Abstract:
An optical sighting device such as a spotting scope, riflescope or binocular having optics that direct the desired visible wavelength light to the eye or other sensor/detector while removing certain undesirable wavelengths such as infrared (IR) or ultraviolet (UV). In one configuration for a spotting scope having two front surface mirrors that fold the optical path to offer greater magnification and performance in a compact housing, the optics are modified by replacing one or both of the mirror surfaces with a band pass mirror, known as a cold mirror, the band pass mirror having particular optical transmission/reflection properties to reflect visible light and pass the undesirable wavelengths. Alternately, in a system employing a prism, one or more of the internal reflection surfaces may be formed with a desired coating for removing the IR wavelength light.

Description:
[0001]    This application claims priority to provisional application No. 60/978,713 filed Oct. 9, 2007, hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    The field of present disclosure relates to optical sighting devices such as spotting scopes, riflescopes, binoculars, monoculars, and optical protection systems and devices therefore. 
         [0003]    Lasers are increasingly found in various military, law enforcement and hunting applications for range finding, tactical illumination (e.g., aiming) and target designation. The human eye, as well as optical sensors/detectors and photo receptors can be damaged by exposure to high intensity light. For example, the retina of the eye and the nearby nerves may be damaged when exposed to a laser beam for only a short time. The irradiance of the laser energy perceived by the eyes, and hence the potential for eye damage, is increased when a user employs a spotting scope, binoculars or similar visual telescopic instrument because the laser light is focused by these magnification devices and thus a greater intensity of such laser light reaches the eyes. The present inventor has recognized the potential hazard of using a spotting scope or other optical devices where an intense laser radiation is present. 
         [0004]    Current laser eye protection methods for visual optics involve placing a filter for attenuating the intensity of laser wavelength light from the visible wavelengths in the optical path of the optics. These filters are placed either in front of or just behind the objective lens of the optics and are either reflective or absorptive filters or an optic with an ablative coating. The present inventor has determined that visible light transmission through either type of filter is typically reduced by more than 10 percent, and with light transmission loss through a typical high-quality observation or targeting device, there is an additional 10 percent loss. Therefore a combined loss of greater than 20 percent would be experienced in that approach. 
       SUMMARY 
       [0005]    The present invention is directed to optical sighting devices. In a preferred embodiment, the optical sighting device includes optics that direct the desired visible wavelength light to the eye or other sensor/detector while removing certain undesirable wavelengths such as IR or UV. A preferred configuration may utilize a spotting scope having two front surface mirrors that fold the optical path to offer greater magnification and performance in a compact housing. This two mirror spotting scope is modified by replacing one or both of the mirror surfaces with a band pass mirror, known as a cold mirror, having particular optical transmission/reflection properties to reflect visible light and pass the undesirable wavelengths. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a diagram of a spotting scope lens system according to a first embodiment employing two front surface mirrors folding the optical path. 
           [0007]      FIG. 2  is a graph of the reflective properties for a preferred broadband dielectric reflector for the spotting scope of  FIG. 1 . 
           [0008]      FIG. 3  is a graph of the transmittance properties for a preferred cold mirror of the spotting scope of  FIG. 1 . 
           [0009]      FIG. 4  is a diagram of an alternate spotting scope or riflescope lens system employing a roof prism. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0010]    Preferred embodiments will now be described with reference to the drawings. The preferred embodiments will be described in terms of a lens system for a spotting scope, but alternate embodiments may be employed within binoculars, monoculars, riflescopes and other types of optical viewing mechanisms. 
         [0011]      FIG. 1  illustrates a first preferred embodiment of a lens assembly  10  for a spotting scope comprised of an objective lens assembly  20  for focusing an object in a field of view to an image at a first focal plane  40 ; first and second mirrors  34 ,  36  for folding the optical path; an erector lens assembly  50  or alternate erector system (e.g., prisms) inverting the image and focusing it to a second focal plane  45 ; and an ocular  80  forming an image for the eye of the user. The optical layout of this spotting scope is based generally on the variable power 12−40×60 mm spotting scope available from Leupold &amp; Stevens, Inc. of Beaverton, Oreg. 
         [0012]    In this lens assembly  10 , one or both of the mirrors  34 ,  36  is replaced with a cold mirror, a dichroic reflector which functions as a broadband interference optic. In a preferred configuration, the cold mirror  34  includes a glass plate or other substrate coated on its front surface with a dielectric film stack. The cold mirror optic is selected to have specific optical properties to (1) reflect the visible light spectrum and (2) pass light of certain near infrared and infrared (IR) wavelengths. The cold mirror  34  reflects virtually all the visible light spectrum while allowing the specific near IR and IR spectrum light to pass through. The typical reflectivity of a cold mirror for visible light is about 96%, which yields a 4% loss in intensity. In contrast, a band pass filter located within the optical path to remove infrared light would typically introduce a 10% loss in the intensity of the visible light passing through the optical system. 
         [0013]    A cold mirror may be defined by its transmissive and reflective properties.  FIG. 2  is a graph of the reflective properties of a preferred cold mirror and  FIG. 3  is a graph of the transmittance properties of a preferred cold mirror. Employing this cold mirror as the reflector  34  and/or reflector  36 , greater than about 90% of the visible light between 450 nm and 700 nm is reflected by the mirror (and along the desired optical path and on the order of 85% or more of the IR radiation between 725 nm and 1200 nm is transmitted through the mirror and thus is not reflected. It is understood that the higher (greater than 1200 nm) wavelength IR radiation is also transmitted through the mirror. Thus the cold mirror having these properties of high reflectance in the visible range and low reflectance in specific IR range may be utilized for reducing the amount of IR radiation reaching the eye of the user while maintaining low loss levels in the visible range. 
