Narrow band and wide angle hemispherical interference optical filter

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Drawing sheet — no readable text.

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design and therefore, of high reliability while being cost

NARROW BAND AND WIDE ANGLE effective.

HEMISPHERICAL INTERFERENCE OPTICAL These and other objects of the invention will become FILTER more readily apparent from the ensuing specification

STATEMENT OF GOVERNMENT INTEREST

when taken in conjunction with the appended claims and accompanying drawings.

The invention described herein may be manufactured BRIEF DESCRIPTION OF THE DRAWINGS and used by or for the Government of the United States of America for governmental purposes without the O FIG. 1 shows a cross-sectional view of the wide ac payment of any royalties thereon or therefor. ceptance anger filter having a narrow passband.

BACKGROUND OF THE INVENTION

FIG. 2 shows a modification of this concept.

DESCRIPTION OF THE PREFERRED

A wide variety of optical filters have been fabricated EMBODIMENT which selectively pass or block wavelengths of interest. 15

Some conventional wide acceptance angle filters, usu Referring now to the drawing, the wide acceptance ally referred to as color filters, are relatively broadband angle filter 10 includes a hemispherically-shaped shell and have passbands of 50 nanometers and more. An 11, selected from optical quality glass or quartz. The other type of optical filter is generally referred to as an shell has a radius R which equidistantly locates a photo interference filter and functions to control the spectral 20 detector 12 from the inner surface of the shell. As ex composition of radiant energy partially by the effects of plained below, the diameter, d, of the photodetector can its interference. These filters are fabricated by deposit be selected to provide for different passbands. ing thin layers of dielectric materials, each with a differ Hemispherically-shaped shell 11 is provided with a ent index of refraction, on a polished glass substrate and coating 13 of desired dielectric composition and thick are well known and have been widely used in a good 25 ness to systematically transmit only a particular fre number of applications, see for example the article enti quency of incident optical wavelength by the well tled "Interference Filters' The Optical Industry and known optical interference phenomenon. Optical inter Systems Directory (1984) pages E-65 through E-67. They ference has been established as being the additive pro can allow narrow passbands and high transmittance to cess whereby the amplitude of two or more overlapping the selected wavelength at normal incident. The filter is 30 waves are systematically attenuated and/or reinforced. transmissive at shorter wavelengths at angles off the An interference filter fabricated in the accordance with normal incident (the larger the angle the shorter the the teachings of this invention has an appropriate coat wavelength). Other classes of filters are characterized ing for controlling the spectral composition of received by birefringent filters, atomic resonance filters, and so radiant wavelength by the effects of its interference. forth. Generally, these filters tend to be overly complex 35 The coating used in this filter can be made up of thin and limited in applications, partially because they are layers of dielectric materials to allow narrow passbands expensive. with high transmittances. A wide variety of such coat Thus, a continuing need exists in the state-of-the-art ings can be selected from commercially available sub for an optic filter which is cost effective and has narrow stances to provide for the passing of a discrete wave band and wide acceptance angle. length A incident at a normal attitude with respect to the SUMMARY OF THE INVENTION surface of the coating on the hemispherically-shaped shell. A typical coating might be zinc sulfide (n=2.2)

The present invention is directed to providing a hemi and cryolite(n=1.35). The layers thickness is of the spherically shaped optical filter that produces a wide desired wavelength (N).

