Abstract:
A weapon sight can be mounted on a weapon. According to still another aspect of the invention, the sight takes a selected action if it detects the presence of a selected color within radiation originating externally of the sight.

Description:
TECHNICAL FIELD OF THE INVENTION  
       [0001]     This invention relates in general to a device that facilitates accurate aiming of a firearm and, more particularly, to a firearm sight that is mounted on the firearm, and through which a user observes a potential target.  
       BACKGROUND OF THE INVENTION  
       [0002]     Over the years, various techniques and devices have been developed to help a person accurately aim a firearm, such as a rifle or a target pistol. One common approach is to mount a sight or scope on the firearm&#39;s barrel. A person then uses the sight or scope to view an intended target in association with a reticle, often with a degree of magnification. Although existing firearm sights of this type have been generally adequate for their intended purposes, they have not been satisfactory in all respects.  
         [0003]     For example, some pre-existing sights have included the capability to record an image showing a target and/or a reticle, and to later display one or more of these recorded images. However, when these recorded images are displayed, it is possible for a safety hazard to occur. For example, if the recorded image is presented on an electronic display that is separately used to show actual targets, a user may mistake the recorded image for an actual target, and may then discharge the weapon in the belief that he or she is shooting at something in the recorded image, when the weapon is actually pointed at some other person or thing. Moreover, even if the user does not intentionally discharge the weapon while viewing recorded images, there is always a risk of accidental discharge. Consequently, if the user is distracted while viewing recorded images, or gives the weapon and sight to another person who is distracted or who is not familiar with weapon safety, the weapon may be inadvertently pointed in a direction that presents a safety hazard.  
         [0004]     A different consideration is that hunting regulations in most states stipulate that hunting is allowed only during the time from one-half hour before sunrise to one-half hour after sunset. The intent of these regulations is to prevent the unsafe practice of shooting in very low light levels, where the actual identity of a target may be questionable. The level of illumination at one-half hour before sunrise and at one-half hour after sunset is sometimes referred to as “civil twilight”, and falls in a luminance range of 0.1 to 1.0 foot-candles. This luminance range corresponds to a cloudless sky. Other conditions can cause the illumination level to drop below that of civil twilight at almost any time during the day, for example where there is a dense cloud cover, or where a hunter is in a dense forest. There is no easy way for hunters and game wardens to determine actual levels of illumination, and this is why states have adopted the compromise approach of defining allowable hunting conditions in terms of dusk and dawn, rather than in terms of actual levels of illumination. Existing sights provide hunters with no assistance in detecting or avoiding actual low light conditions that can present potential safety hazards.  
         [0005]     Still another consideration is that virtually all states have a hunting regulation that requires hunters to wear a fluorescent orange garment above the waist while hunting. This color does not occur naturally in any big game animals, or in their environment. The fluorescent orange color is thus intended to be a visual cue to a hunter that a person is present, rather than a potential animal target. Even where such a garment is present, the patch of orange color may be partly obscured by other objects in the scene, or may be very small if the hunter is a significant distance from the person wearing the garment. In either case, the presence of the orange color in the scene may be inadvertently and unintentionally overlooked by a hunter, resulting in a potentially dangerous situation for the person wearing the garment. Existing rifle sights provide hunters with no assistance in detecting fluorescent orange to avoid potentially dangerous hunting situations.  
       SUMMARY OF THE INVENTION  
       [0006]     According to one aspect of the invention, a method and apparatus relate to a weapon-mountable sight having a display and involve presenting selected information on the display only when a detector portion indicates that the sight has an orientation that meets an orientation criteria.  
         [0007]     According to a different aspect of the invention, a method and apparatus relate to a weapon-mountable sight and involve: using a detector portion to determine a level of ambient illumination external to the sight; and taking a selected action in response to a determination that the level of ambient illumination is less than a selected level of illumination.  
