Title:
Color selective reflector
United States Patent 2412496


Abstract:
This invention relates to color selective reflectors of the type which bear transparent, substantially non-absorbent interference films. It has been previously proposed to provide color-selective reflectors which include multilayer interference films, alternate layers being of material of...



Inventors:
Dimmick, Glenn L.
Application Number:
US60246145A
Publication Date:
12/10/1946
Filing Date:
06/30/1945
Assignee:
RCA CORP
Primary Class:
Other Classes:
359/589
International Classes:
G02B5/28
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Description:

This invention relates to color selective reflectors of the type which bear transparent, substantially non-absorbent interference films.

It has been previously proposed to provide color-selective reflectors which include multilayer interference films, alternate layers being of material of high and low index of refraction, respectively. In reflectors of this type, each layer has a thickness of one-quarter of the wavelength of light of a predetermined color; the low index layers tend to reduce reflection from the surface bearing the film, while the high index layers increase reflection. The reflector as a whole is thus given selective characteristics, that is to say, it transmits a substantial proportion of light of one color, while reflecting a substantial proportion of light of another color.

Reflectors of the character described are, however, extremely sensitive to the angle which the incident beam makes with the interference film.

Thus, as may be seen by reference to Figure 2B of the annexed drawings, a reflector of this type will transmit 89 percent of light of 5000 Angstrom units when the incident beam is normal to the film but only 27 percent of light of that color when the incident beam is at an angle of 450 of the film.

Another disadvantage of reflectors of the type indicated is that they have a peaked characteristic, as may again be seen by reference to Figure 2B. In many applications of selective reflectors (for example in the monitoring device described in my Patent No. 2,314,392) it is desirable that the reflector shall have a high transmission value over a band extending from 4000 Angstroms to 5000 Angstroms, but it is practically impossible to obtain such a wide band characteristic with films all the layers of which are a quarter of a wave thick.

It is an object of the invention to provide an improved multi-layer color selective reflector and particularly a reflector which is less subject to variation in color with angle and which may be given a wider band-pass characteristic than similar reflectors of the prior art.

A particular object is the provision of an improved blue-transmitting yellow-reflecting selective reflector.

These objects are achieved by the use of an interference film consisting of alternate half-wave high index and quarter-wave low index layers. In my copending application (RCV-8883) for a "Dichroic reflector," Serial No. 436,998, filed March 31, 1942, and assigned to the same assignee as the instant application, there is described an interference film comprising a pair of half-wave high-index layers separated by a quarter-wave low-index layer. The advantages of this construction are not limited to a three layer film nor are they limited to the particular materials employed.

The invention may be better understood from a consideration of the following description of two embodiments thereof when read in conjunction with the annexed drawings in which: Figure 1A is a greatly enlarged sectional view of a selective reflector bearing a six-layer film, according to the invention, Figure 1B is a graph showing the transmission characteristics of the reflector of Fig. 1A, Figure 1C is a chart showing the manner of construction of the device of Fig. 1A, Figure 2A is a greatly enlarged sectional view of a selective reflector according to the prior art, of the same materials and in all other respects similar to the reflector of Fig. 1A except that the high index layers are each a quarter-wave instead of half-wave thick, Figure 2B is a graph showing the transmission characteristics of the reflector of Fig. 2A, Figure 2C is a chart showing the manner of construction of the device of Fig. 2A, Figure 3A is a greatly enlarged sectional view of a selective reflector bearing a seven-layer film according to the invention, Figure 3B is a graph showing the transmission characteristics of the reflector of Fig. 3A, and Figure 3C is a chart showing the manner of construction of the device of Fig. 3A.

The reflector shown in Fig. 1A is one designed to reflect a substantial proportion of light from the yellow portion of the visible spectrum and to transmit a substantial proportion of light from the blue portion of the spectrum. It includes a support 10 which may be of glass or other transparent material and which may be assumed for purposes of illustration to have an index of refraction of the order of 1.5.

A surface 11 of the support bears a transparent interference film composed of six successively superimposed layers designated, respectively, by the reference numerals 12, 13, 14, 15, 16 and I7.

The first, third and fifth of these layers each have an effective optical thickness of one quarter of the wave length of light to be transmitted by the device. These three layers may be of cryolite which has an index of approximately 1.3 and are sometimes referred to in this specification and in the accompanying claims as "low index layers." The second, fourth and sixth layers each have an effective optical thickness of one-half of the wavelength of light to be transmitted by the device, and may be of zinc sulfide which has an index of refraction of about 2.1. The three layers last mentioned are referred to in this application as "high index layers." Cryolite and zinc sulfide are mentioned as examples of suitable materials, it being necessary only that the half wavelength layers shall have a greater index of refraction than the quarter wavelength layers.

The various layers of the interference film may be applied, and their thickness controlled in the manner described in myPatentNo.2,338,234. From the data of Fig. 1C it will be seen that in the production of the embodiment of Fig. 1A, a control beam at an angle of 45 degrees to the surface I and a filter having maximum transmission at 4350 Angstroms were employed. The angle of 45 degrees is not critical.

Cryolite is evaporated onto the surface under treatment, and the reflection therefrom becomes progressively less until it is about 10 percent of that from the original untreated glass surface.

