US 3675069 A
A point light source is disclosed having a spark gap in which the arc is confined in a groove and is viewed end-on through an aperture in a mask.
Claims available in
Description (OCR text may contain errors)
United States Patent Crossen July 4, 1972  POINT LIGHT SOURCE  Inventor: Kenneth R. Crossen, New York, N.Y.
 Assignee: EG&G, 1nc., Bedford, Mass.
 Filed: Jan. 6, 1971  Appl. No.: 104,383
52 u.s.c1 ..3l3/268,313/198,3l3/204, 356/86 511 mu ..I-10lj1/88 5s FieldolSearch ..3l3/197,199,190,198,204, 313/214, 250, 258, 243, 268; 356/86; 240/13  References Cited UNITED STATES PATENTS 2,152,992 4/1939 Franciseta]. ..313/204 3,144,551 8/1964 Webb et a1 ..356/86 X 3,418,514 12/1968 Sternberg ..356/86 X 2,252,508 8/1941 Hoff ..356/86 FOREIGN PATENTS OR APPLICATIONS 793,705 4/1958 Great Britain ..313/l97 Primary Examiner-Alfred L. Brody Att0rney-Ralph L. Cadwallader and Lawrence P. Benjamin  ABSTRACT A point light source is disclosed having a spark gap in which the arc is confined in a groove and is viewed end-0n through an aperture in a mask.
5 Claims, 12 Drawing Figures P A'TENTEDJUL 4 I972 sum 2 OF 3 KENNETH R. CROSSEN ATTORNEYS PATENTEnJuL 41912 3,675,069
sum 3 or s KENNETH R. CROSSEN /A/l EN7'0/? ay (4M4 W ATTORWF s POINT LIGHT souncr:
The present invention relates to point light sources and in particular to a spark gap of novel design adapted for use a point light source.
Certain phenomena involving high-speed motion are best studied photographically, the exposure being produced by an arc source of very short duration. Examples of such phenomena are the flight of a bullet and the deformations occuring on impact. However, the normal photographic technique does not reveal certain aspects of these events. For example, a photograph taken in a manner analogous to ordinary flash photography cannot record the compression wave produced by a bullet in flight because the photographic contrast is inadequate. The requirements for a light source intended to yield high quality shadowgraphs are both stringent and mutually contradictory. The time duration of the flash must be short in comparison to the time scale of the phenomenon under study. In the case of sonic compression waves, this means a duration on the order of a few microseconds at most. Failure to keep the duration down will result in motion blur, as when photographing a moving train.
In fact, no point light source can actually be dimensionless. The dimensions must be made small in relation to the distance from the subject to the source. Larger source dimensions lead to deterioration of resolution of the photographic record proportional to the increased dimensions. The last basic requirement is sufficient light output in candle-power-seconds to expose the film at the chosen distance between film and source. In general the subject should be kept very close to the film to maintain high resolution.
Besides these basic requirements there are a host of other practical problems which beset the user of a poorly designed point light source. Inconsistency of output light power will lead to a waste of film (usually expensive 8 inches X inches plates) and possibly the loss of valuable data. Similarly, waste occurs if the point light source self-fires or will not fire at the proper moment. Self-firing or arcing across paths other than I the intended path can lead to eventual destruction or uselessness of the light source.
The most important requirements for a point light source are high light output power, short time duration, and small dimensions. These requirements are so related that some compromises are necessary. A related factor is that of the afterglow, meaning the slow decay in light output after firing of the point light source. Heating of the gas by the spark causes slow decay in light output. To obtain higher light output requires greater energy input (all other things being equal). This heats the gas in the arc to a greater extent and the gas continues to radiate for a longer time after the peak light output has passed. Constriction of the arc path also causes a longer time duration, as shown by an arc traveling in a capillary tube and one traveling in a groove in a glass plate. In these examples, both gaps were filled with air, powered by the same capacitors at the same voltage, and were the same length. The are in the capillary tube had a time duration about three times longer than the arc in the groove. Attempts to cover the open groove with a glass plate were unsuccessful because the glass plate was blown off. Subsequent tries made with the glass plate clamped on in various ways led to a shattering of the plate upon firing of the spark, demonstrating the explosive pressure of the arc. Although constriction increases the time duration, the are turned comers without any appreciable effect.
