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The Phoropter provides an optically additive lens system and an optical Corneal Aligning Device, both essential for a true additive effective power determination.
- The additive lens system refers to the addition of lens powers within the Phoropter.
- The effective power combination of two or more lenses cannot be obtained accurately by simple addition of their individual powers.
- Allowances myst be made which depend in an intricate way on the powers, the curves, the thicknesses, the index of glass, and the air space separating the lenses.
- Accordingly, in the Phoropter, two essential features have been incorporated to ensure the accuracy of the lens prescription.
Specially computed lenses and lens separations such that their designated powers can simply be added together to give effective power of any possible combination.
A means for placing this additive lens power system at a specified distance from the eye.
- When either of these elements is neglected, the corrective accuracy of the lens system is impaired, particularly with regard to combinations of high power lenses.
- The distance at which the spectacle lens is generally worn is considered to be 13.75mm from the apex of the cornea to the ocular surface of the lens.
- With this as the standard, the posterior lens surface of the Phoropter must be placed at a distance of 13.75mm if the Phoropter reading is to be directly applied to spectacle lens power.
- In the Phoropter, this condition is obtained when zero setting of the sight in the Corneal Aligning Device is lined up with the apex of the cornea.
- To establish proper distance between the patient’s eyes and the instrument, adjust the position of the Forehead Rest using the knurled Forehead Rest Knob.
Note: Make certain that the patient’s forehead is resting firmly against the headrest.
- This adjustment will move the patient’s eyes nearer to or or farther from the instrument.
- From the front of the instrument, look into the Corneal Aligning Device. The upper and lower pointers should be in exact alignment with the solid black line visible on the mirror. This is the zero point indicating a 13.75mm distance from the apex of the patient’s cornea. Also visible are three hash marks, each representing 2mm additive distance.
- With the patient’s forehead positioned against the headrest, adjust the headrest to position the apex of the cornea at the zero line (13.75mm from the lenses).
- If, with the headrest retracted, the apex of the cornea appears nasally from the zero line, simply add the distance to 13.75mm (This figure is the total distance from the cornea to the strong sphere, or the vertex distance).
- The scale reading of the Corneal Aligning Device is used with the Correction Factor Table to determine the correction factor for the power reading.
Note: The correction factor is always added to the Phoropter reading as a plus quantity.
If the Phoropter reading is +8.00D and the Corneal Aligning Device scale indicates an additional 4mm, the correction factor according to the table is +0.27. Therefore, the power go the correcting lens is obtained by adding +0.27 to +8.00 diopters, which equals +8.27 diopters, when the spectacle lens is worn at 13.75mm from the cornea.
If the Phoropter reading is -11.50D, and the Corneal Aligning Device indicates an additional 5mm, it is necessary to interpolate to obtain the correction factor. Interpolating between -11.00 and -12.00, the correction factor according to table is +0.62. Therefore, the power of correction lens is obtained by adding +0.62 to -11.50, which equals -10.88 diopters, when the spectacle lens is worn at 13.75mm.
- The tables can also be applied in the case where the spectacle lenses are to be worn at one distance, the test is made at another distance, and neither distance is at 13.75mm.
Note: First, assume the Corneal Aligning Device indicates an additional 4mm; that the spectacle lenses are to be worn at 12mm instead of 13.75mm; and that the Phoroptor reading is +13.00 diopters. In this case, fitting distance of 12mm is subtracted from the refracting distance of 17.75mm (13.75mm plus 4mm), the result being 5.75mm. In the table for plus Phoropter readings at the horizontal row corresponding with +13.00 diopters, the value of 5.75 falls between the 5mm and 6mm columns. By interpolation, the addition is found to be 1.05D. Hence, the power of the spectacle lens should be +13.00 plus +1.05 for a total of +14.05 diopters.
- All tests for distance (static retinoscopy, subjective, photometry) are generally made with the Vergence Levers in the extreme outward position.
- At this setting the lens systems are parallel. (For shorter test distance than 20 feet, compensatory adjustments may be made by moving the levers inward).
- All tests for near (dynamic retinoscopy, amplitude of accommodation, dynamic cross cylinder, positive negative relative accommodation, photometry) are generally made with the Vergence Levers in the inwards (converged) position.
