The Aperture Stop Aremac

As was presented in the previous subsection, an aremac, by way of its focus tracking, gives a new purpose to focusable information displays. This embodiment of the aremac is well suited to most situations where there is subject matter of interest against a background of less interest, or there is only the background.

However, subject matter of interest sometimes appears at a variety of depths, in a typical scene, such that there may be some desire for increased depth of field.

One important attribute of previously known viewfinders, as well as other kinds of displays, is the lack of depth of focus control. Although many of these prior devices have some kind of focus control, they lackdepth of focus control.

Accordingly the author proposes an extended depth of focus aremac that is a device that works more like a camera in reverse than a typical display. The reason it is more like a camera in reverse is that it includes a lens having an aperture, iris, and the like, such that there is somedepth-of-field control or extension (i.e., depth-of-focus control). Thus the aremac is endowed typically with two controls:

one for focus, and another for depth of focus. Of course, one, the other, or both of these controls may be preset or automatically controlled so that they don’t need to be adjusted by a user of the aremac.

An important difference between a camera and this aremac is that a camera can have a single lens group that has an aperture stop. In the limit, as the aperture size is reduced, the aperture stop becomes a pinhole camera. However, we cannot do the same for a display or viewfinder. Inserting aperture stops into the optics of viewfinders and headworn displays will cause reduction in field of view, unlike the effects observed in inserting aperture stops into the optics of cameras. This is because cameras collect light passing through a single point, called the center of projection (COP), whereas the aremac must produce rays of light through an external point, namely the eye. In particular, cameras may be modeled as a single point (the COP), whereas displays are better modeled as somewhat planar surfaces (more like a television screen). In order to get an aperture stop effect, we could put an aperture stop at the eye location (perhaps wear an opaque contact lens that had a small transparent hole in it), but this approach is often not convenient or practical.

Therefore one approach to creating an aremac with extended depth of focus is to use a plurality of lens groups. A simple design for such an aremac is illustrated

(c)

Focus controller Vergence controller

Focus and zoom controller Display controller Zoom control signal

Sensor

Lens group

Focuser Diverter Eye

Aremac lens group Aremac Aremac focuser Eyeglass frame

W T

TTW TW W

Real light Virtual light (a)(b)

Focus controller Focus analyzerEye focus sense proc.

IR PS

SLM Backlight Eyefocus measurer Eyefocus diverter Eye Figure3.16AremacdepthtrackingintheEyeTapsystemisachievedbyhavingthearemacandcamerabothfocused togetherbyasinglefocuscontrolinput,eithermanualorautomatic.Solidlinesdenotereallightfromsubjectmatter, anddashedlinesdenotevirtuallightsynthesizedbythearemac.(a)Aremacfocuscontrolledbyautofocuscamera.When thecamerafocusestoinfinity,thearemacfocusessothatitpresentssubjectmatterthatappearsasifitisinfinitely Whenthecamerafocusesclose,thearemacpresentssubjectmatterthatappearstobeatthesameclosedistance. AzoominputcontrolsboththecameraandaremactonegateanyimagemagnificationandthusmaintaintheEyeTap condition.RaysoflightdefiningthewidestfieldofviewaredenotedW.Raysoflightdefiningthenarrowestfieldof aredenotedT(for‘‘Tele’’).Notethatthecameraandaremacfieldsofviewcorrespond.(b)Aremacandcamerafocus bothcontrolledbyeyefocus.Aneyefocusmeasurer(bywayofabeamsplittercalledthe‘‘eyefocusdiverter’’)obtains anapproximateestimateofthefocaldistanceoftheeye.Boththecameraandaremacthenfocustoapproximately samedistance.(c)Focusofrightcameraandbotharemacs(includingvergence)controlledbyautofocuscameraon side.Inatwo-eyedsystemitispreferablethatbothcamerasandbotharemacsfocustothesamedistance.Therefore oneofthecamerasisafocusmaster,andtheothercameraisafocusslave.Alternatively,afocuscombinerisused averagethefocusdistanceofbothcamerasandthenmakethetwocamerasfocusatequaldistance.Thetwoaremacs, aswellasthevergenceofbothsystems,alsotrackthissamedepthplaneasdefinedbycameraautofocus.

