US3047762A - Electroluminescence - Google Patents

Description

July 31, 1962 s. v. EDENS ETAL 3,047,762

ELECTROLUMINESCENCE Filed Sept. 2'7, 1960 i Y Y Y 200 300 400 soo e00 700 800 900 I000 n00 I200 7 EXCITATION FREQUENCY, CYCLES PER SECOND LUMEN PER VOLT- AMPERE EFFICIENCY PER 0.5 DEGREE SQLID ANGLE POTENTIAL VOLTS 4 5 IO ll TIME, MILLISECONDS so s2 s5 SAMUEL V. EDENS CLARENCE I. GOODRICH, JR.

wE LEY H. SEALS BY ATTORN EY nnnauwannnwvluvm United States Patent Ofiice 3,047,7fi2 Patented July 31, 1962 3,047,762 ELECTRQLUMlNESCENCE Samuel V. Edens, Blacklick, and Clarence I. Goodrich, In, and Wesley H. Seals, Columbus, @hio, assignors to North American Aviation, inc.

Filed Sept. 27, 196i), Ser. No. 58,729 19 Claims. (Cl. 313-108) This invention concerns electroluminescence excitation and particularly relates to a method and apparatus arrangements for exciting lamps and other such devices having electroluminescent materials to especially obtain an improved elficiency in the conversion of electrical energy to light.

We have discovered that the electrical energy to light conversion efficiency typically associated with electroluminescent lamps and the like may be significantly increased if a form of excitation is utilized wherein the voltage characteristic of the applied electrical energy is essentially that of repeated, periodic, spaced-apart energy pulses of preferred voltage shape. More particularly, we advocate that electroluminescent lamps and the like be excited by an electrical energy source that continuously generates repeated, time-spaced energy pulses which each have a near-instantaneous rise time from a reference voltage value to a pre-selected peak voltage value, which each have a zero time duration :at such peak voltage value, and which each have a subsequent rapid voltage fall off from such peak voltage value and a comparatively short fall time to the initial reference voltage value. The timewidth of each periodically repeated energy pulse, measured near or along an abscissa line positioned close to the reference voltage level, is generally a small fraction of the time period determined by the established pulse frequency. Numerous advantages and unobvious results are obtained in connection with the practice of our invention.

An important object of this invention is to provide a method and apparatus for exciting electroltnninescent lamps and the like to develop therein an increased effioiency in the conversion of electrical energy to light in comparison to known methods and forms of electroluminescence excitation.

Another important object of this invention is to provide a method and apparatus for exciting electroluminescent lamps and the like in a manner which establishes an output light brightness change that is a direct function of a corresponding input voltage change and which has a straight line relation over a substantial lamp brightness range.

Another object of this invention is to provide a method and apparatus for combination with known electroluminescent lamps and the like to operate such electroluminescent devices at increased output light brightness when compared to the brightness obtained using conventional methods and apparatus at a similar operating frequency and at a corresponding peak-to-peak operating voltage value.

Another object of our invention is to provide a method and apparatus for electroluminescence excitation which can be effectively applied to electroluminescent lamps and the like to permit the use of comparatively higher pealoto-peak operating voltage values for improved light output without establishing an increase in the level of danger of possible electrical shock.

Still another object of this invention is to provide a method and apparatus for exciting electroluminescent lamps to develop :a linear output brightness change which is a straight line function of a corresponding pulse frequency change throughout a substantial pulse frequency range.

Another object of our invention is to provide a method and apparatus for electroluminescence excitation which may be effectively utilized to optimize the lumen per watt efficiency for an electroluminescent device within given peak-to-peak operating voltage and pulse frequency values.

Other objects and advantages will become apparent during consideration of the description and drawing portion of this application.

In the drawings:

FIG. 1 is a schematic and sectional illustration of an apparatus arrangement which may be utilized in the practice of this invention;

FIG. 2 graphically illustrates the voltage characteristics which are preferably developed in connection with this invention;

FIG. 3 provides information regarding the electrical energy to light conversion efficiency typically associated with this invention and regarding electrical energy to light conversion etliciencies associated with known methods and conventional apparatus for electroluminescence excitatic-n;

FIG. 4 schematically illustrates a portion of an alternate apparatus arrangement which may be used in connection with our invention; and

FIG. 5 schematically illustrates an alternate power supply apparatus arrangement which may be used in practicing our invention.