- With a distance PD of 64mm, moving both levers from the extreme inward position converges the instrument apertures for the near test at 16in. At the same time, aperture separation is decreased by 4mm.
- For PD settings greater than 64mm, the instrument apertures are slightly under converged; reducing the PD adjustment by 1mm or less compensates for it.
- For PD settings less than 64mm, the instrument apertures are slightly over converged; this is corrected by slight outward adjustment of the levers.
Note: Do not attempt to fully converge the instrument below 55mm distance PD.
- All sphere powers, plus and minus, can be introduced into the lens aperture in steps of 0.25D by rotation of a single lens dial, (+0.12D sphere in Auxiliary Lens Knobs can be used to refine the spherical correction to 1/8th F steps).
- The operation is simple Rotation of the Weak Sphere Dial downward (i.e clockwise for the left eye, counterclockwise for the right eye) introduces more plus power or less minus.
- Rotation of the Weak Sphere Dial upward introduces more minus power or less plus.
Note: Plus powers are indicated by black numbers; minus powers, by red numbers.
- An automatic pick-up system links the two sphere power dials so that whenever a change in power is required in the Strong Sphere Dial, it is automatically moved by the Weak Sphere Dial. Thus, one may dial completely through the +16.75D to -19.00 range in 0.25D steps by rotation of only the Weak Sphere Dial.
- However, high power may also be introduced quickly and easily by means of the Strong Sphere Control when desired.
- The Strong Sphere Control introduces sphere power in 3.00D steps and can often be used as a time saver.
To obtain a power of +2.75D (starting from zero) the practitioner could add plus power in the quarter diopter steps by rotating the Weak Sphere Dial downward until +2.75D shows on the sphere power scale.
A quicker way: rotate the Strong Sphere Control nasally one index position to introduce a value of +3.00. Rotate Weak Sphere Dial one index upward to reduce power to +2.75D.
To obtain a power of +7.00D (starting at zero) the quickest way: rotated Strong Sphere Control nasally two index positions to introduce a value of +6.00D. Rotate Weak Sphere Dial four index positions downward to increase power to +7.00D.
To obtain a power of -3.5D (starting from zero) the quickest way: rotate Strong Sphere Control temporally one index to introduce a value of -3.00D. Rotate Weak Sphere Dial two index positions upward to increase power to -3.50D.
- In the Cylinder Lens Dials, which are controlled by turning the Cylinder Power Knobbs, the powers range from 0.00 to -6.00D for intrusments containing minus cylinders and from 0.00 to +6.00D for instruments containing plus cylinders.
- Cylinder power can be changed un steps of 0.25D throughout the full range by means of the Cylinder Power Knob
- To increase power, knobs are turned clockwise.
- A pair of 0.12D cylinders in accessory cells permits refinement to 1/8th D steps.
- A appear of 2.00D cylinders in accessory cells extends cylinder power range to 8.00D.
- Large 360° protractors around the Cylinder Axis Knobs mark the position of the axis from 0° to 180° in steps of 5°.
- The Rx axis reading is taken from the scale at the Cylinder Axis Knob.
- The axis scale around the aperture is provided for reference during retinoscopy.
- The Cylinder Axis Knob (concentric with power knob) can be continuously turned clockwise or counterclockwise to set axis of the cylinder in any meridian from 0° to 180°.
- The Auxiliary Lens Dial is controlled by turning the Auxiliary Lens Knob. The Phoropter provides a selection of 10 auxiliary lenses plus two open apertures.
- Beginning at “O” (open aperture) at the top of the scale, the lenses will index into position in the following order as you turn the Auxiliary Lens Knob clockwise:
- Each Rotary Prism Unit (loupe) has a range of 20.
- Paired, prisms give 40 in any base direction.
- The scale is marked in broad divisions of one prism diopter ().
- The Rotary Prism Unit and the Cross Cylinder Unit are attached to the Turret Assembly. The Turret Assembly rotates to locate either the Rotary Prism Unit or the Cross Cylinder Unit in front of the patient’s eye. When testing for prism rotate the Rotary Prism Unit in front of the eye. With the Rotary Prism Unit in front of the patient’s eye, the unit can be oriented to determine base up, base down, base in, base out prism.