85

Eye

Aremac Aperture

Image stop plane

Figure 3.17 Aremac based on aperture stop between reversed petzval lens groups. The use of two lens groups in a viewfinder permit the insertion of an aperture stop in between. This results in a viewfinder having extended depth of focus. As a result no matter at what depth the eye of a user is focusing on, the image plane will appear sharp. The nonshooting eye (the eye not looking through the camera) can focus on subject matter at any distance, and the camera eye (the eye looking into the aremac) can assert its own focus without eyestrain. Clearly, in some sense, the aremac is a nonassertive viewfinder in that it does not impose a strong need for the user’s eye to focus at any particular distance.

0 40 mm

18 mm

f ₌ 8 mm f ₌ 4 mm

910 920

Image plane

Eye

Aperture stop

12 mm 9 mm

Figure 3.18 Aremac with distal aperture stop. As will be seen later in this chapter, an important first step toward making the apparatus covert is moving the aperture stop out from within the optics, and bringing it further back toward the image plane. The dimensions of the preferred embodiment are indicated in millimeters, and the focal lengths of each of the lenses are also so indicated. Again the eye is indicated in dashed lines, since it is not actually part of the aremac apparatus.

in Figure 3.17, where the eye is depicted in dashed lines, since it is not part of the aremac.

The author has found it preferable that the optics be concealed within what appear to be ordinary eyeglasses. The aperture stop, being dark (usually black), tends to be visible in the eyeglasses, and so should best be removed from where it can be seen. Figure 3.18 depicts an embodiment of the aremac invention in which the aperture stop is more distant from the eye, and is located further back toward the display element. In this way the aperture stop is conveniently made as part of the display element housing, which reduces cost and makes the system more easily manufacturable.

A Practical Embodiment of the Aperture Stop Aremac

Folded optical path designs are commonly used in virtual reality displays. A folded optical path may also be used in the device described in this paper (see

Camera

Aperture stop

Eyeglass temple side piece Image plane

Lens

groups Diverter

Eye d

d

Figure 3.19 Aremac with distal aperture stop and folded optical path. Folding the optical path moves elements of the display system out of the way so that most of the elements can be concealed by the temple side piece of the eyeglasses. Moreover the folded design takes most of the aremac out of the way of the user’s view of the world, such that less of the user’s vision is obstructed. A camera that is concealed in the nosebridge of the eyeglasses can look across and obtain light reflected off the backside of the folding optics. The folding is by way of 45 degree mirrors or beamsplitters, the one closer to the eye being called a ‘‘diverter.’’ This diverter diverts light that would otherwise have entered an eye of the user of the device, into the camera. The distance from the eye to the optical center of the diverter is called the ‘‘EyeTap’’

distance, and is denoted by the letterd. This distance is equal to the distance from the camera to the optical center of the diverter. Thus the camera provides an image of rays of light that would otherwise enter an eye of the user of the device. So the effective center of projection of the camera is located in the left eye of the user of the device. Since the camera and aremac can both have depth of focus controls, both can be in focus from four inches to infinity. In this way the apparatus can easily operate over the normal range of focusing for the healthiest of human eyes.

Fig. 3.19), where the folding has an additional purpose, namely one of the folds as the additional function of a diverter to divert light sideways into the eye. The purpose of the diverter is threefold:

• To get the optics, and display element out from in front of the eye and to allow portions of the apparatus to be moved closer to the user’s head. This reduces the moment of inertia, for example, and makes the apparatus more covert and more comfortable to wear.

• To get more of the apparatus out of the user’s field of view so that less of the user’s vision is obstructed.

• To facilitate the insertion of a camera, in such a way that the camera can receive light that would otherwise pass through the center of projection of a lens of an eye of the user of the apparatus.

Ordinarily lenses and other vitreous or vitrionic elements embedded in the front glass portion of eyeglass lenses can be hidden, owing to the similar vitreous appearance of the eyeglass lens and the lens group to be concealed therein.

However, the design can be further optimized so that the lens group closest to

the eye is reduced in size and visibility at the expense of making the further lens group larger and otherwise more visible. Since the further lens group can be concealed at the edge of the eyeglass lens, by the frames of the eyeglasses, such a design provides a very useful trade-off.

An aperture stop, by its nature, is far more difficult to conceal in an eyeglass lens than transparent optics. Aperture stops are generally black. A large black disk, for example, with a small hole in it, would be hard to conceal in the main central portion of an eyeglass lens. Therefore the use of a distal aperture stop helps by getting the aperture stop back far enough that it can be concealed by the frames of the eyeglasses. Preferably the frames of the eyeglasses are black, so that the aperture stop which is also preferably black, along with a housing to prevent light leakage, will all blend in to be concealed within the eyeglass frames.