as referenced above , fig1 shows an optec 360 retinoscope 10 having thumb slide 12 , condensing lens 14 , and lamp 16 . lamp 16 includes a linear filament designed to create the &# 34 ; streak &# 34 ; reflex or reflection seen by the practitioner from the retina of the eye of the patient being examined . slide 12 moves approximately 1 . 6 cm along handle 18 so that , in its upper position , the filament of lamp 16 is approximately 5 . 0 cm from lens 14 , which has power of + 20 . 00 d . in its lower position , therefore , the filament of lamp 16 is approximately 6 . 6 cm from lens 14 . in use , light rays emanating from lamp 16 are reflected by mirror 19 approximately 45 ° into the patient &# 39 ; s eye . the practitioner can view the rays backscattered from the patient &# 39 ; s retina through a small opening 20 in mirror 19 , effectively focusing the backscattered rays into his pupil . in essence , the phoropter or trial frame lenses subsequently placed before the patient are designed to place the patient &# 39 ; s eye in focus with the practitioner &# 39 ; s eye peering through opening 20 . fig3 - 4 illustrate a modified streak retinoscope 22 of the present invention . retinoscope 22 may be a modified optec 360 retinoscope 10 ( fig1 ) or any other suitable device having a displaceable slide 24 or some other means for moving a lamp relative to a lens . as shown in fig3 - 4 , retinoscope 22 includes a potentiometer 28 coupled to slide 24 , providing means for converting displacement of the slide 24 along handle 30 into an electrical resistance . this resistance can in turn be measured by ohmmeter 32 connected to potentiometer 28 and used by a computer 36 or other appropriate mechanism to calculate the optical power necessary to correct a patient &# 39 ; s error . merely by appropriately combining the resistance measured by ohmmeter 32 with the patient &# 39 ; s current prescription using known equations , computer 36 can rapidly and easily determine the optical correction needed for an overrefracted patient . fig4 details the coupling between potentiometer 28 and slide 24 . wire 40 directly attaches slide 24 to the recording wire or contact arm 44 of potentiometer 28 so that , as slide 24 is displaced ( upward or downward ) along handle 30 , contact arm 44 moves in a corresponding manner . accordingly , potentiometer 28 tracks movement of slide 24 , indicating its deviation from a nominal position . those skilled in the art will recognize that other means may be used to sense the position of slide 24 along handle 30 , including mechanisms electrically or optically coupled to slide 24 or uncoupled but otherwise capable of providing the necessary information . a momentary switch 48 or other suitable device may be included as part of computer 36 ( fig3 ), retinoscope 22 , or elsewhere in the circuitry to provide means for indicating the point at which the practitioner determines that a displacement measurement needs to be recorded . to refract a patient &# 39 ; s eye using retinoscope 22 , the practitioner need merely assume a ( fixed ) position a known distance ( e . g . 50 cm ) from the patient . for a patient having a solely spherical error between approximately - 1 . 75 d and + 2 . 75 d , for example , activating retinoscope 22 with slide 24 in its upper position initially provides to the practitioner the streak reflexive image shown in fig5 a . because no astigmatic error is present in this example , neither the width nor intensity of the streak varies as collar or sleeve 50 is rotated ± 90 ° . lowering slide 24 widens the reflected streak ( fig5 b - c ) until it fills the patient &# 39 ; s pupil as illustrated in fig5 d . again , because the patient has no astigmatic error in this example , rotating sleeve 50 diminishes neither the width nor intensity of the streak ( fig5 e ). at this point momentary switch may be depressed , providing computer 36 information concerning the distance slide 24 has been displaced from its upper position . for a patient having a ( solely ) spherical refractive error of - 2 . 00 d , a retinoscope 22 located 50 cm from the patient &# 39 ; s eye , and slide 24 in its upper position , the practitioner will initially view the images of fig5 d - e . accordingly , no further refractive effort is needed and the initial position of slide 24 is immediately converted into an electrical resistance and transmitted to computer 36 . for a patient having a myopic ( solely ) spherical refractive error greater than - 2 . 00 d , the images of fig5 d - e are likely not attainable for working distances of 50 cm or greater . to accommodate these larger spherical errors , the practitioner can place a phoropter or trial frame lens of , for example , between - 3 . 