         [0008]     According to still another aspect of the invention, a method and apparatus relate to a weapon-mountable sight, and involve taking a selected action in response to detection of a selected color within radiation originating from externally of the sight.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     A better understanding of the present invention will be realized from the detailed description that follows, taken in conjunction with the accompanying drawings, in which:  
         [0010]      FIG. 1  is a diagrammatic perspective view of an apparatus that is a digital rifle sight embodying aspects of the present invention;  
         [0011]      FIG. 2  is a block diagram of the digital rifle sight of  FIG. 1 , and shows some internal components thereof;  
         [0012]      FIG. 3  is a diagrammatic view of an internal display that is a component of the rifle sight of  FIG. 1 , as seen by the eye of a person using the sight;  
         [0013]      FIG. 4  is a diagrammatic view of a switch panel that is a component of the rifle sight of  FIG. 1 , and that has a plurality of manually operable switches;  
         [0014]      FIG. 5  is a diagrammatic view of an external display that is a component of the rifle sight of  FIG. 1 , with a recorded image displayed thereon;  
         [0015]      FIG. 6  is a diagrammatic view of the internal display while the rifle sight is being used to view a scene having a low level of ambient illumination; and  
         [0016]      FIG. 7  is a diagrammatic view of the internal display while the rifle sight is being used to view a scene that includes a person wearing a fluorescent orange garment.  
     
    
     DETAILED DESCRIPTION  
       [0017]      FIG. 1  is a diagrammatic perspective view of an apparatus that is a digital rifle sight  10 , and that embodies aspects of the present invention. Although the sight  10  is sometimes referred to herein as a “rifle” sight, it can actually be used not only with rifles, but also with other types of firearms, such as target pistols.  
         [0018]     The sight  10  includes a rail mount  12  that can fixedly and securely mount the sight  10  on the barrel of a firearm. The sight  10  has a switch panel  13 , with several manually operable switches that are discussed in more detail later. The sight  10  has an external color display  14  that, in the disclosed embodiment, is a liquid crystal display (LCD) of a type commonly found on digital cameras and video cameras for the purpose of viewing images or video clips that have been stored within these cameras. One end of the sight  10  has an eyepiece section  15 .  
         [0019]      FIG. 2  is a block diagram of the rifle sight  10 , and shows some internal components of the sight  10  that are relevant to an understanding of the present invention.  
         [0020]     The sight  10  includes an objective lens section  16  of a known type. In the disclosed embodiment, the lens section  16  has a field of view (FOV) of 5°, but it could alternatively have some other field of view. The lens section  16  optically images a remote scene or target  17  onto an image detector  18 . In the disclosed embodiment, the image detector  18  is a charge coupled device array (CCD array) of a known type, and has 1,920,000 detector elements that each correspond to a respective pixel in each image produced by the image detector  18 , and that are arranged as an array of 1600 detector elements by 1200 detector elements. However, the image detector  18  could alternatively be implemented with any other suitable device, including a device having a larger or smaller number of detector elements, or a type of device other than a CCD array, such as a Complementary Metal Oxide Semiconductor (CMOS) image sensor.  
         [0021]     The image detector  18  produces a sequence of digital color images of the scene  17 , and this sequence of images is supplied to a processing section  21 . Although the image detector  18  of the disclosed embodiment produces color images, the images could alternatively be monochrome images, or black and white images. The processing section  21  includes a processor  22  of a known type, and a memory  23 . The memory  23  in  FIG. 2  is a diagrammatic representation of the memory provided for the processor  22 , and may include more than one type of memory. For example, the memory  23  may include a read only memory (ROM) that contains a program executed by the processor  22 , as well as data that does not change during program execution. The memory  23  can also include some semiconductor memory of the type commonly known as “flash” RAM. A “flash” RAM is a type of memory that is commonly used in devices such as memory cards for digital cameras, and that maintains the information stored therein even when electrical power is turned off.  
         [0022]     The processing section  21  further includes a reformatter  26  of a known type. The reformatter  26  is capable of taking an image generated by the image detector  18 , and reformatting the image to a lower resolution that is suitable for presentation on a display having a lower resolution than the image detector  18 . Images processed by the reformatter  26  are selectively supplied to two display driver circuit  30  and  31 . The display driver circuit  30  drives the external display  14 , and the display driver circuit  31  drives an internal color display  32 . The display driver circuits  30  and  31  can be different channels of a single display driver circuit, but are shown as separate blocks in  FIG. 2  for clarity. In the disclosed embodiment, the color display  32  is a liquid crystal display (LCD) of a known type, and has 76,800 pixel elements arranged as an array of 320 elements by 240 elements. The display  32  could, however, have a larger or smaller number of pixel elements, or could be any other suitable type of display, such as an organic light emitting diode (OLED) display, a liquid crystal on silicon (LCOS) display, or a micro-electro-mechanical system (MEMS) reflective display.  
