AU1132997A - Method and apparatus for reducing strain of the eyes - Google Patents

Description

METHODANDAPPARATUSFOR REDUCINGSTRAINOFTHEEYES

The present invention relates to a method for reducing strain of the eyes in persons required to focus on objects at close range and to apparatus which make use of the method.

A human eye's ability to focus on an object is known as accommodation. In simple terms an unaccommodated eye is focussed on distant objects. Conversely, an highly accommodated eye is focussed on a near object. The unit of measure for accommodation is know as the dioptre (D), and is derived from the reciprocal of the distance between the eye and the object as measured in metres. Hence 1 D (dioptre) of accommodation is required for an object at 1 metre, 2D for one at .5m and 3D at .333 metres. Clearly the measure of accommodation increases dramatically as an object approaches the eye. This is the result of the inverse relationship between accommodation and distance.

All humans have a near point which represents the closest distance that an object can be viewed whilst keeping it in focus. For the majority of persons this near point recedes with age (presbyopia). In a fifteen year old this would be around 10D of accommodation whereas a 60 year old might possess only 1D (hence the need for glasses with advancing age in order to bring the near point closer again).

The act of binocular accommodation is effected by muscles within the eye globe (intraocular muscles) and those surrounding the extremities of the eye (extraocular muscles). The predominant intraocular muscle is called the ciliary muscle which includes three types of smooth muscle within the ciliary body.

In simple terms the ciliary muscle is donut shaped with the lens suspended within the centre of the hole by the suspensory ligaments or zonules. Picturing a round trampoline will help visualise the arrangement. The lens is, within itself, under pressure and in its dissected state would like to be near spherical in shape. However when suspended within the relaxed, uncontracted ciliary body, it is pulled flatter and less convex. Contraction of the ciliary muscle itself does not "squeeze" the lens more convex, it simply reduces the size of the donut hole and with it, the tension in the suspensory ligaments, thus permitting the lens to move towards to its preferred, more convex shape.

In order to accommodate, the ciliary muscle must contract with a certain level of force per each dioptre. This unit force per dioptre is defined as the myodioptre, and increases with accommodation.

It has been shown that in youth, less than 10% of the ciliary muscle's capacity is utilised for near work (@3.5D) and that this utilisation grows exponentially to age 45 where 100% of capacity is required for the same level of accommodation. It has also been shown that long term static muscular force cannot be maintained without fatigue if the exerting force exceeds 10-15% of the muscles maximum capacity, and that this applies equally to the intra and extra ocular muscles of the eyes as well as to the rest of the body.

Bearing the last two paragraphs in mind, it follows that sustained observance of an object such as a display screen at say 2D, for individuals of approximately 25 yrs and on, will result in some level of muscle fatigue because they will require >10-15% of ciliary muscle capacity.

One way in which ciliary muscles can be made to relax or in which strain in the muscles can be reduced is to ensure that the eyes receive regular breaks or to avoid sitting close to the object being viewed. However, the latter usually cannot be avoided when the object is a display screen which displays large amounts of visual detail such as a computer video display unit whereas the former may be impractical to implement regularly in an intensive work place environment.

The present invention therefore is directed to addressing the problem of reducing eye strain when viewing objects at close range especially when regular breaks from viewing the objects cannot or cannot easily be implemented. In addressing this problem the present applicant has recognized that if it is not possible to exercise the eyes by shifting focus away from the object being viewed, then the next best thing may be to exercise the eyes whilst they remain focused on the object being viewed. As previously noted the force exerted by the ciliary muscles increases with accomodation. Thus as an object moves closer to the eye, tension increases. Conversely as the object moves further from the eye the tension is reduced.

Experiments carried out by the applicant appear to support the proposition that the incidence of muscle fatigue is reduced in circumstances where the tension in the ciliary muscles does not remain static but is allowed to vary. This is thought to occur because when muscles are held contracted, supply of blood to the muscles is restricted, thus reducing nutrients to and removal of wastes from the muscles. Varying tension in the muscles improves supply of blood leading to reduction in fatigue due to improved supply of nutrients to and removal of wastes from the muscles.

