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QlidiCH “()0 1 05 Patented July 9, 1946 2,403,685 UNITED STATES PATENT “OFFICE-i 2,403,685 LIGHT FILTER Lawrence T. Sachtieben, Indianapolis, Ind., and William L. Douden, Philadelphia, Pa., assignors to Radio Corporation of America Application March 31, 1941, Serial No. 386,144 2 Claims. 1 This invention relates to a light ?lter consist ing of a plurality of ?lter elements and is par (Cl. 88-105) 2 characteristics in both respects. The expense of depositing such layers is rather high and the cost ticularly adapted for use in light dividers, railway of depositing a su?icient number of layers to se-' signals and similar devices where it is desirable cure an adequate ?ltering e?ect is such as to for the transmissive and re?ective characteristics render them commercially impractical. of the light ?lter to be separately determinable. The ?lter of the present invention is an im Light ?lters have heretofore been made in provement on all of the preceding types of ?lters many varieties, the most common varieties be and involves the use of a solid ?lter material such ing those embodying a transparent colored ma for example as glass having its band pass char terial which could be either solid or liquid or 10 acteristics supplemented by a multi-layer coat even semi-solid. In the liquid ?lters practically ing on the surface thereof which may accentuate any type of solvent and soluble color could be or sharpen the band pass characteristic of the used but these were inconvenient and ine?cient glass ?lter and which may re?ect a color either due to the necessity of a containing cell. Among complementary to that transmitted by the glass, the solid ?lters, those of gelatin and glass or 15 or a color which is one of those transmitted by combination of the two are most usual. The the glass, or both. gelatin ?lters have the disadvantage that only , One object of the invention is to provide an im water soluble colors which do not attack the proved light ?lter. gelatin can be used and the material is very deli Another object of the invention is to provide cate, being readily damaged by moisture or any 20 a light ?lter which is more selective than those mechanical contact. The gelatin ?lters like the heretofore made. liquid ?lters must be enclosed between appro Another object of the invention is to provide a priate protecting plates. The glass ?lters are dichroic light ?lter, the re?ective characteristics the most satisfactory from the mechanical stand of which may be made more or less independent point but only a limited number of colors is avail 25 oi.’ the transmissive characteristics. able and these are difficult to control. Many of Another object of the invention is to provide the ?lters in each of these types have wide trans an improved light divider. mission bands or have several transmission bands .. ' Another object 01' the invention is to provide in different portions of the spectrum. In the a light ?lter for use in a signal light. latter case particularly, the width of one trans 30 Other and incidental objects oi’ the invention mission band cannot be narrowed by increasing will be apparent to those skilled in the art from the thickness of the ?lter or the quantity of a reading of the following speci?cation and an in coloring material because the other transmis spection of the accompanying drawing in which sion band may not be narrowed proportionally Figure 1 is a sectional view of one type 01' and a totally di?'erent color may result. 85 light ?lter, Dichroic ?lters are also known in which a Figure 2 is a sectional view of a second type color is re?ected other than that which is trans of light ?lter, > mitted. For example, a thin ?lm of gold will re Figure 3 shows the application of the improved ?ect a yellowish red light and transmit green. light ?lter to one type of signal light, and Eosin transmits red and re?ects green. etc. Such 40 Figure 4 shows the application of the improved dichroic ?lters, like the transparent ?lters re light ?lter to a light divider. ‘ ferred to above, depend for their characteristics Referring ?rst‘to the form of the invention upon the material of which they are composed. shown in Figure 1, incident light indicated at I It has heretofore been proposed to make a di may enter the ?lter material In from the left chroic ?lter by forming alternate layers of mate 45 and travel in the direction of the solid arrow. rial having a high index and a low index of re The side of the material III which the light traction on the surface of a transparent medium, strikes is preferably provided with a coating ll these layers having a thickness of a quarter of of transparent material which coating may have a wave length of the light which is to be re a thickness of one~quarter wave length for the ?ected. As described, for example, in the Physi 50 light which is to be predominantly transmitted, cal Review for June 1939 at page 1128, such ?lms, by using a su?lcient number of layers, can be made highly re?ective to one desired color or and an index of refraction, preferably of or ap proaching the geometrical mean between that of the ?lter material It and air. The ?lter ma terial It will, for purposes of illustration, be re highly transmissive of another desired color but it is practically impossible to give them band pass as ierred to as glass, although any other solid trans ‘ 0 l l 2,403,885 parent material capable of being appropriately One practical application of this improved light. colored or having an appropriate color may be ?lter is shown in Figure 3, where the invention used. On the exit side of the ?lter material III, is illustrated as applied to a conventional traiiic a plurality of layers of transparent material are signal. provided as indicated at I2. These layers are 5 The greatest danger in the use of colored light alternate layers of low index and high index ma or colored lenses in conjunction with signals, terial, such, for example, as calcium ?uoride hav whether they be road traffic signals or railway ing an index of refraction of approximately 1.3 traffic signals, or signals for other purposes where and titanium dioxide having an index of refrac brilliant lights may strike the signal glasses, is tion of