acceptance angle. A suitable deposition of multiple 45 The coated hemispherically shaped shell provides a dielectric layers is applied onto an optically transparent narrow passband filter at a desired wavelength A and is hemispherical shell. The multilayer dielectric has the referred to as a N filter. The filter exhibits a high trans property to pass a desired wavelength w at normal inci mittance to normal incident A radiation and a high ab dence and can function as a narrow bandpass filter hav sorption or reflection to any other wavelength at nor ing a width as low as 1A. A photodetector located at 50 mal incident. The normal incident A wavelength energy the center of the hemisphere provides representative proceeds in-line to strike photodetector 12 and provide signals when the desired wavelength strikes any loca a representative output signal. Other wavelengths that tion on the coated shell at a normal incident angle. A are shorter than the wavelength of the A filter are trans black absorbing surface disposed about the photodetec mitted through the filter, but not at normal incident. tor will absorb other shorter wavelengths that pass 55 These shorter wavelengths pass through the coating through the coating, so as not to interfere with repre and shell and impinge on a black absorbing surface 14 of sentative signals produced by the desired normal inci diameter 2R that lies in a plane reaching across the dent impinging radiation. Keeping the size of the detec hemisphere. Thus, photodetector 12 being placed at the tor small with respect to the radius of the hemisphere center of the hemispherically shaped M filter detects N. assures that the detected linewidth is narrow. incident radiation only and the rest of the transmitted An object of the invention is to provide an optical light not at normal incident and having wavelengths interference filter having a wide acceptance angle that shorter than N energy is absorbed by black absorbing is responsive only to normal incident light. surface 14. The higher the ratio between the sphere Another object is to provide a filter design having a radius R and the diameter d of photodetector 12 results narrow linewidth that is adaptable to a desire, selected 65 a narrowing of the detected linewidths containing the A wavelength. radiation. A 1 A width of the A filter 13 has a zinc sul Still yet another object is to provide a wide accep fide and cryolite coating with a radius R=25 cm and a tance angle narrow band filter that is uncomplicated in photodetector diameter d = 1 mm.

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Thus, a designer is afforded great flexibility in deter a hemispherically shaped shell being optically trans mining not only the transmitted frequency of interest, missive therethrough;

but also the linewidth or passband of the filter by simply a coating provided on the shell fabricated to transmit changing the dimensions of the sphere and detector incident normal optical radiation of wavelength along with the coating selected. and smaller;

The well known interference filter technology which a photodetector disposed at the radial center of the hemispherically-shaped shell; and currently limits itself to normal incident light only, can a radiation absorbing surface disposed in the same be applied in this concept to provide a wide angle inter plane as the photodetector and sized to extend ference filter. Such a filter can be used with any light O across the diameter of the hemispherically-shaped source and is not limited to collimated laser beams. The shell for absorbing nonincident radiation of shorter wavelength is selected in accordance with the coating wavelength than A.

and other materials used within the conventional inter 2. An apparatus according to claim 1 in which the ference filter technology limitations. The passband is hemispherically-shaped shell has a radius R, the photo selectable by merely redimensioning the configuration 15 detector has an effective diameter d, and the radiation thusly described. As a consequence, a much simpler and absorbing surface has a diameter equal to 2R. more reliable filter is provided that is cost effective coating3. An apparatus according to claim 2 in which the when compared to other filter designs which might of wavelength has a composition to transmit incident radiation have a similar capability. N and blocking and reflecting longer 20 wavelengths and passing incident radiation of wave

Optionally, noting FIG. 2, two hemispheric shells 11 are put together back-to-back as an integrated sphere lengths less than A at other than normal incidence to the surface of the hemispherically shape shell.

that is composed a spherical glass or quartz. An opti 4. An apparatus according to claim 3 in which two cally absorbent surface 14 can be provided that is ab hemispherically-shaped shells are provided back to sorbent to incident light energy on both sides and a 25 back and include at least one photodetector with an photodetector element 12' is selected that can sense oppositely backed black absorption surface to provide radiation from both sides. Both hemispherical or spheri for an omnidirectional A radiation detection capability. cal designs could be fabricated from many small inter 5. An apparatus according to claim 3 in which the hemispherically-shaped shell is composed of many ference filter flats. By suitable application of suitable 30 small coatings, the filter can be made band pass or band stop interference filter flats. in accordance with the needs at hand. 6. An apparatus according to claim 3 in which the

Obviously, many modifications and variations of the neous optically transparent shell is selected from a homoge present invention are possible in the light of the above substance. 7. An apparatus according to claim 3 in which the teachings. It is therefore to be understood that within 35 coating is zinc sulfide and cryolite to pass A radiation of the scope of the appended claims the invention may be 1 A, practiced otherwise than as specifically described. the radius R=2.5 cm

I claim: and the diameter d = 1 mm.

1. A wide acceptance angle filter for transmitting to provide a 1 A passband.

normal incident N optical radiation comprising: 40 : :k k k :k