         [0023]     The eyepiece section  15  ( FIG. 1 ) of the sight  10  includes eyepiece optics  36  of a known type. The eyepiece optics  36  permit the internal display  32  to be comfortably viewed by an eye  37  of a person who is using the sight  10  in association with a firearm. In the disclosed embodiment, the eyepiece optics  36  have an FOV of 15°, but could alternatively have some other suitable FOV. In addition, the eyepiece optics  36  of the disclosed embodiment could optionally be omitted for applications that allow a person to directly view the display  32  with a viewing distance greater than about 8 inches, since comfortable viewing is then possible with little eye accommodation needed.  
         [0024]     The sight  10  includes an accelerometer  41  that has an output coupled to the processing section  21 . In the disclosed embodiment, the accelerometer  41  is a device that can be obtained commercially as part number ADXL105 from Analog Devices, Inc. of Norwood, Mass. Although the disclosed embodiment implements the accelerometer  41  with the Analog Devices-ADXL105 device, the accelerometer  41  could alternatively be implemented with any other suitable device. The accelerometer  41  is a micro-electro-mechanical system (MEMS) device, and serves as a highly sensitive sensor that can detect the relatively small shock wave caused when a firing pin strikes a cartridge within a firearm on which the sight  10  is mounted. In addition, as discussed later, the accelerometer  41  is also responsive to the force of gravity.  
         [0025]     When a firing pin strikes a cartridge, it triggers combustion of the gunpowder or other propellant within the cartridge, so as to expel a bullet or other projectile from the cartridge and firearm. Consequently, a relatively small shock wave is produced when the firing pin strikes the cartridge, and this small shock wave is promptly followed by a significantly larger shock wave or recoil that is produced by the combustion of the gunpowder and the expulsion of the bullet. The latter shock wave is several orders of magnitude larger than the shock wave produced when the firing pin strikes the cartridge. The accelerometer  41  has the sensitivity and bandwidth needed to detect the relatively small shock wave produced when the firing pin strikes the cartridge, but also has the durability needed to withstand the much larger shock wave produced by the ensuing combustion within the cartridge.  
         [0026]     The output signal from the accelerometer  41  has a frequency spectrum for the small shock wave that is significantly different from the frequency spectrum for the ensuing large shock wave. Consequently, the processing section  21  can distinguish a shock wave that represents the firing pin striking a cartridge from a shock wave that represents some other type of event, such as combustion within a cartridge. For example, in order to identify the small shock wave, the processing section  21  could apply a fast Fourier transform (FFT) to the output of the accelerometer  41 , filter out frequency components that are outside a frequency band of approximately 5 KHz to 10 KHz, and then look for a pulse in the energy between 5 KHz and 10 KHz.  
         [0027]     The sight  10  includes a gyroscope  43 , with an output that is coupled to the processing section  21 . The gyroscope is referred to herein as a rate gyro. In the disclosed embodiment, the rate gyro  43  is implemented with a MEMS device that is available commercially as part number ADXRS150 from Analog Devices, Inc. Although the disclosed embodiment uses the Analog Devices ADXRS150 device, it would alternatively be possible to implement the rate gyro  43  with any other suitable device.  
         [0028]     The rate gyro  43  is capable of detecting angular movement of the sight  10  about a not-illustrated vertical axis that is spaced from the rate gyro  43 . Thus, the rate gyro  43  is a highly sensitive device that is effectively capable of detecting movement of the sight  10  in directions transverse to a not-illustrated center line of the objective lens section  16 .  
         [0029]     The sight  10  includes a removable memory card  46  that, when present within the sight  10 , is operatively coupled to the processing section  21 . In the disclosed embodiment, the memory card  46  is a memory card of the type commonly used in digital cameras. However, it would alternatively be possible to use any other suitable device for the removable memory card  46 .  
         [0030]     The sight  10  includes a battery  51  that, in the disclosed embodiment, is a replaceable battery of a known type. However, the battery  51  could alternatively be a rechargeable battery. The sight  10  also includes an external power connector  52  that can be coupled to an external source of power, such as a converter that converts alternating current (AC) to direct current (DC).  