A preferred embodiment of the apparatus of our invention is illustrated schematically :and sectionally FIG. 1 and essentially consists of a power supply connected to the conventional electroluminescent lamp designated 10. Lamp ltl is provided with metallic back-up foil 11 serving as one conductive layer, transparent material 12 serving as another conductive layer, intermediate layer 13 of electroluminescent material such as a suitable phosphor, and an overlay face layer 14 of transparent glass. Such component layers of lamp in are fabricated using known techniques and compositions of material. More specifically, lamp 10 may be laminated with a foil 11 fabricated of aluminum, copper, or the like, a transparent material 12 comprised of stannous chloride, and a layer 13 which is a phosphor such as a zinc sulfide derivative that is contained or dispersed in a non-conductive ceramic. Also, the form of electroluminescent lamp illustrated in FIG. 1 derives its structural integrity essentially through the basic rigidity of the overlay glass face designated 14.

The form of power supply illustrated in FIG. 1 in electrically-connected relation to lamp 10 at contact connections 15 and 16 is important in that the objects of our invention are principally obtained thereby. Such power supply is constructed to include a source of direct-current electrical energy (battery) 17, circuit components 18 through 20 for conveniently adjusting or varying the magnitude of the output voltage associated with the energy derived from source 17, circuit components 21 through 28 for developing electrical energy that voltage-wise is characterized by waves that are repeated at a pre-selected frequency and that each have a square form, and circuit components 29 through 32 for converting the energy developed across winding 21 into energy pulses of the preferred voltage-time shape. Components 21 and 29 comprise the primary and secondary windings of a transformer device having core 32. Core 32 is preferably characterized as having a rectangular hysteresis loop so that windings 21 and 29 will exhibit low inductance at saturation.

Circuit components 18 through 20 are essentially provided for convenience in varying the output frequency and voltage of the power supply. The particular square-wave output frequency developed at winding 29 is proportional to the input voltage in accordance with the relation where V is the direct current potential measured across terminals 33 and 34, K is a constant having a value of 4.0, A is the cross-sectional area of core 32, N is the number of conducting turns in primary winding 21, and B is the flux density of core 32. In the embodiment illustrated in FIG. 1, input voltage variation is achieved through the use of transistor 18, fixed resistor 19, and variable low-power resistor 20. The load for practicing this invention is taken from the energy source across terminal points 33 and 34. From a careful consideration of components 17 through 20, it will be noted that a limited current flow is provided in the load circuit and input voltage variation is achieved through use of low power variable resistor 20. The rheostat 39 shown in FIG. 4 may be considered as performing a function similar to that achieved by components 18 through 20. As previously noted, FIG. 4 provides a partial schematic illustration of apparatus which may be used in connection with this invention but essentially details only an alternate form of input voltage control means, Use of a voltage variation means such as circuit grouping 18 through 20 or rheostat 39 is not mandatory to the practice of our invention. For instance, the apparatus arrangement of FIG. does not employ a voltage variation device.

One side of the square-wave-forming circuit is established at connection 34 which is a center tap with respect to taps 35 and 36. The other side of such circuit group is connected by connection 33 to the negative side of energy source 17. This latter connection leads to the collectors of transistors 22 and 25. Transistor components 22 and 25 are preferably selected for highfrequency operation and are characterized as having a very high gain to limit the heat otherwise dissipated by resistors 23 and 26. Such resistors are provided for the purpose of limiting base current in their respective associated transistor device.

A degree of kick-back voltage is desierd in Winding 21 to operate the transistors and achieve an oscillatory action. Because of the above-stated low winding inductance at transformer core saturation, excessive kick-back voltage transients are avoided to thereby minimize the possibility of transistor damage. As a precaution, diodes 24.- and 27 are added to the square-wave-forming circuitry to further prevent the possibility of voltage kick-back transients damaging transistors 22 and 25 Diode 24 and 27 are connected to winding 21 at connections 35 and 36, respectively, and to the negative side of source 17 through connections 37 and 38, respectively. Resistor 23 is provided to initiate the oscillating switching action of components 22 and 25. It is essentially a bias resistor for the base of transistor 25. When the components of circuit grouping 21 through 23 are subjected to source 17, transistor 25 conducts first. Components 22 through 28, in combination with core 32, provide a high overall switching efficiency. The resulting voltage form developed across winding 29 should be as ideal a square- Wave as possible to thereby permit the formation of highly reproducible shaped pulses by components 30 and 31 That portion of the apparatus illustrated in FIG. 1 as being comprised of components 30 and 31 performs a differentiating function and is provided for developing pulses of electrical energy having a proper wave (voltagetime) shape. Resistor 30 and capacitor 31 are combined in an RC circuit to develop a comparatively short time constant for the shaping circuit, and are eifective to impart the preferred wave shape to the electrical energy of square-wave voltage form transmitted to winding 29. Additional details are provided hereinafter with respect to the selection and sizing of components 30 and 31 to develop the preferred shaped voltage pulse utilized in this invention.