Research since the sixties has shown a steady increase in the incidence of a condition known as Myopia (shortsightedness) in western communities from 6% to 40%. Major studies in both Taiwan and the USA confirm these findings and as a result of comparing different environments of subjects (eg. islanders & outdoor workers vs office workers/students) it is commonly believed that the increased incidence of myopia is strongly related to increasing "near work" demands of the modern workplace.

To add weight to this, research which examines different individuals "dark focus", that is, their focal length at rest (in the dark for there is nothing to focus upon) have shown that this distance reduces after periods of sustained near work. The same has also been found to hold for the extraocular muscles and this is known as the "dark convergence".

Furthermore, in some of these studies it has been suggested and qualitatively shown that a dynamic object by comparison, when viewed has a negligible impact upon the "dark focus and convergence" and thus the contention that viewing a dynamic object may have a positive effect upon reducing myopia and/or its possible onset.

According to one aspect of the present invention there is provided a method for reducing fatigue and/or strain in the eyes of a person viewing an object at relatively close range, the method including the step of automatically repositioning the object relative to the eyes at least periodically whereby causing a shift in focus of the eyes and a corresponding shift in tension of muscles associated with the eyes.

In one form the object may be repositioned continuously such as in a cyclic fashion between a first position relative to the eyes and a second position which is sufficiently different to the first position to cause a shift in focus of the eyes. The step of repositioning the object may include changing the apparent position of the object as distinct from the actual position of said object. In some embodiments this may be achieved without physically moving the object but rather by changing the path length of light travelling between the object and the eyes. In some embodiments this may be achieved by means of movable reflecting surfaces such as mirrors.

According to a further aspect of the present invention there is provided apparatus for reducing fatigue and/or strain in the eyes of a person viewing an object at relatively close range, said apparatus including means for automatically repositioning the object relative to the eyes at least periodically whereby causing a shift in the focus of the eyes and a corresponding shift in tension of muscles associated with the eyes.

The apparatus may include means for continuously repositioning the object such as in a cyclic fashion between a first position relative to the eyes and a second position which is sufficiently different to the first position to cause a shift in focus of the eyes.

To minimize deviations from the optimum viewing position the first and second positions may be chosen such that the vector average of the two positions coincides substantially with a preferred viewing position for the object. In one form the first and second positions may be substantially colinear with the eyes of the person viewing the object. Where an optimum viewing position is approximately 550mm from the eyes, the first and second positions may be substantially 475mm and 625mm respectively in one embodiment.

The apparatus of the present invention may include means such as a platform for supporting an object to be viewed by a person. The apparatus may include means for repositioning the supporting means at least periodically. In one form the repositioning means may include means for moving the supporting means substantially uniformly between first and second positions. Movement between the first and second positions may be continuous or it may be periodic, ie. it may be punctuated with static periods. Movement between the first and second positions may take place at any suitable speed. The speed of movement may be user adjustable. In one form the speed may be adjustable between 0 - 15mm/second approx.

In another form of the present invention the repositioning means may include means for moving the supporting means such that the rate of change of force exerted by the ciliary muscles is substantially constant. It may be shown that the radial force (F) of contraction exerted by the ciliary muscles is

D \ 2

F =

K where D is dioptres and K is an age dependent constant. The dependence of K on age may be represented by the expression K = a - bA + CA² where A = age in years, a = 0.675, b = 0.2 and c = 0.000147. Because D = 1/X where X is the displacement from the eye in meters, the force when expressed as a function of displacement is equal to:

F = (1)

K² X²

The rate of change of force will be a constant C dF dF dX if = C (2) dt dX dt

where V is the speed of the supporting means.

dF is obtained by taking the derivative of equation (1) dX dF (4)

2 „3 dX K ' X Hence the speed (V) of the supporting means expressed as a function of displacement (X) is equal to:

2 3

V = ~^{CK X} (5)

2

The acceleration (a) of the supporting means expressed as a function of displacement is equal to _{a =} _dV_ ₌ _dV dX_ dV_ dt dX ^* dt ^" dX ⁽⁶⁾

dV is obtained by taking the derivative of equation (5) dX

dV_ = 3CK²X² (7) dX

combining equations (5), (6) and (7)

a = -3CK²X² . C K² X³ = 3C²K⁴X⁵ 2 -2 4

The above calculations show that the rate of change of force exerted by the ciliary muscles will be substantially constant if the speed (V) and acceleration (a) of the supporting means are made proportional to the third and fifth powers respectively of the displacement (X) of the supporting means.