approximately 3.0. These ?lms, as pointed 10 the danger of what is usually referred to as a out in the publication above referred to, may have “false clear.” This danger is even greater than a thickness of one-quarter of a wave length of the danger of a completely inoperative signal. the light which is to be primarily re?ected. The If a signal is obviously completely inoperative, characteristics of this ?lter layer I! must be it will, of course, be treated as a caution or chosen with due regard to the characteristics of 15 danger signal. However, a signal may be inopera the ?lter layer III. For example, the ?lter layer tive, or it may even be operative, and light from l0 may be a glass which, when subjected to or an extraneous source may strike the “clear" sig dinary white light, appears to transmit green but nal and be re?ected back, giving the impression which actually transmits various portions of the that the signal is set at clear when this is not spectrum including the red and which gives a 20 actually the case. For example, in the type of subjective effect of green. In such cases, the ?lter traffic signal shown in Figure 3, if the lenses layer l2 might be chosen to re?ect the red light I4, l5 and I6 are of the conventional type with which the ?lter layer in would transmit. In this the upper lens l4 red, the middle lens l5 yellow, case, if white light were incident on the right side of the ?lter and traveled toward the left, the light would appear to be green, while, if the white light were incident on the left side of the ?lter, as indicated at I, the light re?ected as indicated by the dotted arrows R would be the shade of red transmitted by the ?lter layer In, and the re ?ected light would appear to be red. It is not necessary in all cases that the light transmitted and the light re?ected shall be true complementaries. These colors may be different colors which are reasonably close to each other in the spectrum, or they may be purely subjec tive complementaries, as, for example, subjec and the lower lens l6 green, as usual, light from the extraneous source l3 such, for example, as the sun, might strike the green signal 16 and be re?ected back from the inner surface of the lens, giving an approaching motorist the im pression that the green signal was actually lighted when, as a matter of fact, the red signal might be lighted but might not be visible to him due to his position or due to the greater apparent in tensity of the green signal. If, under this cir cumstance, he attempted to cross the intersection at high speed while the tra?ic signal in the other direction was actually green, the results might be disastrous. tive yellow can be produced from a mixture of In the application of our invention to this situ green and red light, and a ?lter transmitting such ation, the red signal I4 may be of the usual type a subjective yellow could be provided with a re 40 or may be treated on both surfaces with one ?ecting layer I2 which re?ected either the green or red transmitted by the ?lter layer It and transmitted in preponderance of the other color. In this case, the ?lter layer 10 which would pro duce a subjective yellow would not determine either the re?ected or transmitted color but would determine only which colors were to be separated quarter wave length nonre?ecting coating, such as indicated at H in Figures 1 and 2, or it may be treated on either surface with a multilayer coating, such as indicated at I2, which will re ?ect a certain percentage of red light. In this case, any light from this signal glass, whether it I be transmitted or re?ected, will be red. The red by the multilayer I2. ‘re?ecting coating will, of course, decrease the In the form of the invention shown in Figure 2, of the red to a certain extent, but the complex layer of the multilayer I2 is located 50 transmission this disadvantage may be minimized by placing on the incident side of the ?lter l0 which may be provided with a nonre?ecting one-quarter wave length layer II on the opposite side. In this ar rangement, the light-selective effect may be se cured as to light incident from the left independ ent of the bandpass characteristics of the ?lter l0, while the light transmitted from right to left is determined by the combined characteristics of the layer l2 and the body "I. In this arrange ment, as compared with the ?rstarrangement, the body In might, for example, be of such color as to transmit a spectrum green without any appreciable transmission at other wave lengths, while the layer l2 might be composed ~of sub layers of such thicknesses as to predominantly transmit the same color of green as that selected by the body In and to selectively re?ect at R the complementary colors. a re?ecting coating on the inner surface of vthe lens and making it of such a thickness that the center of its transmission band is not directly in the center of the transmission band of the red glass used. The yellow or caution signal l5 may likewise be treated with the nonre?ecting coating I I on both surfaces or may be left untreated, since the worst that re?ected light can accomplish is to produce a false caution signal. If it is desired, however, to distinguish between transmitted ‘light and re ?ected light, the inner surface of the lens l5 may be coated with a multilayer l2 which will re?ect red light to which the yellow or amber lens 15 is transparent. Most yellow ?lters appear yellow, not because they transmit only yellow but be cause they absorb only the blue end of the spec trum, permitting such a mixture of colors in the It will be apparent that the particular colors remainder of the spectrum to pass as to produce referred to with each of the examples given above 70 the visual appearance of yellow. In other words, are merely illustrative and that the body I0 may the light transmitted by a yellow ?lter is usually be chosen from the available materials to have a subjective yellow rather than a spectrum yel any desired transmission characteristics, while . low. If a red-re?ecting layer is placed on the the layer l2 may be chosen in either of the re inner surface of this lens, it will not materially lations thereto indicated above. 