00 d and - 12 . 00 d before the patient ( or use the patient &# 39 ; s existing prescription lens ) and continue lowering slide 24 until the images of fig5 d - e are obtained . again , at that point the practitioner can simply activate computer 36 to record the displacement information obtained through potentiometer 28 . in this case , however , the power of the phoropter , trial frame , or existing prescription lens must be included in the final corrective calculation ( either as a separate input to computer 36 or manually after the displacement information is converted into the refractive error ). alternatively , the practitioner can move toward the patient , decreasing the distance between the retinoscope and eye under examination , until he views the images of fig5 d - e . this decreased working distance must be determined and appropriately factored into the value obtained from computer 36 , however . for a patient having a ( solely ) spherical error greater than ± 12 . 59 d , the images of fig5 d - e are similarly not likely to be obtained at a working distance of 50 cm . the practitioner in such a case can place a phoropter or trial frame lens of , for example , between + 3 . 00 and + 12 . 00 before the patient ( or again use the patient &# 39 ; s existing prescriptive lens ). with this lens in place , the practitioner can continue lowering slide 24 until the images of fig5 d - e are obtained , at which point he can activate computer 36 to record the displacement information obtained through potentiometer 28 . as in connection with the prior example , the power of the phoropter , trial frame , or existing prescription lens must be included in the final corrective calculation . fig5 f - i and 6a - d illustrate reflections viewed for a patient having a cylindrical error in addition to the spherical errors mentioned in examples a - d . in fig5 f - i , the axis of the patient &# 39 ; s spherical error is 180 ° , while in fig6 a - d the axis is 45 °. for the patient having a cylindrical error principally in the 180 ° meridian , the practitioner determines the spherical error in the same way as discussed above . upon rotating sleeve 50 by ± 90 °, however the image of fig5 f is obtained and the angular orientation of the streak ( i . e . 180 °) is noted or estimated by the practitioner . the practitioner again lowers slide 24 ( fig5 g - h ) until the streak fills the pupil ( fig5 i ), at which point computer 36 is utilized to record the displacement of the slide 24 . the noted cylinder axis can then be included with the measurements to produce a final corrective prescription . embodiments of retinoscope 22 can also incorporate lens discs or carriers to permit lenses of other powers to be substituted for or combined with lens 14 . for example , including a disc of spherical lenses in + 0 . 50 d increments capable of being optically aligned with opening 20 would enhance the practitioner &# 39 ; s ability to use retinoscope 22 accurately at any working distance from 0 - 100 cm . incorporating a distance finder into retinoscope 22 would additionally permit electronic measurement of the working distance for input into computer 36 , while electrically or otherwise coupling the lens disc to the computer would allow direct input of the added spherical power into the computer 36 for use in later calculations . other embodiments of retinoscope 22 function opposite the manner described earlier , recording , for example , the distance slide 24 is displaced from its lower position . these embodiments are designed to accommodate the operating principles utilized in some commercial retinoscopes , in which the light rays from lamp 16 are focused at infinity when slide 12 is completed lowered . yet other embodiments of retinoscope 22 contemplate permitting slide 24 to move more than 1 . 6 cm , providing a greater range of dioptric powers available for refraction . the foregoing is provided for purposes of illustrating , explaining , and describing embodiments of the present invention . modifications and adaptations to these embodiments will be apparent to those of ordinary skill in the art and may be made without departing from the scope or spirit of the invention .
techniques and retinoscopic apparatus for measuring or determining a patient &# 39 ; s optical error are disclosed . the techniques include overrefraction and can be performed from a fixed position , avoiding the practitioner &# 39 ; s need to move back and forth relative to the patient &# 39 ; s eye . equipment associated with the apparatus is adapted to record the location of the retinoscope slide during the examination to provide information concerning the optical error present in the patient &# 39 ; s eye .