The supporting means may be adapted to support any object or article which is to be focussed on at close range for long periods at a time. In one form the supporting means may be adapted to support a video display unit associated with a personal computer. In another form the supporting means may be adapted to support printed matter such as a text book.

In an alternative embodiment of the present invention the apparatus may include means for altering the apparent position as distinct from the actual position of an object viewed by a person. The altering means may include one or more light reflecting means. The light reflecting means may include one or more reflecting surfaces such as mirrors. In one form the altering means may include a pair of offset parallel mirrors not unlike the mirrors in a periscope. The altering means may be interposed to intercept the line of sight of the person viewing the object. A first mirror may be positioned to reflect an image of the object to a second mirror. The second mirror may be positioned to reflect the image of the object from the first mirror into the eyes of the person viewing the object. At least one mirror may be movable so as to alter or change the path length of light travelling between the object and the eyes. Where the light reflecting means includes one or an odd number of mirrors the image of the object may be reversed from left to right. For example where the object includes a video display monitor the direction of the scan or raster of the monitor may be reversed to reverse the displayed image.

Preferred embodiments of the present invention will now be described with reference to the accompanying drawings wherein: Fig. 1 shows a perspective view of apparatus according to one embodiment of the present invention;

Fig. 2 shows a perspective view of a modified form of apparatus;

Fig. 3 shows the apparatus of Fig. 1 or 2 with the cover plate removed;

Fig. 4 shows a schematic diagram of a limit sensor circuit associated with the apparatus of Figs. 1 and 2;

Fig. 5 shows a schematic diagram of a drive circuit associated with the apparatus of Figs. 1 and 2;

Fig. 6 shows a representation of apparatus according to another embodiment of the present invention; Figs. 7a, 7b and 7c show examples of speed versus distance plots;

Fig. 8 shows a schematic diagram of a drive circuit associated with the apparatus of Fig. 6;

Figs. 9(a) to 9(d) show flow diagrams associated with the drive circuit of Fig. 8; and Fig. 10 shows a conceptual diagram of apparatus according to a further embodiment of the present invention. The apparatus of Fig. 1 includes a base 10 and a platform 11 movably mounted thereon. Platform 11 is adapted to reciprocate or move back and forth along base 10 between preset outer limits. The outer limits may be defined via optical means as described below. The apparatus includes an on/off switch 13 for activating operation of the apparatus and a variable control element 14 for adjusting the speed of movement of platform 11. The apparatus includes a power cord 15 including plug 16 correcting the apparatus to a source of mains power. Plug 16 is adapted to be received by a (switched) power outlet socket associated with a personal computer or the like. Platform 11 includes a bearing surface 12 for receiving an object or article which is to be focussed on at close range, such as a computer video display unit or monitor or a book stand or the like.

Fig. 2 shows a modified bearing surface 20 associated with platform 11 which includes a partly spherical socket 22 adapted to receive a ball shaped base associated with the monitor of a typical personal computer.

Fig. 3 shows the apparatus of Fig. 1 disassembled to reveal the driving mechanism associated with the apparatus. The platform of the apparatus includes a lower frame 30 and cover 31. Frame 30 has mounted near its corners four wheels 32, 33, 34, 35 for rollably supporting the platform upon base 36. Base 36 includes grooves 37, 38, 39, 40 for cooperating with wheels 32, 33, 34, 35 respectively. Grooves 37-40 are profiled to provide guidance for wheels 32- 35. The platform is driven via wheels 32, 33. Wheels 32, 33 are drivably coupled to axle 41. An electric D.C. motor 42 is drivably coupled to axle 41 via gear wheels 43, 44. Gear wheel 43, is mounted to the rotor of motor 42 and gear wheel 44 is mounted to axle 41. Electric motor 42 is driven by a control unit 45 mounted underneath cover 31. Control unit 45 includes optical sensors 46, 47. Sensor 46, 47 are positioned above openings 48, 49 respectively in frame 30 and are adapted to sense changes in contrast in the surface of base 36.

