Lamp with high color-discrimination capability
United States Patent Thornton, Jr.  
 LAMP WITH HIGH COLOR-DISCRIMINATION CAPABILITY William A. Thornton, .lr., Cranford, NJ.  
 Westinghouse Electric Corporation, Pittsburgh, Pa.  
 Filed: Aug. 10, 1973 Appl. No.: 387,391  
 Inventor:  
 Assignee:  
 References Cited UNITED STATES PATENTS 12/1937 Moers ..3l3/225 1/1968 Johnson ........3l3/225 Apr. 1, 1975 [57] ABSTRACT A lamp and method producing a generally purplish light having-three constituent emissions in specific wavelength ranges. The light provides for optimum or near optimum color discrimination, and small color differences are 2% to 3 times more easily seen under this light than under daylight. The three emissions are located principally within the 400-470 nm, 500-550 nm and 610-680 nm wavelength ranges and the proportions are such that blending the three emissions produces visible light having, in 1C1 coordinates, an x&#34; in the range of about 0.27 to 0.45 and a y&#34; in the range of about 0.09 to 0.25.  
 7 Claims, 6 Drawing Figures 1 l l l l l .5 .6 THE (lJl-CHROMATICITY DlAGRAM OF THE 101 SYSTEM &#34;mii&#39;iirl&#39;ififl H975 $875,453  
 YELLOWISH I RED PURPLE THE (x,y)-CHROMATICITY DIAGRAM OF THE ICI SYSTEM 560 660 WAVELENGTH (NM) LAMP WITH HIGH COLOR-DISCRIMINATION CAPABILITY CROSS REFERENCE TO RELATED APPLICATIONS In copending application, Ser. No. 96,744, filed Dec. 10, 1970, which application is a continuation-in-part of Ser. No. 742,291, filed July 3, 1968, by the present inventor and owned by the present assignee, is disclosed a device substantially generating only three specific emissions (green, red, and blue) to efficiently produce white light with excellent color rendition of illuminated objects. In addition to specifying the wavelength ranges for the three emissions, this copending application also discloses combinations of specific phosphors in accordance with that invention. The instant invention does not generate white light (as defined in that copending application) and maximizes color discrimination (as opposed to the copending application which provides a preferred color rendition and a high color rendition index).  
  Copending application Ser. No. 279,562, filed Aug. 10, 1972 as a continuation-in-part of the aforementioned Ser No. 96,744 by the present inventor and owned by the present assignee discloses a luminescent discharge lamp using three narrow-band-emitting phosphors to produce white light within the concept of the aforementioned copending application Ser. No. 96,744, but limited to certain specific phosphors and specific ranges of composition. The instant invention again maximizes color discrimination, rather than color rendition and color preference as in this copending application.  
  In copending application, Ser. No. 279,561, filed Aug. 10, 1972 by the present inventor and owned by the present assignee, is disclosed a luminescent discharge lamp using at least one wide-band halophosphate phosphor supplemented with at least two narrow band phosphors. That combination of wide-band narrow phosphors provides white light with good color rendition at generally high efficiency and at reduced cost as compared to the aforementioned copending applications Ser. No. 96,744 and Ser. No. 279,562 (the use of a relatively inexpensive wide-band phosphor enabling the amount of relatively expensive phosphors to be reduced). As compared to this copending application as well as the aforementioned copending applications), the instant invention differs in that it provides a light other than a white light and that it maximizes color discrimination, rather than color rendition.  
 BACKGROUND OF THE INVENTION This invention relates to devices for the generation of light for near optimum or optimum color discrimination.  
  There are certain visual tasks which require easy discrimination among colors with only small differences in the colors, for example a wiring task with color-coded wires having numerous colors. Apparently, in the prior art, no lamp had ever been designed specifically to optimize color discrimination. The assumption had apparently been made that the maximum color discrimination is provided by white light and particularly by white light having a continuous spectrum. Apparently it had been assumed that the optimum color discrimination was provided by daylight or something quite close thereto.  