         [0014]    In an alternate configuration, both mirror  34  and mirror  36  may comprise cold mirrors. In such a configuration, the intensity loss will be somewhat higher than the single cold mirror combination. For example, presuming a 4% loss in the visible light wavelength, combined reflectivity would be 96%×96%=92.16%. However, by providing the reflective surfaces of both mirrors  34  and  36  being cold mirrors, combined reflectivity for IR wavelength light would be 15%×15%=2.25%. Thus for the dual cold mirror system, on the order of 97.75% of the IR light is prevented from reaching the ocular  80  via operation of the two cold mirrors  34 ,  36 . 
         [0015]    In the dual cold mirror configuration, the mirrors  34 ,  36  may comprise the same or similar dielectric coatings or each mirror may comprise a different coating having unique optical properties or reflectivity frequency response. For example, one of the mirrors may effectively remove IR wavelength light (e.g., 85% transmissive) at 725 nm +/−5 nm while the other mirror may effectively remove IR wavelength light (e.g. 85% transmissive) at 750 nm ±5 nm. In another example, one mirror may have a lower removal percentage in a band at a particular wavelength as in the following table: 
         [0000]                                              TABLE A                       IR Reflectivity @ 725 nm   IR Reflectivity @ 750 nm                                    Mirror A   50%   20%       Mirror B   15%   45%       Total:   7.5%    9%                    
Thus by way of two mirrors, the Mirror A compensates for the removal deficiency of Mirror B at the 750 nm band and Mirror B compensates for the removal deficiency of Mirror A at the 725 nm wavelength band whereby in excess of 90% of IR light intensity is removed at each of the 725 nm and 750 nm wavelength bands. Thus by a combination of cold mirrors, a high range (greater than 90%) or ultra high range (greater than 95%) of IR light across the IR spectrum may be removed.
 
         [0016]    The coating is selected/designed depending upon several factors, including: mirror materials, the angle of incidence θ i , and the wavelengths of the light desired to be removed. 
         [0017]    In a preferred single cold mirror configuration, it is preferred that the cold mirror transmit (and thus remove from the optical path) at least 70% or 80% of IR light from 725 nm to 1200 nm (or 750 nm and 1100 nm). Some of the common laser devices applications include: YAG laser (1500 nm and 1064 nm) typically used in military vehicle applications; and laser diodes (905 nm and 808 nm) typically used for laser rangefinder applications. By removing the IR light of this wavelength range, laser light produced by these laser devices is removed. 
         [0018]      FIG. 4  illustrates an alternate embodiment of a lens assembly  110  that may be particularly useful for a binocular, spotting scope or riflescope. This optical layout is based on a configuration that supports binoculars, spotting scopes and riflescopes available from Leupold &amp; Stevens, Inc. of Beaverton, Oreg., but according to a preferred embodiment, the roof prism is modified as described in the following. As modified, the binocular is comprised of an objective lens assembly  120  for focusing an object in a field of view to an image at a focal plane  145 ; a roof prism  130  for inverting the image  140 ; and an ocular lens assembly  180  for focusing and forming an image for the eye of the user  190 . 
         [0019]    The roof prism  130  illustrated in  FIG. 4  is in the form of a modified Schmidt-Pechan prism. The prism  130  includes six internal reflective surfaces  136  and  134   a ,  134   b  with one or more of these reflective surfaces comprising cold mirrors of similar optical properties as described in the previous embodiment. It is noted that in  FIG. 4  the roof mirrors  134   a ,  134   b  are not separately illustrated. The cold mirror reflects light of wavelengths in the visible range with high efficiency and allows light in the IR wavelength range (about 85%) to pass through effectively removing a large portion of the IR light from the optical path. Reflective surface  136  may be a preferred surface for a single cold mirror application, but by including cold mirrors at both surfaces  134   a ,  134   b  with each surface passing 85% of IR light, a combined 97% of the IR light intensity may be removed. 
         [0020]    Other types of prisms may be employed with dielectric coatings on certain surfaces to serve as a cold mirror. A Porro prism may have two reflective surfaces that may accommodate a dielectric coating. A double Porro prism may have four reflective surfaces suitable for dielectric coating. An Abbe-Koenig prism may have two reflective surfaces for dielectric coating. An Amici prism is another type of roof prism having one reflective surface that may be suitable for dielectric coating. 
         [0021]    The undesirable IR wavelength light that is transmitted through the cold mirror is preferably routed to a black body behind the cold mirror, the black body designed to absorb or trap the IR light. Since the IR light is likely not to be a sustained pulse, it is expected that the total energy of the IR radiation will be somewhat low and thus should not generate significant heat. 
         [0022]    Thus preferred optical systems and configurations have been shown and described. While specific embodiments and applications for have been shown and described, it will be apparent to one skilled in the art that other modifications, alternatives and variations are possible without departing from the inventive concepts set forth herein. Therefore, the invention is intended to embrace all such modifications, alternatives and variations.