Base 36 includes reflecting zones 50, 51 defining end limits for movement of the platform relative to base 36. The control unit 45 is adapted to reverse the direction of rotation of motor 42 when sensor 46 detects light from zone 51 and again when sensor 47 detects light from zone 50. Details and operation of control unit 45 are described below with reference to Figs. 4 and 5.

Fig. 4 shows a limit sensor circuit including latching flip flop 52 configured via NOR gates 53, 54. NOR gates 53, 54 may comprise an integrated circuit type 7402. Flip flop 52 is driven into either of its stable states via opto circuits 55,56 respectively. Opto circuit 55 includes op amp 57 which may comprise an integrated circuit type LM311. Input terminals 2, 3 of op amp 57 are fed via photo diode D1 of opto sensor 46. The sensitivity of photodiode D1 is controlled via variable gain resistor VR1. Opto sensor 46 includes light emitting diode LED1. LED1 and photo diode D1 are oriented so that when opto sensor 46 is above reflecting zone 51 light from LED1 is reflected from zone 51 into photodiode D1. This causes photodiode D1 to conduct and the output of op amp 57 rises. A high on the output of op amp 57 causes a corresponding high at input 8 of flip flop 52 and a low at output terminal A of flip flop 52. When the output at terminal A is low, the output at terminal B will be high and vice versa.

The configuration and operation of opto circuit 56 is similar to that of opto circuit 55, although photodiode D2 and light emitting diode LED2 of opto sensor 47 are oriented so that when opto sensor 47 is above reflecting zone 50 light from LED2 is reflected from zone 50 into photodiode D2. The latter causes photodiode D2 to conduct and the output of op amp 58 rises causing a corresponding high at input 12 of flip flop 52 and a low at output terminal B of flip flop 52.

Fig. 5 shows a variable drive circuit including transistors Q1-Q6, diodes D3,D4 and dc motor 42. When input C which is connected to output A of flip flop 52 is high, transistors Q2 and Q4 conduct pulling low the positive terminal of motor 42. This causes diode D3 and transistor Q5 to conduct and current flows through motor 42 from its negative to its positive terminals. Motor 42 then runs in its reverse direction.

When input D which is connected to output B of flip flop 52 is high, transistors Q1 and Q6 conduct pulling low the negative terminal of motor 42. This causes diode D4 and transistor Q3 to conduct and current flows through motor 42 from its positive to its negative terminals. Motor 42 then runs in its forward direction. The speed of motor 42 is controlled via voltage regulator 59 including variable resistor VR2.

Regulator 59 may be an integrated circuit type LM350. Transistors Q1.Q2 may be BC 3104's, transistors Q3, Q5 may be TIP32's and transistors Q4.Q6 may be TIP 31 's. Diodes D3.D4 may be IN4004's. Motor 42 may be a model GB 3535D manufactured by the Densitron Coφoration.

The variable drive circuit of Fig. 5 may be adapted to displace platform 11 such that the rate of change of force exerted by the ciliary muscle is substantially constant. This may be done by replacing voltage regulator 5a and/or variable resistor VR2 via a digitally controlled potentiometer such as an EXICOR device type X9314. The latter may be programmed with resistance tap settings based on values of speed (V) and displacement (X) calculated from equation (5). A transducer for measuring displacement (not shown) may be mounted on or otherwise associated with platform 11. The displacement transducer may provide displacement data for controlling the position of a wiper element of the digitally controlled potentiometer. Because absolute values of velocity are not critical to operation of the present invention the speed/displacement relationship expressed in equation (5) may be normalized by a suitable choice of constant (C) to provide a desired terminal velocity for platform 11 , eg. 10mm. per second when the displacement (X) is 625mm. The latter will be obtained with an initial velocity of approximately 4.4mm per second when the displacement (X) is 475mm. The drive circuit may be made more accurate by utilizing a control system with a feedback loop. The feedback loop may utilize displacement (X), velocity (V) and/or acceleration (a) variables to control the displacement of platform 11. Fig. 6 shows an alternative mechanism for driving the apparatus of Fig. 1 or Fig. 2. The driving mechanism of Fig. 6 includes a rack 60 which may be attached to or formed with base 10. A gear wheel 61 is arranged to mesh with rack 60. Gear wheel 61 is rotatably mounted on platform 11 and is driven via a DC motor 62. A gearbox (not shown) may be interposed between motor 62 and gear wheel 61. The mechanism of Fig. 6 includes control module 63 for controlling movement of motor 62. Control module 63 includes processing means such as a microprocessor or microcontroller for controlling the speed of motor 62. The speed of motor 62 may be controlled according to a desired speed vs. distance profile. The speed vs. distance profile may in a particularly preferred embodiment be determined according to equation (5). However, in some embodiments a different speed profile such as a substantially uniform speed may be adopted. The desired speed profile may be stored in a memory associated with the processing means. Distance information may be provided to control module 63 via slotted wheel 64 and associated sensor 65 and via reference sensor 66.