 SUMMARY OF THE INVENTION The lamp and method of this invention provide for optimum or near optimum color discrimination and small color differences are 2% to 3 times more easily seen than under daylight. Contrary to the prior art, the invention produces neither white light, nor a continuous spectrum, nor light closely resembling daylight. The instant invention provides the device or method, which blends together a first light constituent having an emission principally confined to the 400-4 nm wavelength range (blue), a second light constituent having an emission principally confined to the 500-550 nm wavelength range (green), and a third light constituent having an emission principally confined to the 610-680 nm wavelength range (red). The relative light intensities of the first, second and third light constituents are controlled to produce visible light of [CI coordinates with an x value in the range of 0.27 to 0.45 and a y value in the range of 0.09 to 0.25.  
  Preferably, the device is a discharge lamp. When a fluorescent discharge lamp is used, the phosphors contained therein are preferably strontium chloroapatite activated by divalent europium, zinc silicate activated by manganese, and magnesium fluorogermanate activated by manganese.  
 BRIEF DESCRIPTION OF THE DRAWINGS The invention can be best understood by reference to the exemplary embodiment illustrated in the following:  
  FIG. 1 is a graph of the [CI chromaticity diagram demonstrating the difference in chromaticity between the light of the instant invention and the white light of of the prior art (the [CI color system is described in detail in the Handbook of Colorimetry&#34; by Arthur C. Hardy, The Technology Press, Massachusetts Institute of Technology, 1936).  
  FIG. 2 shows the chromaticities of the eight standard test colors under daylight on a u,v-diagram (the u,vdiagram is described in Projective Transformations of lCl Color Specifications&#34; by D. L. MacAdam in the Journal of the Optical Society of America, Volume 27, page 294, I937).  
  FIG. 3 shows a portion of the u,v color diagram with color discrimination octagons of a lamp of the instant invention, of daylight and for several lamps of the prior art.  
  FIG. 4 shows the contours of source colors which, when produced in accordance with the instant invention, provide equal improvement in color discrimination (with values normalized to a daylight value of FIG. 5 shows an approximate spectral power distribution for an ideal illuminant.  
  FIG. 6 is an elevational view, shown partly in section, of a low pressure mercury discharge lamp coated with a substantially homogeneous mixture of phosphorous.  
 DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to FIG. I there is shown the area (enclosed by the dot-dash line) of x,y color coordinates of light of the device or method of the instant invention. It can be seen that such light is generally purplish in color and differs significantly from the light in the area enclosed by the dashed line and designated white (the white&#34; area was similarly defined in the aforementioned copending applications which were limited to SUCT&#39;I white source colors). The point marked D on FIG. 1 represents daylight. It can be seen that the source color of the instant invention is also significantly different from that of daylight. Thus, surprisingly, the optimum source color for color discrimination is not that of daylight nor even what would normally be referred to as white&#34;.  
  FIG. 2 is a u,v color diagram, which is used because it is more uniform than the x,y diagram (that is even distances anywhere on the u,v diagram correspond approximately to the same perceived color differences). The octagon on FIG. 2 is produced by using the eight ClEl test-colors specified in the CRI procedure (as set forth in the publication of the International Commission of Illumination, identified as publication CIE No. 13, E-] .3.2, 1965). The eight test colors were illuminated by the illuminant, the chromaticities plotted on the color diagram and lines drawn connecting the adjacent points, to produce an octagon. This was done on FIG. 2 using average daylight, as the illuminant. The lengths of the sides of the octagon are approximately proportional to the perceived color differences between adjacent colors, and thus generally, the larger the octagon, the larger the perceived color differences, and the easier the discrimination between colors.  
  FIG. 3 shows the color discrimination octagons for a number of sources. The daylight octagon is labeled D. It can be seen that octagons of the typical lamps of the prior art, as represented by the cool white (CW), warm light (WW), and high pressure sodium (NA) octagons, are smaller than the octagon for daylight, thus indicating that color discrimination is easier under daylight than any of these other illuminants. It has been discovered, however, that much larger octagons can be obtained by devices and methods of the instant invention and, for example, a fluorescent lamp with strontium chloroapatite activated by divalent europium, zinc silicate activated by manganese, and magnesium fluorogermanate activated by manganese produces the octagon labeled l.  