Thus, the present invention provides a point light source of novel design in which a practical compromise between contradictory requirements has been achieved. Other features and advantages of the invention will become apparent upon perusal of the following description in conjunction with the accompanying drawings in which:
FIG. I is a schematic of an ideal circuit useful in explaining the present invention;
FIGS. 2 and 3 graphically illustrate the effect on light output of shortening the gap between the main electrodes of a spark FIGS. 4 and 5 graphically show the light output and time duration for various gap lengths of a spark gap;
FIGS. 6 and 7 are useful in describing the effect of channeling the arc of a spark gap in a groove;
FIGS. 8 and 9 illustrate one embodiment of the present invention; and
FIGS. 10, 11 and 12 illustrate still another embodiment of the present invention.
FIG. 1 illustrates an ideal circuit with ideal components and switch for discharging energy through a spark gap having a resistance R As will be apparent, electrically the degree of damping and the resonant frequency affect the time duration of the arc. Shortening the gap reduces R modifying the light output in two ways.
First, the energy discharged by capacitor C will be dissipated by R, and R Decreasing R thus decreases the energy dissipated in the gap. Moreover, only a small fraction of this energy converts to light energy, the remainder being wasted as heat energy dissipated in the gap.
Second, if the switch is closed, capacitor C discharges and the gap fires, with the light output peaking and then oscillating as it decays exponentially. FIGS. 2 and 4 illustrate the effect on light 3 of shortening the gap between the main electrodes of a practical spark gap, such as that hereinafter described. Note that with the shorter gap, the afterglow increases, thus increasing the time duration, while the peak light output decreases. As seen in FIGS. 4 and 5 very small variations in gap length have a remarkable effect on the light output, time duration and reliability of the practical spark gap. Note that the optimum gap lengths for peak light output and for minimum time duration coincide at about thirty-five sixtyfourths inch.
FIG. 6 illustrates a spark gap having main electrodes 20 ccmented on one side of glass plate 22 with trigger electrode 24 afi'ixed on the opposite face of plate 22. Trigger electrode 24 essentially is the switch of FIG. 1. A trigger pulse which may be on the order of 4OKV, applied between trigger electrode 24 and one of the main electrodes 20, couples capacitively with the air in the gap between tips 26 of main electrodes 20, ionizes the air therebetween, and capacitor C (FIG. 1) discharges between the main electrodes producing a discharge arc and a flash of light. This construction prevents arc-over between trigger electrode 24 and either of main electrodes 20 by reason of the insulation provided by glass plate 22. However, each time the gap is fired, the arc takes a different path between tips 26 that cannot be reliably fixed in position. Thus, if a mask, not shown, with an aperture is placed behind trigger electrode 24, there will be drastic variations in light output through the aperture.
Placing electrodes 20 in groove 28 as shown in FIG. 7 fixes the location of the are between tips 26 of electrodes 20. The are tends to remain in groove 28 even when electrodes 20 are lifted out a full one'sixteenth of an inch. To date I have found that the most satisfactory method of making groove 28is to cut into a flat plate of glass with a thin carbide wheel. If a mask with an aperture five sixty-fourths by four sixty-fourths inch is placed behind trigger electrode 24 in FIG. 7, opposite the arc, and if the arc length is roughly thirty-five sixty-fourths inch, only one-seventh of the arc will be seen through the aperture, meaning that only 14 percent of the available light will pass through the aperture. This disadvantage led to the embodiment of the present invention illustrated in FIGS. 8 and 9.
FIGS. 8 and 9 illustrate glass plate 30 as having grooves 32 and 34 cut in opposite faces and opposing each other, with groove 36 cut at the end. Main electrode 38 lies in groove 32 and main electrode 40 lies in groove 34. Glass plate 42 is affixed with screws or cement to plate 30. Aperture 44 opposite groove 36 is formed with masking tape 45. Note that aperture 44 is large enough for both legs of the arc to be seen end-on as it forms between main electrodes 38 and 40 and passes around the hairpin comer at groove 36. Glass plate 42 also provides insulation between trigger electrode 46 and main electrodes 38 and 40. Note that the tip of trigger electrode 46 is centered at the center of aperture 44. In this embodiment light output increased three times with no change in time duration.
FIGS. through 12 illustrate a preferred embodiment of the point light source of the present invention. FIG. 10 is a perspective view of the assembled point light source; FIG. 11 illustrates details of construction with mask 50 removed; and FIG. 12 illustrates a bottom view of the assembled point light source; Electrical banana plug terminal 52 connects by lead 72 to main electrode 54 and terminal 56 connects by lead 76 to main electrode 58. Likewise, lead 60 connects to trigger electrode 62.