Sensor 66 is mounted to mark the start (or end) of travel of platform 11 and provides a known reference point relative to which a calibrated platform position can be found. Slotted wheel 64 is mounted directly to the shaft of motor 62. A typical slotted wheel with a diameter of approximately 15mm may have between 30 to 50 slots. As wheel 64 rotates a beam of light associated with sensor 65 is alternately transmitted and blocked by the slots in wheel 64 to an associated receiver. Software associated with control module 63 monitors the status of sensors 65, 66 to determine a current location of platform 11. In a simple design example the circumference of slotted wheel 64 may be set to 50mm with 50 slots. This achieves a resolution of 1 mm distance from reference per slot.

The speed profile may be determined by a number of factors including distance from the reference point, mode of operation and user adjustments (eg. age, distance, average speed etc.). One or more speed profiles may be precalculated and stored in memory in the form of a look up table. Fig. 7a shows a typical speed vs. distance profile over 150mm travel distance of platform 11. The speed vs. distance information in Fig. 7a can be transferred to a look up table. The software associated with control module 63 may read the information stored in the table and set the correct speed at the required distance. Because memory space is limited the speed information may be plotted for a number of sample points along the distance scale. Typically 30 sample points is adequate to reflect the desired speed profile. Fig. 7b shows 30 sample points plotted at 5mm increments. Because in the preferred embodiment speed increases exponentially with distance, the speed gap between successive distance points increases quite rapidly towards the end of the distance scale. Such a stepped effect may be unacceptable to the user. To provide a smoother transition in speed between adjacent distance points the same 30 sample points can be repositioned to minimize the size of the steps between adjacent speed zones. Fig. 7c shows the same 30 points at varying distances from the reference. The actual sample points may be optimized according to well known mathematical principles and will not be discussed herein.

Fig. 8 shows a schematic diagram of the control module 63. The control module includes microcontroller 80. Microcontroller 80 may be a single chip device such as a circuit type Z86E30. A non-volatile memory (EEPROM) 81 is connected to microcontroller 80 for storing system configuration parameters such as average motor speed, mode of operation and age of user.

System configuration data is inputted to microcontroller 80 via button switches S1 - S4. Switches S1 to S4 perform mode selection, up, down and adjustment functions respectively. An LCD display panel 82 is adapted to display data associated with the operation of the control module.

Microcontroller 80 includes an internal memory (RAM) for storing operation program code and speed data either in the form of a look up table or as an analog function. Microcontroller 80 controls the speed of a 12 volt DC motor 83 by modulating the duty cycle of pulses generated at a fixed frequency. This arrangement allows motor 83 to achieve maximum torque. The speed controlling pulses are generated on lines 1 , 27 of microcontroller 80 and are amplified via buffer transistors T5, T6 respectively. The speed and direction of motor 83 is controlled via a H-bridge circuit including transistors T1 - T4. When motor 83 is to be driven in a forward direction microcontroller 80 activates transistor T4 via line 26. This pulls the collector of transistor T4 to a low value and allows current to flow from transistor T1 , via the motor terminals marked (+) (-) and via transistor T4 to ground. It is to be noted that the direction of current flow is from the (+) to the (-) terminal of motor 83. When motor 83 is to be driven in a reverse direction microcontroller 80 activates transistor T3 via line 28. This pulls the collector of transistor T3 to a low value and allows current to flow from transistor T2 via the motor terminals and via transistor T3 to ground. It is to be noted that the direction of current flow is from the (-) to the (+) terminal of motor 83. The speed of the motor in the forward and reverse directions is determined by the duty cycle of pulses passing through the bases of transistors T1 and T2 respectively.