At this point, the reflection is a minimum and the layer is a quarter-wave thick. Zinc sulfide is then evaporated on top of the cryolite layer. The reflection increases steadily and reaches a maximum of approximately 750 percent of the original reflection. At this point the zinc sulfide is a quarter-wave thick, and this is the point at which evaporation of the layer is ordinarily terminated in the prior art. In the present invention, however, the zinc sulfide is further applied, and reflection now falls to a minimum of about 52 percent, at which the layer is a half-wave thick.

Successive layers of cryolite and zinc sulfide are applied in accordance with the directions in the chart (Fig. 1C), each low index being applied to a thickness of a quarter-wave length and each high index layer to a thickness of a half-wave length.

Transmission curves for the device of Fig. 1A are given in Fig. 1B. These curves were taken with a spectrophotometer from a device built as shown in Fig. 1C. The solid line of Fig. 1B shows the transmission when the incident beam is normal to the film, and the broken line gives the transmission when the film is at an angle of 45 degrees to the light path. It will be observed that movement of the incident beam with respect to the film through an angle of 45 degrees shifts the transmission beam fairly constantly by about 140 Angstrom units towards the blue end of the spectrum.

It will also be seen that transmission is maintained substantially constant at the blue end of the spectrum between 4000 and 5000 Angstrom units. The desirability of this characteristic in many applications has already been referred to.

Fig. 2A is a sectionalview of a selective re- 6 flector of the prior art, and Fig. 2B gives transmission curves of that reflector for comparison with those of Fig. 1B. The device of Fig. 2A is of the same materials and in all other respects similar to that of Fig. 1A except that the high 6 index layers are a quarter-wave instead of halfwave thick, and the device was therefore designed to have maximum reflection at 6000 Angstroms instead of minimum reflection at 4350 Angstroms.

Both devices are yellow by reflection and blue 7 by transmission, but variation of the angle of the incident light shifts the transmission curve of the device of Fig. 2A by as much as 600 Angstrom units. Moreover, this shifting is not nearly as constant as with the reflector of the pres- 7 ent invention. In other words, the reflector-of the invention is only about one-quarter as responsive to variations in angle as the reflector of the prior art. It will also be seen that there are wide variations in the transmission of the prior art reflector at the blue end of the spectrum in the region from 4000 to 5000 Angstrom units.

The multi-layer film of the invention is not limited to the precise six-layer film shown in Fig. 1A, nor to the materials employed in that embodiment, nor to cases where the low index layer is next to the glass. Fig. 3A is a sectional view of a seven-layer film according to the invention composed of alternate half-wave layers of zinc sulfide and quarter-wave layers of material which is believed to be thorium oxi-fluoride, with the high index layer in this case next to the glass.

The material of which the quarter-wave low index layers are composed is that described in my copending application for "Reduction in reflection from glass," Serial No. 470,583, filed December 30, 1942, and assigned to the same assignee as the instant application. This material is believed to be thorium oxi-fluoride with the formula ThOF2, and has an index of refraction (when deposited by evaporation in a high vacuum) of approximately 1.5. The difference between the indices of refraction of thorium oxifluoride and of zinc sulfide is therefore not quite as great as that between cryolite and zinc sulfide; to produce the same variation in transmission (from red to blue) as in the case of the device of Fig. 1A, seven layers are needed instead of the six of the embodiment first described.

Figs. 3B and 3C give respectively transmission curves and instructions for the production of the device of Fig. 3A. It is apparent from an inspection of the two curves of Fig. 3B that the characteristics before referred to are almost as good in the case of the embodiment of Fig. 3A as in that of Fig. 1A.

There has thus been described a color-selective reflector comprising a transparent support and a multi-layer film on a surface of the support, the film being composed of alternate half-wave layers of higher index than the support, and quarter-wave layers of lower index than the halfwave layers. The construction is of particular advantage for reflectors required to transmit blue and reflect yellow light, where variation of color with angle, and efficiency of transmission at the blue end of the visible spectrum are factors to be taken into account.

I claim as my invention: 1. A color selective reflector having low response to changes in the angle of incident light, said reflector comprising a transparent support and a transparent interference film consisting of 0 at least four layers superimposed on a surface of said support, alternate layers of said film being of a material of higher index of refraction than that of said support and each having an effective optical thickness of one-half the wavelength of 5 light to be transmitted, the remaining layers of said film being of a material having an index of refraction at least as low as that of said support, each layer having an effective optical thickness of one-quarter of said wavelength, and the outer0 most layer of said reflector being a one-half wavelength layer of said high index material.

2. A selective reflector according to claim 1 wherein one of said low index layers is next adjacent said surface.

5 3. A selective reflector according to claim 1 wherein one of said high index layers is next adjacent said surface.

4. A selective reflector according to claim 1 wherein said support is of glass having an index of refraction of the order of 1.5.

5. A selective reflector according to claim 1 wherein said support is of glass having an index of refraction of the order of 1.5 and said low index layers are of cryolite and said high index layers are of zinc sulfide.

6. A selective reflector according to claim 1 wherein said support is of glass having an index of refraction of the order of 1.5 and said low index layers are of thorium oxyfluoride and said high index layers are of zinc sulfide.

q 7. A selective reflector according to claim 1 wherein said support is of glass having an index of refraction of the order of 1.5 and said film consists of six layers whereof the first, third and fifth are of cryolite and the second, fourth and sixth are of zinc sulfide.

8. A selective reflector according to claim 1 wherein said support is of glass having an index of refraction of the order of 1.5 and said film consists of seven layers whereof the first, third, fifth, and seventh are of zinc sulfide and the second, fourth, and sixth are of thorium oxifluoride.

GLENN L. DIMMICK.

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