the lamp that would change would be total operating voltage (i.e., arc drop), total power, and total light output, all of which would increase linearly with increasing length, since every incremental length of the positive column is the same as every other.
There are three important processes that occur within the plasma discharge in every discharge lamp: (1) the gain of energy by the electrons in the plasma from the axial electric field established when a predetermined voltage is applied across the electrodes, and its redistribution among the electrons to establish an electron energy distribution: (2) the energy losses to the mercury and the starting gas (e.g. argon) by these electrons in a manner such that only the high-energy fraction (&gt;4.66 ev) is capable of creating excited atoms that will generate useful radiation, while all electrons (low energy as well as high) can lose energy via useless elastic collisions with gas and mercury atoms, wasted in the form of heat; and finally, (3) the escape of energy of excited atoms in the form of radiation, which in the case of the preferred ultraviolet radiations requires multiple emissions and reabsorptions before the radiation reaches the wall. It can be seen that the fraction of high-energy electrons in the distribution controls the balance between useful energy loss (creation of excited atoms) and wasted energy loss (elastic collisions).
In prior art lamps because of the essential equivalence of every increment of length of the positive column, all three of these processes occur simultaneously in every increment of volume of the column. Moreover, it is well known that since these three processes are not simultaneously optimized by the same choices of discharge parameters, the design of all such discharge lamps is inevitably a compromise balancing favorable changes in one process against unfavorable changes is either or both of the others. It is desirable, then to provide a means of separating at least one of these three discharge processes spatially from the other two, and causing it to take place in a different portion within the discharge envelope, so that the conditions for energy input and energy dissipation may be independently optimized.
The advantages of this separation of functions so far as practical lamps are concerned are two fold, especially with regard to compact fluorescent lamps. First they permit the achievement of relatively high arc drops in limited length of tube, without requiring very small diameter tubes. High efficiency requires that the positive column component of the discharge voltage be large in comparison to the electrode loss, a condition relatively easily achieved with multiply constricted lamps fabricated according to the preferred dimensional ratios outlined hereinafter. The achievement of electron energy distributions having a much enhanced fraction of high energy electrons also permits operation of the lamps at much higher power input per unit volume while still remaining in the approximately linear domain of output versus current. This means that high specific light levels can be reached in small volume lamps at good efficiency, an extremely important consideration for compact lamps. Also, under comparable conditions of high loading, lamps according to the present invention become substantially more efficacious than their straight tubular counterparts.
Major limitations in the prior art discharges result from the fact that the energy distribution of the electrons in a discharge optimized to the best degree hitherto possible is approximately a Maxwellian EQU f(E)=(2/kT.sqroot..pi.).epsilon..sup.1/2 e.sup.-.epsilon. ; .epsilon.=E/kT
where
E=electron energy PA1 k=Boltzmann's constant PA1 T=electron temperature.
At the typical electron temperature in a prior art fluorescent lamp, it is a characteristic of the Maxwellian distribution that only approximately one percent of the electrons have energy enough to create an excited atom, whereas all of them can make wasteful elastic collisions with gas atoms. It is possible, of course, to arrange conditions such that the electron temperature of the distribution is higher, in order to have a higher fraction of the electrons with energies greater than 4.66 ev. This is accomplished, however, only by reducing the diameter, or decreasing the gas filling pressure, both of which result in more rapid loss of high-energy electrons to the walls, thereby adversely affecting the efficiency. Finally, in actual lamp conditions, the losses of high-energy electrons from the distribution (either as a result of excitation of mercury or as a result of losses to the walls) are sufficiently rapid that the high-energy tail of a Maxwellian distribution cannot be fully populated by energy input processes; in short, the already small fraction of electrons that are able to do useful work is reduced still further.
A further disadvantage of this deficiency results from the fact that low-energy electrons can collide with excited mercury atoms and remove their energy, creating high-energy electrons and atoms in the lowest or ground state from which they cannot radiate. In the limit of very large collision rate (i.e., high electron density), it is plain that the fraction of atoms that can be in the excited state and emit radiation is dependent only on the fraction of high-energy electrons versus the fraction of low-energy electrons, and becomes independent of the total number density of electrons, and therefore independent of discharge current density. In this domain, useful output becomes a constant, while losses continue to increase, so that efficiency declines drastically with increasing current density.
As a consequence of this effect, if current density is increased from low values toward high in a fluorescent lamp, the number of excited atoms (and consequently the radiation output) increases linearly at low currents, and approaches a constant value essentially independent of current at high values of current density. The only mechanism hitherto available to the lamp designer to ameliorate this condition has been to reduce tube diameter and gas pressure to permit higher electron temperature and consequent higher fraction of high energy electrons, with the disadvantages already outlined above.
Fluorescent lamps have been made in the past which attempt to increase the voltage drop across the lamp by means of indentations or grooves in the envelope. Examples of such lamps having a plurality of individual indentations formed in a periodic manner along the envelope to increase the effective arc stream length are shown in U.S. Pat. Nos. 2,916,645; 2,973,447; and 3,098,945. These configurated lamps which have elongated tubular envelopes with non-circular cross-sections are generally complicated and consequently rather expensive to manufacture.
Another lamp is shown in U.S. Pat. No. 3,988,633 in which a plurality of separate and continuous grooves are used to increase the radiation of the lamp by altering the wall recombination rate of the plasma ions with the phosphor. The additional voltage generated in this type of lamp is not substantial i.e., much less than 0.25 volts per groove.
The prior art did not recognize the importance of minimizing the thickness of the constricted portion and of having as sharp a transition as possible, the more closely to approach the condition of a step-function change in potential in a very short distance.