Diode pairs D2, D3 are provided to flatten back emf spikes caused by the inductive load associated with motor 83. Motor position feedback to microcontroller 80 is provided via opto coupler

84 comprising LED 85 and photo transistor 86. A beam of light passing between LED 85 and photo transistor 86 is interrupted via slotted wheel 64 as described above. As slotted wheel 64 rotates the beam of light is alternately transmitted and blocked by the slots in the wheel. The operating software can interpret these signals to determine the distance travelled by the platform.

Start position feed back to microcontroller 80 is provided via optocoupler 87 comprising LED 88 and photo transistor 89. A mask (not shown) is arranged to block passage of light between LED 88 and photo transistor 89 when the platform has reached the end (or start) of its travel. The operating software can interpret this signal for internal direction sequencing and platform reference calibration.

The control module includes a power supply shown generally at 90. Power supply 90 includes IC regulators 91, 92 for regulating the voltage supply to motor 83 and microcontroller 80 respectively. At power up the software may be initialized and the previous user settings retrieved from the non-volatile memory 81. As the position of the platform may be unknown, the platform may be moved until the reference location is found.

The control module may have two modes of operation namely a constant mode and a variable mode. In constant mode the speed of the motor (and platform) may be substantially uniform throughout its entire travel. In variable mode the speed of the motor (and platform) may be varied according to a speed vs. distance profile stored in the RAM of microcontroller 80 eg. a profile as dictated by equation (5). The variable mode may include refinements to system configuration parameters such as the ability to adjust the speed v. distance profile to take account of the age of the user and/or the distance of the user from the platform. Other system configuration parameters which may be user modified to suit personal comfort levels include average speed of the platform. Modifications to system configuration parameters may be performed by adjusting the initial speed and/or the values of coefficients C and K in equation (5).

The operating software may be broken down into several sub-tasks for each function. Basic operation may be achieved by sequencing through a task list. Each task on the list may service a specific function in the control module with overall operation being controlled by a procedure task. The procedure task may set absolute speed of motor 83 from precalculated look up tables stored in the RAM memory of microcomputer 80. Depending on the mode of operation the software may use the look up table and settings. As the platform moves from the reference point the speed may be read from the table and the required motor speed updated instantaneously. Precision interrupt routines may set pulse width modulation (PWM) duty cycle to achieve the required motor speed.

The following steps 93 to 98 refer to the main task flow diagram shown in Fig. 9(a). Step 93-lnitialize CPU

At power up the software may initialise the microcontroller's ports, timers and variables. The previous users settings may be retrieved from non volatile memory. As the position of the monitor is unknown, a flag may be set to inform the procedure task to move the monitor until the reference location is found.

Step 94-Button task

The button task may monitor the input buttons. If a button is pressed, this task may inform the procedure task the button type and transition (on or off). All button debounce periods may be handled in the button task. Step 95-Buzzer task

The buzzer task may operate the buzzer output. This task may be informed by other tasks to sound for a period of time. The buzzer task may operate the buzzer for the required period then turn off. Step 96-Over current task The overcurrent task may monitor the motors level of current. If the current rises above a defined level for a extended period of time, the task may inform the procedure tasks for corrective action. Step 97-Reference point task

The reference point task may monitor the reference point optical sensor input. If end of travel is detected for a short period of time this task may inform the procedure task for corrective action. Step 98-Procedure task

The procedure task may control overall operation of the control module. Information may be gathered from the preceding tasks and used to determine the mode of operation.

Figs 9b to 9d are flow diagrams showing the interrupt routines for motor PWM frequency, motor PWM period and motor distance respectively.

Fig. 10 shows an embodiment of the present invention in which the apparent position as distinct from the actual position of a video display monitor is altered or repositioned. In Fig. 10 a person 100 is viewing screen 101 of video display monitor 102 via mirrors 103,104. The apparent path length between the eyes of person 100 and screen 101 is the sum of path lengths P1 , P2, P3. Path length P2 may be increased or decreased via a lifting/lowering mechanism (not shown) attached to mirror 104 and/or monitor 102. Where a lifting/lowering mechanism is applied to both mirror 104 and monitor 102, it should be arranged to move mirror 104 and monitor 102 (vertically) in opposite directions, ie. away and towards each other. The purpose of the lifting/lowering mechanism is to vary, preferably is a cyclic fashion, path length P2.