Referring now to constructional details, an insulating epoxy adhesive is shown sealing Pyrex base plate 64 to PVC boards 66 and 68. Banana plug terminal 52 and 56 are mounted on PVC board 66. A mass 70 of insulating epoxy adhesive seals lead 72 between banana plug terminal 52 and main electrode 54 to Pyrex glass plate 64 and PVC board 66 as illustrated, holding lead 72 permanently in position. Similarly, a mass 74 of insulating epoxy adhesive holds lead .76 firmly fixed in position and therefore main electrode 58 in held firmly fixed in position. Likewise, mass 78 of insulating epoxy adhesive holds trigger electrode 62 firmly in position and seals lead 60 to the bottom of PVC board 66. Note that Pyrex glass base plate 64 has groove 80 cut completely across it. As illustrated, main electrode 54 is disposed at the bottom of groove 80 while the tip of main electrode 58 is disposed at the bottom and front edge of groove 80. Mask 50 is affixed to PVC board 68 by screws made of insulating material such as teflon and spaced therefrom by insulating washers (not shown). The tip of trigger electrode 62 is on the bottom surface of base plate 64 and located approximately half way between the tips of main electrodes 54 and 58.
It will be noted that in the embodiment of FIGS. 10 through 12 the aperture in mask 50 can be reduced to approximately one half of the area of aperture 44 in FIGS. 8 and 9. Even with this reduction in aperture size, the light output was more than double. The embodiment of FIGS. 10 through 12 has some advantage over the embodiment of FIGS. 8 and 9. For example, the difficulty of cutting a dangerously thin groove around a corner is avoided. Further, two grooves are not cut opposite each other on both sides of a base plate. This avoids the danger that during operation the spark may puncture through the remaining septum of base plate. Additionally, in FIGS. 8 and 9 glass plate 42 becomes pitted with use and the amount of light output is reduced. This is avoided in the embodiment of FIGS. 10-12 because aperture 51 is an open hole. In this mask 50 may be made of thin black bakelite. In operation it is found that the bakelite is not at all damaged by the are heat. In test of the embodiment of FIGS. 10-12, aperture 51 in mask 50 remained constant in dimension after more than 200 firings. 7
In testing the embodiment of FIGS. 10 through 12, a 0.05 microfarad capacitor charged to 16 kilovolts was caused to be discharged between the main electrodes by a 40 kilovolts trigger pulse. The are produced had a time duration of onehalf microsecond and an output of 0.72 candle power second. When a 0.0039 microfarad capacitor charged to 16 kv was discharged between the main electrodes an arc was produced that had a time duration of 0.2 microseconds and an output of 0.06 candle power second.
Although I have described my invention with great particularity and have disclosed some embodiments, my invention may take any of a great number of configurations and those shown are merely examples and all configurations employing the principle of my invention are aimed to fall within the spirit and scope of my invention as defined in the following claims.
1. A point light source comprising:
dielectric material having first and second planar surfaces that are perpendicular to each other and intersect in a first line;
a groove cut in the first planar surface and extending to the second planar surface; a first main electrode disposed in the groove and fixed in position;
a second main electrode affixed to the second planar surface and disposed with its tip adjacent the end of the groove and extending just above the bottom of the groove;
the centers of the tips of the first and second main electrodes defining a second line located within the groove; and
a planar mask affixed and spaced parallel to the second planar surface and having an aperture near the second main electrode and aligned with said second line.
2. A point light source as in claim 1 and in which the distance between the tips of the main electrodes is approximately thirty-five sixty-fourths of an inch.
3. A point light source as in claim 4 further comprising a trigger electrode disposed on a third planar surface parallel to the first planar surface with itstip approximately half way between the main electrodes.
4. A point light source as in claim 3, in which the tips of the main electrodes and of the trigger electrode are exposed to air, the remaining metallic portion of each such electrode that would be exposed being insulated with a mass of insulating epoxy adhesive.
5. A point light source as in claim 1 in which the mask is black bakelite.
zg g gy UNITED STATES PATENT @FFECE CERTIFECATE F CGEQTEN Patent No. '3 ,675 ,069 Dated July a, 1972 Invento1-( Kenneth R. Crossen It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:
Column 1 l ine after use" insert -as-.
Column 2, ine 22 delete I and insert --3--.
Column 2, I ine 23, delete "3" and insert -output--.
Signed and sealed this 29th day of May 1973 (SEAL) Attest:
EDWARD M.FL ETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents