Abstract:
An apparatus comprises a light source which produces a first radiation within a first wavelength range and which produces a second radiation within a second wavelength range when a property of the light source is changed. The apparatus further comprises an optical device which receives the first radiation and attenuates the first radiation to emit a first attenuated radiation having a first perceived brightness. The optical device also receives the second radiation and attenuates the second radiation to emit a second attenuated radiation having a second perceived brightness. The first and second perceived brightnesses are approximately equal.

Description:
TECHNICAL FIELD 
     This disclosure relates in general to apparatus and techniques for managing the effects of spectral shift in a light source and, more particularly, to apparatus and techniques for managing the effects of a spectral shift in a light source within an optical sight. 
     BACKGROUND 
     Optical sights are used for various purposes, one example of which is mounting a sight on a weapon in order to help a user accurately aim the weapon. The optical sight takes image information from a distant scene, and presents this image information within a field of view which is visible to the eye of a user. 
     In reflex gun sights, a positioning marker or image, such as cross-hairs, a luminous dot, or other type of reticle is superimposed onto the visible field of view to aid a marksman in aiming the weapon and hitting the target within the field of view. The positioning marker can be generated using a light source such as a light emitting diode (LED). Changes in one or more physical properties of the light source, such as drive current or temperature, can cause a spectral shift in the light emitted from the light source. Additionally, individual units of the same nominal light source can have spectral shifts relative to the nominal source. This shift can produce undesirable effects such as changes in perceived brightness or a shift in the hue of the light. A change in perceived brightness is due to the eye&#39;s variable sensitivity to the color, or wavelength, of the light. As the emitted wavelength spectrum of the light source shifts toward wavelengths to which the human eye is more visually sensitive, the eye perceives this shift as a brightening of the light source. As the emitted wavelength spectrum of the light source shifts toward wavelengths to which the human eye is less visually sensitive, the eye perceives the shift as a dimming of the light source. These spectral shifts can also influence the perceived hue of the light source. The effects of spectrum shift also occur when mixing multiple color light sources to create computer or other visual displays. In a reflex gun sight, the effects of spectral shift can cause the reticle to appear too bright, distracting the user from the field of view, or too dim, causing the user difficulty in discerning the reticle against the field of view. 
     New apparatus and methods are needed to overcome the effects of spectral shifting. 
     SUMMARY 
     In one exemplary aspect, an apparatus comprises a light source which produces a first radiation within a first wavelength range and which produces a second radiation within a second wavelength range when a property of the light source is changed. The apparatus further comprises an optical device which receives the first radiation and attenuates the first radiation to emit a first attenuated radiation having a first perceived brightness. The optical device also receives the second radiation and attenuates the second radiation to emit a second attenuated radiation having a second perceived brightness. The first and second perceived brightnesses are approximately equal. 
     In another exemplary aspect, a sighting apparatus comprises an elongated housing including a first end and a second end defining an optical axis. The second end is configured to admit external light from an external scene. The sighting apparatus also includes a light source within the housing which produces a first light within a first wavelength spectrum and which produces a second light within a second wavelength spectrum when a property of the light source is changed The apparatus also includes an optical device which combines the external light with the first light to create a first combined light and attenuates the first combined light to emit a first attenuated light having a first perceived brightness. The optical device also combines the external light with the second light to create a second combined light and attenuates the second combined light to emit a second attenuated light having a second perceived brightness, wherein the first and second perceived brightnesses are approximately equal. 
     In another exemplary aspect, a method comprises projecting from a light source a first radiation within a first wavelength range and responsive to a change in a property of the light source, projecting from the light source a second radiation within a second wavelength range. The method also includes receiving at an optical device the first and second radiations. The method also includes conveying from the optical device an attenuated form of the first radiation having a first perceived brightness and conveying from the optical device an attenuated form of the second radiation having a second perceived brightness. The first and second perceived brightnesses vary by less than 10%. 
     Further aspects, forms, embodiments, objects, features, benefits, and advantages of the present invention shall become apparent from the detailed drawings and descriptions provided herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is diagrammatic depiction of an apparatus which is an optical sight for a weapon. 
         FIG. 2  is graph depicting a narrowband spectrum of radiation and the transmission/reflection characteristics of a nominal filter. 
         FIG. 3  is a graph depicting a narrowband spectrum of radiation, a photopic luminous response curve, and the change in perceived brightness of the narrowband radiation. 
         FIG. 4  is a graph depicting a narrowband spectrum of radiation, a photopic luminous response curve, the transmission/reflection characteristics of a filter for attenuating shorter wavelengths, and the change in perceived brightness of the narrowband radiation for both the filtered and unfiltered narrowband radiation. 
         FIG. 5  is a graph depicting a narrowband spectrum of radiation in both nominal and shifted positions, the transmission/reflection characteristics of a filter for attenuating the radiation, and a model square filter. 
         FIG. 6  is a graph depicting broadband spectrums of radiation conveyed by two different filters. 
         FIG. 7  is a graph depicting the filtering of two narrowband light sources. 
     
    
    
     DETAILED DESCRIPTION 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments, or examples, illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Any alterations and further modifications in the described embodiments, and any further applications of the principles of the invention as described herein are contemplated as would normally occur to one skilled in the art to which the invention relates. 
       FIG. 1  is a diagrammatic view of an optical apparatus  10  which in this embodiment may be a sight for a weapon. For example, the sight  10  could be mounted on a rifle, in order to assist a marksman in aiming the rifle at a target within distant scene.  FIG. 1  does not depict all of the structure of the sight  10 , but only selected components which facilitate an understanding of the present invention. 
     The sight  10  has a housing, which is represented diagrammatically in  FIG. 1  by a broken line  12 . Lines  14  represent visible light radiation which embodies an image of a remote external scene  16 . In this embodiment, the radiation  14  may be a broadband or “white light” radiation. The external scene  16  could be any of a wide variety of different things, and is therefore depicted diagrammatically in  FIG. 1  by a broken line. 
     The sight  10  also includes a light source  18  which may include, for example, an LED, a laser, or a liquid crystal display (LCD), powered by a power supply  20 , such as a battery. Line  22  represents visible light radiation which embodies an image of a marker. The image may be a colored dot, a set of cross-hairs, alpha/numeric text, or any other figure that may be used as a position marker or to provide information that may be helpful to a user. In this embodiment, the radiation  22  may be a relatively narrowband or monochromatic color light. In other embodiments, as will be described, the light source or multiple light sources may emit multiple narrowband or monochromatic light radiations. 
     The sight  10  also includes an optical device  23  which includes a lens  24  and a filter  26 . The lens  24  is located within the optical path of both the broadband radiation  14  and the narrowband radiation  22 . The lens  24  may carry the filter  26 . In this embodiment, the filter may be applied to a single surface of the lens, but in alternative embodiments of optical devices, the filter may be spaced apart from the lens, applied to multiple surfaces of the lens or embedded on or in a layered optical device. In this embodiment, filter  26  is a dichroic combining filter. The filter  26  may be implemented as a bandpass or edge filter. 
     In this embodiment, the filter  26  reflects a portion of the narrowband radiation  22 , creating reflected radiation  30  and transmits a portion of the broadband radiation  14 , creating transmitted radiation  28 . Conveyed radiations  28  and  30  are combined and travel through the sight  10  to an eye (or a sensor)  32 . A portion of the radiation  14  shown as radiation  34  is reflected and is not combined with the radiation  30 . 
     For example, if the light source  18  emits a generally monochromatic red light radiation  22  and the filter  26  is selected or tuned to reflect red light, it will convey red light radiation  30  toward the eye  32 . However, it will also reflect red light radiation  34  from the broadband radiation  14  so that the transmitted radiation  28  does not include red light radiation. Consequently, the transmitted radiation  28  may appear dim and overly saturated with green light. 
     It is understood that the sight  10  may also include other optical devices such as other lenses, filters, prisms, light sources, or mirrors. 
       FIG. 2  is a graph that explains the operation of a filter, such as a filter that may be used for filter  26 . The horizontal axis represents wavelength, increasing from left to right. The solid line  40  represents the spectrum of a narrow band light source, such as light source  18 , and the broken line  42  represents the reflection characteristic of a nominal filter. In this embodiment, the nominal filter is selected to reflect the majority of the spectrum from the light source. Even if the spectrum shifts to longer or shorter wavelengths by 10 nm due to changes in a physical property of the light source such as temperature, changes in drive current, part variability, deviation of the peak wavelength from a nominal peak wavelength, or other causes, the nominal filter conveys the majority of the narrow band spectrum toward the eye  32 . Additionally, the nominal filter reflects (as radiation  34 ) the portion of the broadband spectrum  14  that falls within the reflection characteristic 42 of the nominal filter. 
       FIG. 3  is a graph that explains how the perceived brightness of a light source changes as the spectrum shifts. Again, the solid line  40  represents the spectrum of a narrow band light source. The line  44  represents the photopic luminous response of a typical human eye. The line  46  represents the perceived brightness of the unfiltered light source as the spectrum shifts up and down from the nominal position. As shown, when the peak wavelength is shifted between approximately 660 nm and 620 nm, the brightness is increased by a factor of four. A shift in the spectrum of a red or orange light toward the shorter wavelengths would therefore appear to the viewer as an increase in the perceived brightness of the light source, with all other things being approximately equal. 
       FIG. 4  is a graph that explains how the application of an alternative filter (that may be used for filter  26 ) attenuates reflected wavelengths shorter than approximately 630 nm. The lines  40 ,  44 , and  46  represent information as explained above for  FIG. 3 . The line  48  represents the transmission/reflection characteristic of the filter of this alternative embodiment. The line  50  represents the perceived brightness of the filtered light source as the spectrum shifts up and down from its nominal position. As line  50  shows, when the peak wavelength is shifted between approximately 660 nm and 620 nm, the filtered perceived brightness is relatively flat or equal in that it varies by less than approximately 10%. It is understood that an optimal filter with an optimal transmission/reflection curve may be selected or tuned based upon known characteristics of the light source, including the nominal emitted spectrum and the expected shift due to changes in physical properties of the light source. 
       FIG. 5  is a graph that explains the application of still another more optimized alternative filter (that may be used for filter  26 ). The line  52  represents the nominal spectrum of a narrow band light source (such as may be used for light source  18 ). The line  54  represents the transmission/reflection characteristic of a shifted filter of this alternative embodiment. The line  56  represents a model square filter that would give approximately the same effect as the shifted filter represented by line  54 . The model square filter and the filter represented by line  54  are centered on a wavelength offset or shifted from the center of the spectrum represented by line  52 . In this embodiment, the filters are offset away from the shorter wavelengths. The filters may also be configured to reflect a narrower spectrum of light than the filter shown in  FIG. 2 . For example, the shorter edge of the filters (i.e., the edge closest to the shorter wavelengths) may be moved closer to nominal spectrum  52  of the light source, leaving the longer edge of the filters (i.e., the edge closest to the longer wavelengths) unchanged. 
     The line  58  represents a shift in the spectrum of the narrow band light source due to, for example, a change in a physical property of the light source such as temperature, drive current, or any of the factors listed above. When the spectrum shifts, the filters of this embodiment attenuate reflected light radiation  60  in the shorter wavelength region of the spectrum, the region most sensitive to the human eye. The result of this attenuation is that the perceived brightness of the light source remains approximately equal to the perceived brightness when the spectrum was in the nominal position  52 . For example, the perceived brightness may be considered “approximately equal” if the variance is less than 20%, and preferably less than 10%. Line  62  represents another shift in the spectrum of the narrow band light source, this time toward the longer wavelengths. With this shift, the filters attenuate less of the spectrum, but the perceived brightness is still approximately equal to the perceived brightness when the spectrum was in the nominal position  52  and in the shifted position  58 . 