  FIG. 4 shows the locus of points of source colors having an ease of color discrimination of the same multiple of daylight color discrimination. FIG. 4 also shows the approximate area to which the source color of the instant invention is limited (the area within the dot-dash lines as delineated by visible light within the prescribed ranges of x and y).  
  FIG. 5 illustrates what is apparently approximately the ideal spectral distribution. This distribution would produce the optimum in color discrimination. Surprisingly, this distribution is not a continuum, as has apparently been assumed heretofore, but consists of emission principally in three relatively narrow ranges of wavelengths.  
  Thus, a device which efficiently generates light and illuminates objects in a manner which provides for substantially improved color discrimination can comprise a three component light generating medium and means for energizing the medium to a visible light generating condition. The first component of the medium, when energized, should exhibit a visible emission located principally (and preferably substantially) in the 400-470 nm wavelength range. The second component of the medium, when energized, should exhibit a visible emission located principally (and preferably substantially) in the 500-550 nm range. The third component of the medium, when energized, should exhibit a visible emission located principally (and preferably substantially) in the 610-680 nm wavelength range (as used herein, principally means more than 50% and substantially means more than of the visible energy is emitted in the prescribed range). The relative portions of the three components of the light generating medium should be such that when their emissions are blended, there is produced light of predetermined x and y values of 1C] coordinates, the at value being in the range of about 0.27 to 0.45 and the y value being in the range of about 0.09 to 0.25. This produces a purplish source color within the area enclosed by the dot-dash lines as indicated on the x,y diagram of FIG. 1. Preferably the x should be between about 0.29 and 0.37 and the y between about 0.13 and 0.19. The optimum values of 0.32 for x and 0.16 for y provide (when resulting from the aforementioned three emissions) about three times the color discrimination of daylight.  
  it is preferable that the wavelength ranges of the component emissions be relatively tightly defined (however the lack of efficient sources of some of these components provides a practical problem). In the shorter wavelength range it is preferable that the emission be located substantially in the 400-435 range, and the optimum is a line emission at about 430 nanometers. It will be noted that the divalent activated strontium chlorapatite phosphor, whose emission is not located substantially in the preferred range of 400-435 nm, is still quite useful as it is more efficient than available phosphors whose emissions are located substantially in the preferred range. In the long wavelength range it is preferable that the emission be located substantially in the 630-670wavelength range and the optimum is a line emission at about 660 nm. In the middle wavelength range the optimum is a line emission at approximately 530 nm. FIG. 5 shows the approximate spectral power distribution for an ideal illuminant and illustrates the relative magnitude of the three emissions for the ideal chromaticity.  
  There are a number of different types of sources of such emissions, including discharge lamps, lasers, electroluminescent devices, and devices using RF excitation. Preferably the emission is produced by means of a discharge lamp.  
  One type of discharge lamp which is quite satisfactory is the conventional low pressure mercury (fluorescent) lamp. Such a lamp can provide a three-component-light-generating medium in the form of a mixture of phosphors. A number of phosphors can be used as components of the mixture of phosphors. The following three examples of phosphor mixtures each provide an x value of approximately 0.32 and a y value of 0.16.  
  Thirty-eight grams of strontium pyrophosphate activated by divalent europium, blended with 2 grams of zinc silicate activated by divalent manganese, and 8 grams of magnesium fluorogermanate activated with manganese with a valence of 4 plus;  
  Twenty-three grams of strontium chloroapatite activated with divalent europium, blended with 2.5 grams of zinc silicate activated with divalent manganese, and 34 grams of magnesium fluorogermanate activated with (4 plus) manganese;  
  Ten grams of strontium chloroapatite activated with divalent europium, blended with 3 grams of zinc silicate activated by divalent manganese and 12 grams of yttrium vanadate activated by trivalent europium.  