         [0031]     As mentioned above in association with  FIG. 1 , the sight  10  has a switch panel  13  with a plurality of manually operable switches. These switches include a power switch  57 , and also include several other switches  58 - 65  that are each coupled to the processing section  21 , and that are discussed in more detail below. The battery  51  and the external power connector  52  are each coupled to inputs of the power switch  57 . When the power switch  57  is respectively actuated and deactuated, it respectively permits and interrupts a flow of current from the battery  51  and/or the connector  52  to circuitry  71  that is disposed within the sight  10 , and that requires electrical power in order to operate. The circuitry  71  includes the image detector  18 , the processing section  21 , the display drivers  30  and  31 , the external display  14 , the internal display  32 , the accelerometer  41 , the rate gyro  43 , and the memory card  46 .  
         [0032]     The sight  10  has a connector  81  that is coupled to the processing section  21 . The connector  81  can be used to upload image data or video data from the sight  10  to a not-illustrated computer, as discussed later. In addition, the connector  81  can be used to download an electronic reticle from a computer to the sight  10 , as also discussed later. In the disclosed embodiment, the physical configuration of the connector  81 , as well the protocol for transferring information through it, conform to an industry standard that is commonly known as the Universal Serial Bus (USB) standard. However, it would alternatively be possible to use any other suitable type of connector and communication protocol, such as a standard serial connector and communication protocol, or a standard parallel connector and communication protocol.  
         [0033]     The sight  10  includes a further connector  82 , through which video information can be transferred from the sight  10  to an external device, in a manner conforming to an industry video standard that is commonly known as the National Television Standards Committee/Phase Alternating Line (NTSC/PAL) standard. In the disclosed embodiment, the connector  82  is a standard component of the type commonly known as an RCA jack. However, it could alternatively be any other suitable type of connector, and information could be transferred through it according to any other suitable protocol.  
         [0034]      FIG. 3  is a diagrammatic view of the internal display  32 , as seen by the eye  37  of a person looking into the sight  10  through the eyepiece optics  36 . In a normal operational mode, the display  32  presents a view of the scene  17 , as captured by the image detector  18  through the objective lens section  16 . The scene  17  is shown diagrammatically in  FIG. 2  by broken lines.  
         [0035]     The processing section  21  superimposes a reticle  101 - 105  on the image of the scene  17 . In the disclosed embodiment, the reticle includes a small center circle  101 , and four lines  102 - 105  that each extend radially with respect to the circle  101 , and that are offset by intervals of 90°. The reticle  101 - 105  is a digital image that is downloaded into the sight  10  through the USB connector  81 , and that is stored by the processing section  21  in a non-volatile portion of the memory  23 . The reticle can have almost any configuration desired by a user. In particular, a reticle with virtually any desired configuration can be created by a user in a separate computer, or obtained by the user from the sight manufacturer or a third party through a network such as the Internet. The new reticle can then be downloaded electronically in digital form through the connector  81 , and is stored in the memory  23  of the processing section  21 .  
         [0036]     The processing section  21  takes the reticle that is currently stored in the memory  23 , and digitally superimposes the reticle on images that will be sent to the display  32 . In  FIG. 3 , the reticle  101 - 105  has been superimposed on the image in a manner so that the reticle is centered on the display  32 . However, the position where the reticle appears on the display  32 , and thus the position of the reticle relative to the image of the scene  17 , can be adjusted in a manner that is described later.  
         [0037]     The processing section  21  can also superimpose some additional information on the image of the scene  17 . In this regard, the lower left corner of the display  32  includes a windage or azimuth adjustment value  111 . As mentioned earlier, the position of the reticle  101 - 105  on the display  32  can be adjusted, in a manner that is discussed in more detail later. The windage adjustment value  111  is a positive or negative number that indicates the offset by which the reticle  101 - 105  has been adjusted either leftwardly or rightwardly from the centered position shown in  FIG. 3 .  
         [0038]     The upper right corner of the display  32  has a battery charge indicator  113  that is divided into three segments, and that is used to indicate the state of the battery  51 . In particular, when the battery is fully charged, all three segments of the battery charge indicator  113  are displayed. Then, as the battery  51  becomes progressively discharged, there will be a progressive decrease in the number of displayed segments of the battery charge indicator  113 .  
         [0039]     The upper left corner of the display  14  presents an image count value  114 , and this count value  114  relates to the fact that the processing section  21  can store images in the removable memory card  46 , as discussed later. The image count value  114  is an indication of how many additional images can be stored in the unused space that remains within the memory card  46 .  