     The filters associated with lines  54  and  56  may be tuned or selected based upon the characteristics of the light source. For example, the integral of the source spectrum weighted by the photopic response and the filter response over the wavelength range may be solved to be approximately constant over source spectrum shifts. 
     The graphs of  FIGS. 2-5  have depicted spectrums of a narrow band light source, such as light source  18 , without showing the effect of the various filters on external broadband light radiation, such as radiation  14 , which may be combined with the narrowband light radiation as explained above.  FIG. 6  is a graph that explains the effect of the shifted filter of  FIG. 5  on the broadband radiation. Line  64  represents the transmission/reflection characteristic of a nominal, relatively centered filter as described above for  FIG. 2 . Line  66  represents the spectrum of a broadband light filtered by the nominal filter of  FIG. 2 . Line  68  represents the transmission/reflection characteristic of a shifted filter as described above for  FIG. 5 . Line  70  represents the spectrum of a broadband light filtered by the shifted filter of  FIG. 5 . Area  72  of the spectrum of line  70  represents additional radiation passed by the shifted filter that may make the broadband light appear brighter and display less of a hue shift. Thus, using the shifted filter of  FIG. 5  as the filter  26  accommodates a spectral shift in the narrowband light radiation without causing a significant change in perceived brightness of the narrowband light radiation. Because the shifted filter may also reflect a narrower spectrum of light than the nominal filter, the reflected broadband radiation  34  also has a narrower spectrum, permitting more of the external broadband light to reach the eye of the user and reducing the hue shift of the transmitted broadband light. 
     In another alternative embodiment, an optical apparatus may include multiple narrowband light sources that combine to form a mixed hue. With a nominal filter that broadly reflects the multiple narrowband radiations, the color of the combined light radiation may shift as one or more of the spectrums shift. This phenomena is depicted in  FIG. 7 . In this embodiment, generally monochromatic green light  74  from a first light source is mixed with generally monochromatic red light  76  from a second light source. The resulting mix of light would have an orange hue. Line  78  represents the mixed and unfiltered, nominal (unshifted) light. Line  80  represents the transmission/reflection characteristic of a filter for filtering both the red and the green light. Line  82  represents the mixed nominal (unshifted) light reflected by the filter represented by line  80 . Line  84  represents a shifted red source with a spectral shift due to changes in temperature, drive current, or another physical property of the light source. The shift primarily affects the red light. The shifted red light causes the unfiltered mixed light to change from a deep orange hue to an orange-green color. Line  86  represents the shifted red light that has been filtered by the filter represented by line  80 . Because the filter attenuates shorter wavelengths in the shifted red light, the mixed light remains primarily orange with minimal amounts of green. Thus, the filter selected or tuned to the specific monochromatic light sources is able to limit the shift in hue that would accompany spectral shift in unfiltered mixed light. 
     In the above recited embodiments, the level of perceived brightness has been based upon the human photopic luminosity curve which is generally based upon daytime levels of light. It is understood that perceived brightness may, alternatively, be based upon a human scotopic luminosity curve that is generally based upon dim-lighting conditions. 
     Although the optical devices described above may be well suited for application in optical sights, they may also be broadly applied to other applications in which spectral shifting of a light source would otherwise result in undesirable changes in perceived brightness or in perceived hue. For example, the techniques and apparatus described herein may be applied to attenuate spectrum shifting in monochromatic or multichromatic light sources used in a wide variety of applications including computer displays, televisions, digital photo frames, projectors, and e-books. 
     Although several selected embodiments have been illustrated and described in detail, it will be understood that they are exemplary, and that a variety of substitutions and alterations are possible without departing from the spirit and scope of the present invention, as defined by the following claims.