In an alternative arrangement mirror 104 may be dispensed with and monitor 102 and mirror 103 may be rotated clockwise through 90° so that screen 101 is substantially horizontal. In the latter arrangement path length P3 may be varied by lifting/lowering monitor 102 and/or mirror 103.

Path lengths P2, P3 may be varied by means of a drive circuit as shown in Fig. 5 or Fig. 8.

Finally, it is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention.

Claims (29)

1. A method for reducing fatigue and/or strain in the eyes of a person viewing an object at relatively close range, the method including the step of automatically repositioning the object relative to the eyes at least periodically whereby causing a shift in focus of the eyes and a corresponding shift in tension of muscles associated with the eyes.

2. A method according to Claim 1 wherein said step of repositioning includes moving the object in a cyclic fashion between a first position and a second position, said second position being sufficiently different to the first position to cause a shift in focus of the eyes.

3. A method according to Claim 1 or 2 wherein said repositioning is performed at a substantially uniform speed.

4. A method according to Claim 3 wherein said speed is user adjustable between 0 to 15mm/second .

5. A method according to Claim 1 or 2 wherein said step of repositioning is performed such that the shift in tension of said muscles is substantially constant.

6. A method according to Claim 1 , 2 or 5 wherein said repositioning is performed at a speed which varies with displacement of said object from a reference position.

7. A method according to Claim 6 wherein said speed is user adjustable.

8. A method according to Claim 7 wherein said speed is adjustable to vary the average speed of said object.

9. A method according to Claim 7 wherein said speed is adjustable to take into account the age of the user.

10. A method according to Claim 7 wherein said speed is adjustable to take into account the average distance of the user from the object.

11. A method according to any one of Claims 6 to 10 wherein said speed is proportional to the third power of said displacement.

12. A method according to any one of Claims 6 to 11 wherein the rate of change of said speed is proportional to the fifth power of said displacement.

13. A method according to any one of the preceding claims wherein said step of repositioning includes repositioning an image of said object.

14. Apparatus for reducing fatigue and/or strain in the eyes of a person viewing an object at relatively close range, said apparatus including means for automatically repositioning the object relative to the eyes at least periodically whereby causing a shift in the focus of the eyes and a corresponding shift in tension of muscles associated with the eyes.

15. Apparatus according to Claim 14 wherein said means for repositioning includes means for moving said object between a first position and a second position, said second position being sufficiently different to the first position to cause a shift in focus of the eyes.

16. Apparatus according to Claim 14 or 15 wherein said means for repositioning repositions said object at a substantially uniform speed.

17. Apparatus according to Claim 16 wherein said speed is user adjustable between 0 to 15mm/second.

18. Apparatus according to Claim 14 or 15 wherein said means for repositioning repositions said object such that the shift in tension of said muscles is substantially constant.

19. Apparatus according to Claim 14, 15 or 18 wherein said means for repositioning repositions said object at a speed which varies with displacement of said object from a reference position.

20. Apparatus according to Claim 19 wherein said speed is user adjustable.

21. Apparatus according to Claim 20 wherein said speed is adjustable to vary the average speed of said object.

22. Apparatus according to Claim 20 wherein said speed is adjustable to take into account the age of the user.

23. Apparatus according to Claim 20 wherein said speed is adjustable to take into account the average distance of the user from the object.

24. Apparatus according to any one of Claims 19 to 23 wherein said speed is proportional to the third power of said displacement.

25. Apparatus according to any one of Claims 19 to 24 wherein the rate of change of said speed is proportional to the fifth power of said displacement.

26. Apparatus according to any one of claims 14 to 25 wherein said means for repositioning includes a platform for supporting said object, said platform being mounted to a base for reciprocating movement thereon.

27. Apparatus according to any one of claims 14 to 25 wherein said means for repositioning includes means for repositioning an image of said object.

28. A method for reducing fatigue and/or strain in the eyes of a person substantially as herein described with reference to the accompanying drawings.

29. Apparatus for reducing fatigue and/or strain in the eyes of a person substantially as herein described with reference to the accompanying drawings.