         [0040]     The top center portion of the display  32  has a capture mode indicator  115 , and a firing pin detection indicator  116 . The capture mode indicator  115  shows which of two capture modes is currently in effect, as discussed later. The firing pin detection indicator  116  indicates whether or not the sight is currently enabled to detect the firing pin striking a cartridge, as discussed later.  
         [0041]     The bottom central portion of the display  32  includes an autoboresight alignment indicator  117 , for a purpose that is not related to the present invention, and that is therefore not described here in detail. An angular error indicator  120  appears in the central portion of the display  32 . The indicator  120  is a circle that is larger than and concentric to the circle  101  at the center of the reticle  101 - 105 . The diameter of the indicator  120  is increased and decreased in response to variation of a particular operational criteria, as discussed later. Depending on the current mode of operation of the sight  10 , the reticle  101 - 105  and the various indicators  111 - 120  may all be visible, or only some may be visible.  
         [0042]      FIG. 4  is a diagrammatic view of the switch panel  13 , and shows each of the manually operable switches  57 - 65  of the switch panel  13 . The types of switches and their arrangement on the panel  13  is exemplary, and it would alternatively be possible to use other types of switches, and/or to arrange the switches in a different configuration. The power switch  57  has already been discussed above, and therefore is not discussed again here.  
         [0043]     The switch  58  is a detect switch. As mentioned earlier, the accelerometer  41  ( FIG. 2 ) is capable of detecting a shock wave that occurs when the firing pin of the firearm strikes a cartridge. Successive manual actuations of the detect switch  58  alternately instruct the processing section  21  to enable and disable this detection feature. When this feature is respectively enabled and disabled, the detection indicator  116  is respectively visible on and omitted from the display  32 .  
         [0044]     The switch  59  is a mode switch. In one operational mode, the processing section  21  of the sight  10  can take a single image generated by the image detector  18 , and store this image in the removable memory card  46 . In a different operational mode, the processing section  21  can take several successive images generated by the image detector  18 , which collectively form a video clip, and store these images in the memory card  46 . Successive actuations of the mode switch  59  cause the processing section  21  to toggle between these two operational modes. When the mode for storing video clips is respectively enabled and disabled, the detection indicator  115  is respectively visible on and omitted from the display  32 . There are two types of events that will cause the processing section  21  to save an image or a video clip.  
         [0045]     First, if the detect switch  58  has been used to enable detection of the firing pin striking a cartridge, the processing section  21  will respond to each detection of this event by saving either a single image or a video clip in the memory card  46 , depending on whether the capture mode that has been selected using the mode switch  59  is the image capture mode or the video capture mode. It will be recognized that, since a video clip is a series of several images, saving a video clip in the memory card  46  will take up several times the storage space that would be required to save a single image. After saving an image or a video clip, the processing section  21  adjusts the image count indicator  114  presented on the display  32 . In particular, if a single image is stored, then the count value  114  will simply be decremented. On the other hand, if a video clip is saved, the value of the indicator  114  will be reduced by an amount that corresponds to the number of images in the video clip.  
         [0046]     The other event that will cause the processing section  21  to save one image or a video clip is manual operation of the switch  64 , which is a capture switch. Whether the processing section  21  saves a single image or a video clip is dependent on the capture mode that has been selected using the mode switch  59 . When the capture switch  64  is manually operated, the processing section  21  selects either a single image or a video clip from the current output of the image detector  18 , and then saves this image or video clip in the memory card  46 . As mentioned earlier, a separate and not-illustrated computer can be coupled to the connector  81 , and the processing section  21  can upload to that computer the images or video clips that are stored in the memory card  46 .  
         [0047]     The switch  63  is a rocker switch that serves as a zoom control switch. Pressing one end of the switch  63  increases the zoom factor, and pressing the other end decreases the zoom factor. In the disclosed embodiment, the zoom is continuous and can range from 1× to 4×. When the disclosed system is operating at a zoom factor of 4×, a center portion is extracted from each image produced by the image detector  18 , where the center portion has a size of 320 by 240 pixels. This center portion is then displayed on the color display  32 , with each pixel from the center portion being mapped directly on a one-to-one basis to a respective pixel of the display  32 .  
         [0048]     When the zoom factor is at 1×, the reformatter  26  essentially takes an entire image from the image detector  18 , divides the pixels of that image into mutually exclusive groups that each have 16 pixels arranged in a 4 by 4 format, averages or interpolates the 16 pixels of each group into a single calculated pixel, and then maps each of the calculated pixels to a respective corresponding pixel of the display  32 . Similarly, when the zoom factor is at 3×, the reformatter  26  essentially takes an image from the image detector  18 , extracts a center portion having a size of about 960 pixels by 720 pixels, divides the pixels of this center portion into mutually exclusive groups that each have 9 pixels arranged in a 3 by 3 format, averages or interpolates the 9 pixels of each group into a single calculated pixel, and then maps each of the calculated pixels to a respective corresponding pixel of the display  32 . As still another example, when the zoom factor is at 2×, the reformatter  26  essentially takes an image from the image detector  18 , extracts a center portion having a size of about 640 pixels by 480 pixels, divides the pixels of this center portion into mutually exclusive groups that each have 4 pixels arranged in a 2 by 2 format, averages or interpolates the 4 pixels of each group into a single calculated pixel, and then maps each of the calculated pixels to a respective corresponding pixel of the display  32 .  
         [0049]     As mentioned above, the zoom from 1× to 4× is continuous in the disclosed embodiment. When the zoom factor is between 1× and 2×, between 2× and 3×, or between 3× and 4×, the reformatter  26  takes an appropriate portion of an image, and then groups, interpolates and maps the pixels of this portion into the pixels of the display  32 , in a manner analogous to that discussed above. Although the zoom in the disclosed embodiment is continuous, it would alternatively be possible for the zoom factor to be moved between discrete zoom levels, such as the four discrete zoom levels of 1×, 2×, 3× and 4×. In addition, although the zoom range in the disclosed embodiment is 1× to 4×, it would alternatively be possible to use some other zoom range.  
         [0050]     With reference to  FIG. 4 , the switch  65  is a four-way reticle switch. Any one of the upper, lower, left or right sides of this switch (as viewed in  FIG. 4 ) can be manually operated in order to respectively indicate a selection of up, down, left or right. Each time the upper side of the switch  65  is actuated, the position of the reticle  101 - 105  is adjusted upwardly with respect to the display  32 , and thus with respect to the image of the scene  17  that is presented on the display  32 . Each such actuation of the switch  65  causes the reticle  101 - 105  to be moved upwardly by a predetermined number of pixels, and the elevation value  112  in the lower right corner of the display  32  is incremented in response to each such adjustment. Similarly, if the lower side of the switch  65  is actuated, the reticle  101 - 105  is adjusted downwardly on the display  32  by the predetermined number of pixels, and the elevation value  112  is decremented. Similarly, actuation of the left or right side of the switch  65  causes the reticle  101 - 105  to be adjusted leftwardly or rightwardly by a predetermined number of pixels on the display  32 , and causes the windage value  111  in the lower left corner of the display  32  to be either incremented or decremented.  
         [0051]     As mentioned above, the sight  10  is capable of capturing and storing either single images or short video clips. In order to view these stored images or clips, the user presses the view switch  62 , thereby causing the processing section  21  to use the external display  14  to present either the first still image from the memory card  46 , or the first video clip from the memory card  46 .  FIG. 5  is a diagrammatic view of the display  14  with a recorded image displayed thereon. It will be noted that the recorded image includes not only the scene, but also the reticle  101 - 105 , so that the user can see where the reticle was positioned with respect to the scene when the trigger of the rifle was pulled.  
         [0052]     If the memory card  46  contains more than one image or video clip, then an arrow  142  will be visible to indicate that the user can move forward through the images or video clips. The user presses the right side of the reticle switch  65  in order to move to the next successive image or video clip. Except when the user is viewing the first image or video clip, an arrow  141  will be visible to indicate that the user can move backward through the images or video clips. The user presses the left side of the reticle switch  65  in order to move backward through the images or video clips. The view indicator  142  will be visible except when the user is viewing the last image or video clip, and the view indicator  141  will be visible except when the user is viewing the first image or video clip. The view mode is terminated by pressing the switch  62  a second time, in order to turn off the external display  14  and thereby conserve battery power.  
         [0053]     As is well known to persons who use rifles and similar weapons, care must always be used to avoid pointing the rifle at anyone or anything that the user does not intend to shoot, in case there is an accidental discharge of the rifle. The sight  10  is designed to reduce the likelihood that the rifle may be inadvertently pointed in a direction that presents a safety hazard. In particular, the sight  10  includes the external display  14 , in order to avoid displaying any recorded images from the memory on the internal display  32 . This avoids a situation in which a hunter might mistake a recorded image on the internal display  32  for an actual view of the target, and then discharge the firearm in the belief that he or she was shooting at something in the recorded image, when in fact the rifle was actually aimed at something or someone else.  
         [0054]     A further consideration is that, even with the presence of the external display  14 , there could still be a potential safety hazard if a user became distracted while viewing recorded images on the display  14 , and inadvertently pointed the rifle in a direction that presented a safety hazard. A similar scenario is that the user might inadvertently point the rifle in an unsafe direction while trying to orient the sight  10  so that another person can see the images on the display  14 . Or the user might hand the rifle with the sight  10  to that other person, in order to allow the person to have a good view of images presented on the external display  14 . That other person might then point the rifle in an unsafe manner, either because the person was distracted by the displayed images, and/or because the person simply was not suitably familiar with the basic principles of safe weapon handling.  
         [0055]     The sight  10  is designed to also avoid this latter type of hazard. More specifically, as mentioned above, the accelerometer  41  is very sensitive and can detect the force of gravity. Consequently, as the sight  10  is progressively moved from a position where the rifle barrel is horizontal to a position where the rifle barrel is pointing vertically upwardly, the output signal of the accelerometer  41  will have a force component due to gravity that progressively increases. Based on that force component, the processor  22  of the sight  10  does not present any images on the external display  14 , unless an optical centerline of the sight  10  (which extends generally parallel to the barrel of the attached rifle) is within 10° to 20° of a vertical reference. Consequently, the rifle barrel will be pointing almost directly upwardly wherever the external display  14  is actuated and showing any recorded image information.  
         [0056]     Although the sight  10  uses the accelerometer  41  to determine its orientation, it would alternatively be possible to use any other suitable sensor arrangement to detect orientation. As one example, it would be possible to use a group of conventional mercury switches having different orientations.  
         [0057]     The switch  61  serves as an angle rate switch that can be operated to enable and disable the display of an angular error rate, as sensed by the rate gyro  43 . In particular, successive manual actuations of the switch  61  will alternately enable and disable this function. When this function is respectively enabled and disabled, the angular error indicator  120  is respectively visible on and omitted from the display  32 . When this function is enabled, the processing section  21  monitors the output of the rate gyro  43 . Typically, a user will be aiming the firearm and attempting to keep the reticle center  101  accurately centered on a portion of the scene  17  that is considered to be a target.  
         [0058]     If the user happens to be holding the firearm very steady, then the rate gyro  43  will detect little or no angular motion of the sight  10  and the firearm, or in other words little or no transverse movement thereof. Consequently, the processing section  21  will present the indicator  120  as a circle of relatively small diameter, in order to indicate to the user that the firearm is being relatively accurately held on the selected target. On the other hand, if the user is having difficulty holding the firearm steady, then the rate gyro  43  will detect the greater degree of angular movement of the firearm and the sight  10 . Consequently, the processing section  21  will display the indicator  120  with a larger diameter, thereby indicating that the reticle center  101  is not being held on the target as accurately as would be desirable.  
         [0059]     In the disclosed embodiment, the change in the diameter of the indicator  120  is continuous. In other words, a progressive increase in the amount of angular movement of the firearm and the sight  10  results in a progressive increase in the diameter of the indicator  120 . Conversely, a progressive decrease in the amount of angular movement of the firearm and sight results in a progressive decrease in the diameter of the indicator  120 . The user will therefore endeavor to squeeze the trigger of the firearm at a point in time when the reticle center  101  is centered on the target, and when the indicator  120  has a relatively small diameter that indicates the firearm is currently being held very steady.  
         [0060]     The remaining switch  60  on the switch panel  55  is a boresight switch, and is used to enable and disable an autoboresight alignment mode. When this mode is respectively enabled and disabled, the autoboresight alignment indicator  117  is respectively visible on and omitted from the dismay  32 . As indicated earlier, the autoboresight alignment function is not related to the present invention, and therefore is not described here in detail.  
         [0061]     Hunting regulations in most states stipulate that hunting is allowed during the time from one-half hour before sunrise to one-half hour after sunset. The intent of these regulations is to prevent the unsafe practice of shooting in very low light levels, where the actual identity of a target may be questionable. The level of illumination at one-half hour before sunrise and at one-half hour after sunset is sometimes referred to as “civil twilight”, and falls in a luminance range of 0.1 to 1.0 foot-candles. This luminance range corresponds to a cloudless sky. Other conditions can reduce ambient illumination to a level below that of civil twilight at almost any time during the day, for example where there is a dense cloud cover, or where a hunter is in a dense forest. There is no easy way for hunters and game wardens to determine actual levels of illumination, and this is why states have adopted the compromise approach of defining allowable hunting conditions in terms of dusk and dawn, rather than in terms of actual levels of illumination.  
         [0062]     The image detector  18 , based on its sensitivity and integration time, can give a direct measure of the actual levels of illumination present in scenes viewed through the sight  10 . Consequently, the processing section  21  analyzes the images received from the image detector  18 , in order to determine the ambient level of illumination within the detected scene. In the disclosed embodiment, the processing section  21  averages the brightness of all of the pixels in a given image, and then compares the calculated average to a predetermined threshold that corresponds to civil twilight. Alternatively, however, any other suitable technique may be used to make this analysis. If the processing section  21  determines that the calculated average brightness is above the predetermined threshold, indicating that the level of ambient illumination is greater than civil twilight, then the sight  10  is operated in a normal manner. On the other hand, if the processing section  21  determines that the calculated average brightness is below the threshold, then the processing section displays a warning.  
         [0063]     More specifically,  FIG. 6  is a diagrammatic view of the internal display  32  while the sight  10  is being used to view a scene having a low level of ambient illumination. After calculating the average level of brightness for the displayed image, and determining that the calculated average is below the predetermined threshold, the processing section  21  displays the image with the addition of a warning  201 . In the disclosed embodiment, the warning  201  is the alphanumeric phrase “LOW LIGHT LEVEL”. In order to attract the attention of the user, this warning can be displayed in a color such as red, and/or can be made to blink. This warning notifies the user that light levels are low, thereby reminding the user that target recognition may be questionable and that hunting conditions may be unsafe. A responsible hunter will not want to shoot in these conditions.  
         [0064]     Although the warning  201  in the disclosed embodiment is the alphanumeric phrase “LOW LIGHT LEVEL”, it could alternatively be some other alphanumeric phrase, a symbol such as a circle with a slash through it, or a combination of a symbol and an alphanumeric phrase. In addition, as discussed above, the disclosed embodiment responds to detected low light levels by displaying the warning  201  in association with the detected image. Alternatively, however, it would be possible for the processing section  21  to respond to the detection of a low light level by inhibiting the display of any image of any scene. In that case, the processing section could display the warning  201  (without any image), or could simply disable the presentation of any information on the display  32 .  
         [0065]     Virtually all states have a hunting regulation that requires hunters to wear a fluorescent orange garment above the waist while hunting. This color does not naturally occur in any big game animals or their environment, and is intended to be a visual cue to a hunter that a person is present, rather than a potential animal target. Even where such a garment is present, the patch of orange color may be partly obscured by other objects in the scene, or may be very small if the hunter is a significant distance from the person wearing the garment. In either case, the presence of the orange color in the scene may be inadvertently and unintentionally overlooked by a hunter, resulting in a potentially dangerous situation for the person wearing the garment.  
         [0066]     As a safety measure, the processing section of the sight  10  monitors images received from the image detector  18  for any pixels therein that represent a fluorescent orange color in the scene. If this color is detected, then the processing section  21  superimposes a warning on the image. In this regard,  FIG. 7  is a diagrammatic view of the internal display  32  while the sight  10  is being used to view a scene that includes a person wearing a fluorescent orange garment. In response to detection of the fluorescent orange color, the processing section  21  superimposes a warning  221  over the portion of the image where the fluorescent orange color was detected. In the disclosed embodiment, the warning  221  is a circle with a slash. In order to attract the attention of the user to the warning  221 , the warning can be presented in a color such as red, and/or can be made to blink.  
         [0067]     As discussed above, the warning  221  in the disclosed embodiment is a symbol in the form of a circle with a slash. Alternatively, however, the warning  221  could be some other symbol, an alphanumeric phrase, or a combination of a symbol and an alphanumeric phrase.  
         [0068]     Although one embodiment has been illustrated and described in detail, it will be understood that various substitutions and alterations are possible without departing from the spirit and scope of the present invention, as defined by the following claims.