  Other phosphors which can also be used in blends include: magnesium arsenate activated by (4 plus) manganese or yttrium oxide activated by trivalent europium for long wavelength (red) emitters; zinc magnesium silicogermanate activated with divalent manganese, yttrium silicate activated with trivalent terbium, or magnesium gallate activated with divalent manganese as middle wavelength (green) emitters; and calcium tungstate as a short wavelength (blue) emitter.  
  With reference to FIG. 6, there is shown a low pressure mercury vapor fluorescent lamp, wherein a conventional, elongated, tubular, soda-lime glass envelope has operative discharge sustaining electrodes 12 at opposite ends. The discharge sustaining material comprises mercury and inert gas filling as is well known in the art. A layer of a substantially homogeneous mixture of phosphor is disposed on the interior surface of the envelope 10.  
  Another type of discharge lamp which can be used is the high pressure mercury discharge lamp. In this type of lamp, emission components can be provided either by additives within the arc tube or by phosphors coated on the other envelope. If one or more of the three component emissions is supplied by phosphors, the same type of phosphors generally can be used as mentioned for fluorescent lamps above. Such phosphors are, of course. coated on the outer envelope in the conventional manner. In such lamps, a portion of the blue emission is generally contributed by the mercury are.  
  Some of or all of the components can also be supplied by additives to the arc tube. For example, the blue component can be supplied by indium, the green component by thallium, and the red component by cadmium. Lithium can also be used to supply the red component. Mercury can also be used to supply the blue and green emissions, although these are generally in the wrong ratios and a green filter may be required.  
  The instant invention provides a device or a method of generating light which provides for substantially improved color discrimination. While the invention has been explained by describing particular embodiments thereof, it will be apparent to those skilled in the art that modifications may be made without departing from the scope of the invention.  
 I claim:  
  1. A device which efficiently generates light and illuminates objects in a manner which provides for substantially improved color discrimination, said device comprising:  
 a. a three-component, light-generating medium forming an operative part of said device;  
 b. means for energizing said medium to a visiblelightgenerating condition;  
 c. a first component of said medium, when energized, exhibiting a visible emission located principally in the 400-470 nm wavelength range;  
 d. a second component of said medium, when energized exhibiting a visible emission located principally in the 500-550 nm wavelength range;  
 e. a third component of said medium, when energized, exhibiting a visible emission located principally in the 610-680 nm wavelength range; and  
 f. the relative proportions of said components of said light-generating medium being such that when their emissions are blended, there is produced visible light of predetermined x and y values of 1C! coordinates, said at value being in the range of 0.27 to 0.45, and said y value being in the range of 0.09 to 0.25.  
  2. The device of claim 1, wherein said emission of said first component of said medium is located substantially in the 400-435 nm wavelength range, said emission of said second component is located substantially in the 500-550 nm wavelength range, and said emission of said third component of said medium is located substantially in the 630-670 nm wavelength range.  
  3. The device of claim 1, wherein said device is a discharge lamp.  
  4. The device of claim 3, wherein said discharge lamp is a fluorescent lamp and said medium is a mixture of phosphors.  
  5. The device of claim 4, wherein said first component of said medium is strontium chloroapatite activated by divalent europium, said second component of said medium is zinc silicate activated by manganese, and said third component of said medium is magnesium fluorogermanate activated by manganese.  
  6. The device of claim 5, wherein said x value is about 0.32, and said y value is about 0.16.  
  7. The method of efficiently generating light which provides substantially improved color discrimination for illuminated objects, which method comprises:  
  a. blending together l a first light constituent having emission principally confined to the 400-470 nm wavelength range, (2) a second light constituent having an emission principally confined to the 500-550 nm wavelength range, and (3) a third light constituent having an emission principally confined to the 6 l 0-680 nm wavelength range; and  
  b. controlling the relative light intensities of said first, second and third light constituents to produce visible light of predetermined x and y values of [Cl coordinates, said 2: value being in the range of 0.27 to 0.45,  
 and said y value being in the range of 0.09 to 0.25.