One particular electrical energy output developed by the apparatus arrangement of FlC 1 and having the voltage-time pulse shape which is preferred in connection with he practice of this invention is illustrated in FIG. 2. Electrical energy pulses through 49 are shown in FIG. 2 in the manner of an oscilloscope screen presentation, and are developed across connections and 16 of the equipment of FlG. l, have a peak-to-peak voltage range which extends from 300 volts positive to 300 volts negative, and have a frequency of 400 cycles per second. output pulse frequency is considered to be 400 (cycles) per second although 800 separate pulses, positive and negative, are developed each second. In the illus trated electrical energy wave form of PEG. 2 each pulse develops a peak voltage value (positive or negative) from a reference voltage of zero level with a near-instantaneous i 3 time, has a near-zero time duration at the maximum voltage value, and returns to the reference voltage level (zero) with a rapid voltage fall-off and with a comparatively short fall time. The rise of each pulse to its maxivoltage value appears as a near-vertical presentation and as shown in HS. 2 the pulse width, as measured at abscissa level near the reference voltage value, comprises only a small portion of the base time period established by the output pulse frequency. Referring to FIG. 2, the typical pulse width measured at essentially the reference voltage value of zero volts is approximately 0.3 millisecond and constitutes about one-eighth /8) of the base time period of 2.5 milliseconds. The RC circuit components and 31 used to develop pulses through 49 included a resistor 30 having a value of 10K ohms and a capacitor 31 having a value of 0.01 microfarad. The specific resistance and capacitance values stated in this application have reference to an apparatus arrangement which employs an electroluminescent lamp having a lightemitting surface area of approximately 20 square inches. Sucn specific values will change in magnitude when similar electrical energy pulses having a shaped voltage form are developed for electroluminescent lamps of different output area.

The significance of the instant invention is illustrated in H6. 3 wherein a graphical comparison is made of electrical energy to light conversion eificiencies associated with a typical electroluminescent panel lamp excited by conventional techniques in relation to conversion eificien cies associated wtih the practice of our invention. The efficiency values plotted in FIG. 3 were obtained using the apparatus arrangement of FIG. 1. The electroluminescent panel lamp ltl provided in the arrangement was a commercially-available electroluminescent panel lamp which emitted green light.

Curve 50 is established from information obtained through the measurement of lamp brightness developed by applied, repeated, shaped voltage pulses having a constant pealr-to-peak voltage and a constant pulse width but at different output frequencies over a range which extends from 200 cycles per second to 1200 cycles per second (400 individual pulses per second to 2400 individual pulses per second). The RC circuit components utilized to develop shaped voltage pulses having a 0.1 millisecond pulse base width included a 0.001 microfarad capacitor 31 and a 10K ohm resistor 30. Peak-to-peak voltage was maintained at 600 volts (+300 volts to -300 volts) and the root-mean-square voltage measured with a non-true R.M.S. meter at different excitation frequencies varied from volts at 1200 c.p.s. (cycles per second) to 42.5 volts at 800 c.p.s. to 30 volts at 500 c.p.s., to 24 volts at 400 c.p.s., and 13 volts at 200 c.p.s. Lumen per voltampere efficiency (based on a 0.5 degree solid angle of emitted light) varied from approximately 0.06 at 1200 c.p.s. to 0.09 at 800 c.p.s., to 0.12 at 500 c.p.s., to 0.17 at 400 c.p.s., and to 0.30 at 200 c.p.s. True R.M.S. voltage values for a particular curve should remain constant throughout its entire frequency range. This is for the reason that R.M.S. voltage in all cases is a function that is independent of a time value as such.

Curve 51 is comparable to curve 50 except that the pulse width of each repeated shaped voltage pulse was varied to a constant 0.2 millisecond value at its base through the use of RC circuit components which included a 0.0 05 microfarad capacitor 31 and a K ohm resistor 30. A similar 600 volt peak-to-peak voltage was maintained and the frequency varied from 200 cycles per second to 1200 cycles per second. Measured root-meansquare voltage values for observations related to curve 51 varied from 75.0 volts at 1200 c.p.s., to 55.0 volts at 800 c.p.s., to 39.5 volts at 500 c.p.s., to 33.0 volts at 400 c.p.s., to 18.5 volts at 200 c.p.s. Efiiciency values for curve 51, in terms of the ordinate units of FIG. 3, varied from 0.07 at 1200 c.p.s., to 0.105 at 800 c.p.s., to 0.14 at 500 c.p.s., to 0.165 at 400 c.p.s., to 0.28 at 200 c.p.s. It should be noted that the excitation developed in connection with curve 51 provides a generally better measured efiiciency than curve 50 throughout the range of from approximately 400 cycles per second and up.

Curve 52, which also is based upon the features of our invention, was developed from measurements made in connection with the excitation of the hereinbefo-re-identified electroluminescent panel lamp using repeated shaped voltage pulses having an individual pulse width of 0.3 millisecond. Peak-to-peak voltage was maintained at a level of 600 volts and pulse frequency, in cycles per second, was varied over the graphed range. Measured root-mean-square voltage values varied from 100 volts at 1200 c.p.s., to 72 volts at 800 c.p.s., to 52 volts at 500 c.p.s., to 44 volts at 400 c.p.s., and to 24 volts at 200 c.p.s. Lumen per volt-ampere efficiency was determined to vary from approximately 0.065 at 1200 c.p.s., to 0.10 at 800 c.p.s., to 0.12 at 500 c.p.s., to 0.14 at 400 c.p.s., and to 0.15 at 200 c.p.s. The relative efficiency is slightly better than that of curve 50 at frequencies above 500 c.p.s but not quite as good as the efficiency associated with the 0.02 millisecond pulse excitation of curve 51 in the same frequency range. The individual pulse base width, as a percentage of pulse frequency time period, can be computed using a conventional approach. By way of example, the pulse width percentage at 1200 cycles per second varies from 12% to 36% for 0.1 and 0.3 millisecond pulses, respectively, to percentages of 2% and 6% at 200 cycles per second for corresponding pulse widths.

The electrical energy to light conversion efficiencies associated with this invention, as presented in curves 50 through 52, are especially important when considered in light of curves 53 through 55 of FIG. 3. Curve 53 graphs the efficiencies obtained by exciting an electroluminescent lamp with a conventional square-wave voltage excitation, curve 54 represents the efficiencies obtained through use of a conventional sine-wave voltage shape excitation, and curve 55 relates to excitation using a triangular-shaped voltage wave. A 100 volt root-mean-square voltage value was held constant in developing all the information for establishing curves 53 through 55 throughout the indicated frequency range. Peak-to-peak voltage was varied from values of 200 volts for curve 53 to 282 volts for curve 54 to 400 volts for curve 55. The electrical energy to light conversion efficiencies associated with curves 53 through 55 are significantly lower than the efficiency curves 50 through 52 associated with this invention. In the case of square-wave excitation, the lumen per Volt-arnpere efiiciency maintained a. nearly-constant value of approximately 0.01 over the frequency range from 200 c.p.s. to 1200 c.p.s. The conversion efiiciency depicted by curve 54 is a nearly-straight line relation from a value of approximately 0.025 lumen per volt-ampere at 200 c.p.s. to 0.015 lumen per volt-ampere at 1200 c.p.s. Triangularshaped voltage wave excitation established an improved efficiency, but one which varied only throughout the range of from 0.025 lumen per volt-ampere at 1200 c.p.s. to 0.05 lumen per volt-ampere at 200 c.p.s.

Several general observations should be noted. First, the data developed for use in FIG. 3 involved brightness measurements for an electroluminescent panel lamp having a leading power factor. Brightness measurements were made with a conventional brightness meter wherein a particular color response factor of 3.0 was established for measuring the green light emitted by the electroluminescent lamp. Volt-ampere measurements were utilized in developing the values plotted in FIG. 3 as a matter of convenience. The relative efiiciency values of FIG. 3 of the drawing can be corrected to determine absolute lumens per watt efliciency by applying a correction factor. This correction factor takes into consideration total light emitted through a hemisphere, the brightness meter color response factor, and power factor, in accordance with conventional analyses.

FIG. 5 illustrates an apparatus arrangement which may be utilized to obtain the advantages of our invention in connection with a conventional alternating current electrical energy source. As illustrated therein, source 60 may be a typical volt, 60 cycle, single-phase source of electrical energy such as is commonly used in domestic lighting applications. Source '60 provides an output having a sinusoidal wave-form as to its voltage-time relation and is connected to winding 63 by the circuit lines designated 61 and 62. Winding 65, winding 63, and core 64 comprise a suitable transformer coupling device for voltage amplification. Circuit components 66 through 73 are provided in accordance with the teachings of our invention. Capacitor 66 and resistor 67 comprise the differentiating components to develop the herefore-described shaped voltage pulses. A square-wave form electrical energy output is developed at terminals '68- and 69 by Zener diode 70. The shaped voltage pulses are applied to electroluminescent lamp 71 by connections through the line components 72 and '73.

As used in this decription, the term shaped voltage pulse refers to an individual pulse of electrical energy having a voltage-time relation wherein the voltage value has a near-instantaneous rise time from a reference voltage to a peak voltage (positive or negative), has essentially a zero dwell time at the peak voltage value, and has a comparatively rapid voltage fall off and a short fall time from the peak voltage value to the initial reference voltage value. Such shaped voltage pulses are also characterized as having a total time duration at voltages sig nifieantly above the reference voltage level which is less than fifty percent (50%) of the time period established by the output pulse frequency (cycles per second). In the examples given above, pulse width varies for 2% of the period established in connection with a 0.1 millisecond pulse at 200 c.p.s. to 36% of the period established in connection with a 0.3 millisecond pulse at 1200 c.p.s. The term near-instantaneous must be interpreted in light of our observance that an instantaneous rise time would be present if the electroluminescent lamp component of the apparatus arrangement had infinitely small resistance rather than finite capacitance.

The improved efficiency achieved with shaped pulse voltage excitation is not entirely derived from the effected reduced root-mean-square voltage. Although minimum root-mean-square voltage values were achieved in connection with curve 50 an optimum efficiency appears to have been obtained in connection with curve 51 relating to a 0.2 millisecond pulse base width. Greatest lamp brightness, achieved at somewhat reduced efficiency, was developed in connection with curve 52. We have, through the developed data, clearly established that increased brightness can be obtained from commercial electroluminescent lamps as a result of the improved conversion efliciences obtained through practice of our invention. Also, higher peak-to-peak voltages at high excitation frequencies can be applied to commercially-available electroluminescent lamps in connection with the practice of this invention with accidental contact to exposed lamp terminals resulting in little or no electrical shock hazard. This advantage is attributed largely to the reduced energy 7 available at the peak of each shaped voltage pulse of electrical energy. In conventional sine-wave excitation root-mean-square voltage values increase proportionally to applied peak-to-peak voltage and not as a function of frequency.

We have also observed in connection with our invention that an increase in apparatus input voltage is mam? fested by a corresponding increase in lamp brightness. The observed excellent linearity was in connection with an electroluminescent lamp arrangement corresponding to FIG. 1 and utilized 0.6 millisecond base width repeated shaped voltage pulses throughout a range of from 357 cycles per second to 500 cycles per second and with respective input voltage values extending from 25 volts DC. to 35 volts D.C. (measured at terminals 33-34). Use of our electroluminescent excitation invention, therefore, offers advantages in that brightnes changes can now be utilized to measure or detect changes in voltage with true straight-line linearity over substantial input voltage ranges.

It is to be understood that the forms of the invention herein shown and described are to be taken as preferred embodiments of the same, but that various changes in the shape and size of parts may be resorted to Without departing from the spirit of the invention or the scope of the subjoined claims.

We claim:

1. A method of exciting an electroluminescent lamp which includes the steps of applying successive pulses of electrical energy to said lamp, said pulses each being characterized by a voltage-time history having a near-instan taneous rise time from a reference voltage value to a peak voltage value, having a zero time duration at said peak voltage value, and having a comparatively rapid voltage fall off from said peak voltage value to said reference voltage value.

2. The method defined in claim 1, wherein said electrical energy pulses are repeated at a pre-selected frequency, the duration of each of said pulses being substantially less than the time period established by said pulse repetition frequency.

3. The method defined in claim 1, wherein each of said electrical energy pulses is separated from a succeeding electrical energy pulse by a finite period of time at essentially said reference voltage value.

4. A method of exciting an electroluminescent lamp means which includes the step of applying electrical energy pulses to said lamp means, said pulses being characterized by a voltage-time history which extends from a reference voltage value alternately to a positive peak voltage value and to a negative peak voltage value, said voltage-time history including a near-instantaneous rise time from said reference voltage value to one of said peak voltage values, a zero time duration thereat, a rapid voltage fall ofi from said one peak voltage value to said reference voltage value, a near-instantaneous rise time to the other of said peak voltage values, a zero time duration thereat, and a rapid voltage fall off from said other peak voltage value to said reference voltage value.

5. The method defined in claim 4, wherein said pulses are repeated at a frequency which establishes a prescribed time period which is uniform as to successive like pulses, said pulses each having a duration which is substantially less than one-half of said time period.

6. The method defined in claim 4, wherein successive pulses of said electrical energy pulses are separated from each other by a finite period of time at essentially said reference voltage value.

7. A method of exciting an electroluminescent lamp which includes the step of applying successive shaped voltage pulses of electrical energy to said lamp, said shaped voltage pulses each being formed over a voltage range which extends from a reference voltage value to a peak voltage value, and each said shaped voltage pulse being separated by a finite time interval from its succeeding ii shaped voltage pulse at the level of said reference voltage value.

8. A method of exciting an electroluminescent lamp means which includes the step of developing successive pulses of electrical energy which are characterized by a square-wave-voltage form, converting said square-wave voltage form electrical energy pulses into electrical energy pulses having a shaped voltage form, and applying said shaped voltage form electrical energy pulses to an electroluminescent lamp means, said shaped voltage form including an instantaneous rise time from a reference voltage value to a peak voltage value, zero dwell time at said peak volage value, and a rapid voltage fall off from said peak voltage value to said reference voltage value.

9. The method defined in claim 8, wherein said squarewave voltage form electrical energy pulses are repeated at a pre-selected frequency, the duration of each of said shaped voltage form electrical energy pulses at all voltage values other than essentially said reference voltage value being substantially less than one-half the period established by said pre-selected frequency.

10. The method defined in claim 8, wherein said square wave voltage form electrical energy pulses are repeated at a preselected frequency, the duration of each of said shaped voltage form electrical energy pulses at other than approximately said reference voltage value being less than approximately 36% of the repeated period established by said pre-selected frequency.

11. A method of converting a variable input voltage into a proportional variable light brightness and which includes the steps of: converting electrical energy having a potential which is proportional to said variable input voltage into spaced-apart successive pulses of electrical energy, and exciting an electroluminescent material in a lamp by said pulses to thereby produce a variable light brightness, said pulses each being characterized by a near-instantaneous rise time from a reference voltage value to a variable peak voltage value, by a zero duration at said variable peak voltage value, and by a comparatively rapid fall off of voltage from said variable peak voltage value to said reference voltage value.

12. The method defined in claim 11, wherein said spaced-apart pulses of electrical energy are repeated at a variable frequency when said input voltage is varied, said variable frequency being proportional to said variable input voltage over a substantial input voltage range.

13. The method defined in claim 11, wherein said spaced-apart pulses of electrical energy are repeated at a variable frequency when said input voltage is varied, the duration of each said pulse being constant over a substantial frequency range and being substantially less than onehalf the period established by each frequency in said frequency range.

14. Apparatus comprising, in combination: a lamp means having an electroluminescent material which emits visible li ht when excited, a power supply which develops an electrical energy output for exciting said electroluminescent material, and conducting means electrically connecting said power supply to said lamp means electroluminescent material, said power supply developing an electrical energy output which includes successive spacedapart electrical energy pulses characterized by a nearinstantaneous rise time from a reference voltage value to a peak voltage value, a zero dwell time at said peak voltage value, and a comparatively rapid voltage fall off from said peak voltage value to said reference voltage value. 1

15. The apparatus defined in claim 14, wherein said power supply includes a resistance-capacitance circuit portion having a short time constant, said conducting means being directly connected to said circuit portion and to said lamp means.

16. The apparatus defined in claim 14, wherein said spaced-apart pulses are repeated at a pro-selected frequency, the duration of each of said pulses being substantially less than the time period established by said frequency.

17. Apparatus comprising, in combination: first means which repeatedly develops an electrical energy output having a square-Wave voltage form, difierentiating means connected to said first means and which changes each square-wave voltage form electrical energy output received from said first means into an electrical energy pulse which has a near-instantaneous rise time from a reference voltage value to a peak voltage value, zero duration at said peak voltage value, and a rapid voltage fall off from said peak voltage value to said reference voltage value, illuminating means having an electroluminescent material contained therein, and separate means connected to said differentiating means and to said illuminating means to conduct each electrical energy pulse developed by said differentiating means to said electroluminescent material.

References Cited in the file of this patent UNITED STATES PATENTS r 2,843,804 Diemer July 15, 1958 2,895,081 Crownover July 14, 1959 2,937,298 Putkovich May 17, 1960

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