75 affect the color balance of the transmitted light QU'GIMI “UUH 2,408,686 - 5 . and will decrease the transmitted light only a negligible fraction, while, if light from an ex traneous source is re?ected from this rear layer 6 ~ thickness and by applying a layer II which is highly re?ective in the red. The red light re ?ected by the layer I! may be applied to an of the lens, the re?ected light will appear red appropriate monitoring device, as described, for and a person relying upon the signal will be cor example, in Dimmick application Serial No. respondingly warned. 372,811, ?led January 2, 1941, now Patent No. The green or “go” signal I6 is the one which 2,338,234 of January 4, 1944. In this application is the greatest potential source of danger. This of the invention, it will be apparent that the signal may be, as pointed out above, of a mate selectively re?ected layer l2 separates the red rial which transmits subjective green, 1. e., a color 10 light, as in the said Dimmick application, but which appears green to the eye but which also that, instead of all the complementary colors transmits a considerable portion of red. If the being transmitted, the only colors transmitted signal material is of this type, the inner surface are those which are neither re?ected by the layer of this lens l6 may be coated with a red-re?ecting l2 nor absorbed by the ?lter body l0 and, in the layer l2 and, while'light from the lamp I9 will 15 present instance, only the ultra violet is trans appear to be green when transmitted, any light mitted. from an extraneous source, which is re?ected by It will be apparent from the foregoing examples the inner layer of the lens IE, will appear to be that the improved ?lter may be used to transmit red. If the material of which the lens [6 is com one portion of the spectrum and to re?ect the posed is of the type which has a bandpass char 20 complementary portion, or it may be used to acteristic in the green and does not have an ap transmit one portion of the spectrum and to re preciable transmission in the red, the arrangement ?ect a supplementary portion which is not di shown in Figure 2 may be used. In this case, the rectly complementary. As in the case of the red-re?ecting layer is placed on the outer sur green signal referred to above, the light reflected face of the lens and any incident light from ex may be subjectively complementary but objec traneous sources is necessarily re?ected as red tively only supplementary to the light transmitted light, while light from the source I9 is not inter by the ?lter. ’ fered with. It will be apparent that, with either It will be apparent that the invention is not of the foregoing arrangements, it will be impos limited to the use of the selectively re?ecting sible for the signal to show a “false clear.” In coating on only one surface of the ?lter body but stead of the green signal l6 being arranged to that the selective coating may be used if desired re?ect red light, the layer thicknesses may be so on both surfaces thereof. This arrangement is chosen that yellow light will be re?ected, thereby particularly 'desirable where the ?lter body is highly transmissive of the color which the selec is quite possible, of course, for the signal to show 35 tive re?ector is intended to eliminate and this a false danger signal or caution signal, but this, form of the invention is particularly applicable at the worst, involves only a silght delay While the to the divider arrangement shown in Fig. 4. signal is investigated or more closely observed. We claim as our invention: If the selectively re?ecting surface is to be ap 1. A light ?lter including a selectively light plied to the inner surface of the lens in this case, 40 transmitting ?lter body and a selectively light the glass or other material must be so chosen re?ecting coating on the surface thereof composed that it is capable of transmitting the yellow light, of alternate layers of high and low index trans but in the alternate form, wherein the re?ected parent material so selected and of such thickness surface is applied to the outer surface of the lens, in relation to the light transmitted by the ?lter it may be used if the lens is not [transparent to body as to be predominantly re?ective of a color giving a caution instead of a danger signal. It yellow. A different application of the invention is illus trated in Figure 4, where the improved ?lter is applied to a light divider. In this instance, use is made of the absorptive characteristics of ?lter Ill and the selectively re?ective characteristics of the layer I! to an even greater extent than in the preceding example. This type of light divider may, as an example, be applied to sound-record ing spectrum systems where it is desired to record sound by means of ultra-violet light. The known ultra violet ?lters which are highly transparent to ultra violet and highly opaque to most visible light nevertheless transmit a considerable portion of the deep red, and it has heretofore been very di?icult to eliminate this deep red color without a considerable loss of ultra-violet. When ordi nary sound-recording ?lm is used, the di?iculty from this source is not serious, but, if it is desired to use panchromatic ?lm, it is desirable to elim inate the red light. In the present instance, this is accomplished by making the ?lter body ID of an ultra violet transmitting ?lter of appropriate subjectively complementary to that transmitted by said body and predominantly transmissive of at least a portion of the light transmitted by said body. _ 2. A light ?lter including. a selectively light; transmitting ?lter body’ and a selectively light re?ecting coating on the surface thereof com posed of alternate layers of high and low index transparent material so selected and of such thickness in relation to the light transmitted by the ?lter body as to be predominantly re?ective ' of a color subjectively complementary to that transmitted by said body and predominantly transmissive of at least a portion of the light transmitted by said body, and a transparent layer of material on the other surface of said body of such index of refraction and thickness in rela tion to the light transmitted as to reduce the sur face re?ection of the light transmitted by both said body and the selective coating. LAWRENCE T. SACHTLEBEN. WILLIAM L. DOUDEN.