- The Finger Roll Knob rotates the prims lenses inside the Rotary Prism Unit and varies the magnitude of prism.
When the Finger Roll Knob id located at either the top or bottom of the Rotary Prism Unit, the prism change will be base in or base out as the Finger Roll Knob is rotated.
When the Finger Roll Knob is located on either the left or right side of the Rotary Prism Unit, the prism change will be base up or base down as the Finger Roll Knob is rotated.
Note: With the Rotary Prism set for introducing base in or base out prism, the arrowhead positioned nasally from 0 denotes base in prism.
- The standard cross cylinder cells supplied are +0.25D. These cells are removable and +_0.37D and +0.5D are available and may be substituted.
- The power of the cross cylinder is engraved on the cell.
- Red dots indicate the minus axis and white dots, the plus axis.
- A thumb-operated Roll Knob provides for rapid “flipping” of cross cylinders.
One of the unique features of the Phoropter is that the Cross Cylinder Unit (loupe) lenses are geared together with the correcting cylinder test lenses so that when a change in axis is made in the latter, a corresponding change will automatically occur in the axis of the cross cylinder lenses. This feature relieves the practitioner of the necessity of manually changing the cross cylinder axis each time correcting the cylinder axis changed.
Note: because most practitioners prefer to check the cylinder axis before checking the cylinder power, the procedure is written in this sequence. If you prefer to check power first, reverse the sequence and perform a final power check after the axis check.
- With the tentative sphere and correcting cylinder (determine by retinoscopy and/or the astigmatic chart) in place, the Cross Cylinder Unit positioned before the aperture of the eye being test.
- The patient fixates on the smallest line of legible letters.
- The Cross Cylinder Unit is in the correct position for axis check when the axis of the Finger Roll Knobs (handles) correspond to the axis of the correcting cylinder and the red and white dots are 45° from the correcting cylinder axis.
- Rotate the Turret Assembly to position the Cross Cylinder Unit in front of the main aperture (Roll Knobs should correspond to the axis of the correcting cylinder).
- If the cross cylinder is not in the correct position (i.e. axis 45° from the correcting cylinder axis), the practitioner need only rotate the Cross Cylinder Unit 45° counterclockwise to detent.
- The axis check test is performed in the usual manner with the cross cylinder lens flipped from position I to position II.
a. If the vision is improved in one position, the minus* correcting cylinder axis is rotated toward the position of the red dots in which vision is improved.
Note: As the correcting cylinder axis is rotated, the cross cylinder axis is automatically rotated a corresponding amount. Hence, the practitioner does not have to manually rotate the cross cylinder the same amount as the correcting cylinder for the subsequent rechecks.
b. Again, recheck for axis following any modification made in the correcting cylinder axis and by following procedures as in (a) unit final end-point is reacher.
c. End-point is reached (i.e correcting cylinder axis is correct) when vision is equally impaired by flipping cross cylinder lens from position I to position II.
- To reach the power check position from axis check, the practitioner merely rotates the unit clockwise to the next detent position. Visual confirmation of the correct power check position finds the white dots, or the red dots and letters “P” (power) on the cross cylinder.
Note: Set Cross Cylinder Unit for power check (“P’s” on cylinder unit should be parallel to correcting cylinder axis).
- Since the cross cylinder is in the axis check position, the practitioner merely rotates the unit 45° clockwise to detent for proper power check positioning. The power check test is performed in the usual manner with the cross cleaner lens flipper from position I to position II.
a. If vision is better with red dots perpendicular to correcting minus* cylinder axis, power of correcting cylinder is increased.
b. IF vision is better with red dots perpendicular to correcting minus* cylinder axis, power of correcting cylinder is reduced.
c. End-point is obtained (i.e. correcting cylinder power is correct) when vision is equally impaired by flipping cross cylinder lens from Position I to Position II.
- At the end of each refraction, the Cross Cylinder Unit should be rotated 45° counterclockwise to the detent so that for the next refraction the cross cylinder axis will be positioned 45° from the correcting cylinder axis (i.e Thumb Roll Knobs parallel to arrows on axis knobs.). This presets the instrument for the axis check during the next refraction.
*When plus, instead of minus, correcting cylinders are used, attention is given to the white dots, instead of the red dots on the cross cylinder lens.