OPTO 4101: Refraction1 Subjective verification for refraction MSc Mohammed A.M Aljarousha Department of Optometry Faculty of Allied Health Sciences Islamic University-Gaza

Verification for spherical power

Duochrome test

Typical wavelength of colors VIBGYOR Wavelength/ nanometer Color 410 Violet 450 Indigo 550 Blue 570 Green 580 Yellow 610 Orange 660 Red

Duochrome test The index of refraction varies for different wavelengths of light. The index of refraction is greater for short wavelength light rays than for longer ones. Green light is refracted to greater degree than yellow light, which is refracted to a greater degree than red light, and so on. Incoming rays of white light are dispersed, with blue refracted to a greater degree than red. In an emmetropic eye, therefore, yellow light is focused on the retina, green light is focused anterior to the retina, and red light is focused posterior to the retina. In myopic eye, red is closer to the retina, while in hypermetropic eye green is closer. If the red green test is used and the patient is myopic and insufficiently corrected , then the letters on the red side will stand out blacker, clearer and sharper. They will require more minus power for the green to be as distinct as the red.

On the other hand if the same myope is overcorrected and made artificially hypermetropic, then the letter against the green background appear blacker, clearer and sharper. The sphere is adjusted until letters on both sides are of equal quality. Duochrome test is useful only in refining spherical power, contributing nothing to the determination of cylinder power or axis.

2. Trial and error This method is simply adding +0.25Ds and -0.25Ds in a monocular fashion and asking the patient if the vision is better or worse: If the patient report that it is better without any lens, then he is emmetrope, and his refractions is correct. If the patient report that it is better in adding the +0.25Ds, continue adding plus until no further improvement is achieved. Then add the new addition power for his previous prescription. If the patient responds that vision is worse, remove +0.25Ds or prepare to add -0.25Ds; ask the patient if the letters are sharper and clearer or not? If it is better, continue adding minus until no further improvement is achieved, and then add the new addition power for his previous prescription. - This method of verification completely depends on patient’s answers, so the accuracy will be determined according to the patient observations.

3. Pinhole The pinhole lens is a lens that has a hole in the center of lens, this hole has the effect of reducing the width of the bundle of diverging rays (called a ‘‘pencil of light’’) coming from each point on the viewed object. Normally, the full opening of the pupil admits light. It is the improper bending of the outermost rays in that pencil of light which causes refractive errors such as myopia, hyperopia, presbyopia and astigmatism to be noticeable. Pinhole can brings about clearer vision in all these conditions. By blocking these peripheral rays, and only lettering into the eye those rays which pass through the central portion of the pupil, any refractive error in the lens or cornea is not noticed as much. The pupil may be wide open, but only the central portion is receiving light. The improvement in visual acuity can be striking. So if the vision is improved when looking through the pinhole lens, that’s mean the prescription is not accurate, not completely correct and then verification is necessary to be done.

Verification for cylindrical power ‘‘astigmatism’’

1. Astigmatic fan It is used to verify the axis of trial cylinder lens. On looking at such fan if any of the lines are seen more clearly than the others astigmatism must be present. If the vertical line is clear more than the horizontal meridian it is the less emmetropic one (has the defect). (Inverse relation). more clear line = less emmetropic. Less clear line = more emmetropic. A cylinder lens is placed in front of the eye with its axis horizontal will correct the vertical meridian and when the correct glass power is to be worn all the lines of the astigmatic fan appear equally distinct. The cylinder which thus renders the outline of the whole lines equally clear is the amount of astigmatism and the axis of the cylinder is at right angle to the line which was initially seen most clearly (less emmetropic) according to the sign of the cylinder lens.

2. Staenopic slit It is essentially an accessory in the trial set that consists of an elongated pin hole aperture as a slit cut in an opaque disc, used to detect the axis of astigmatism if present. When put before the eye it allows only rays of light in a particular meridian to enter the eye. If the slit is placed horizontal before the eye the vertical meridian is pin holed and produces a point image. After correction of any spherical ammetropia all meridians in 360 degree can be seen equally on rotating the staenopic slit. In presence of a high astigmatism and before correction , the staenopic slit can detect the axis of correcting cylinder if it's properly positioned. The slit is sometime useful in determining the astigmatism when other devises are unsatisfactory. When the image is equally clear in 360 degree in presence of the slit no astigmatism is present , all lines are equally clear.

N.B. The main difference between the stenopic slit and astigmatic fan is the slit of this device detect the less ametropia or the emmtropic meridian (has the defect) and the axis of the correcting cylinder lens is put on this meridian (at zero power). Ametropia= more clear Ametropia= less clear

3. Rotation of dial screw It is simply done by asking the patient to rotate the screw of the trial frame to rotate the cylindrical lens in the trial frame, the patient will rotate the screw until he see the best clear image. At this point the rotation is stopped, and we can determine the accurate axis of the cylindrical lens.

4. Jackson cross cylinder Cross cylinder is composed of a plus cylinder and a minus cylinder of equal power that are ground on a lens, with their axes at right angles to each other. The commonly used cross cylinders are of ± 0.25D and ± 0.5D. All cross cylinder lenses have a spherical equivalent power of zero (Plano). The cross cylinder may be used to refine the patient’s cylinder axis or power after estimation by either objective (retinoscope) or subjective (astigmatic dial) means. To use the cross cylinder to refine the axis of the patient’s eye, the cross cylinder is placed over the trial lens in such a manner that the axes of the cross cylinder straddle the trial lens axis. For example, if the trial lens has a cylinder axis at 90, the cross cylinder is placed so that one axis is at 45 while the other axis is at 135. The cross cylinder is then rotated 180 (flipped) so that the meridian was originally at 45, is now at 135 and vice versa.

If the patient reports equally blurred vision in each of the positions of the cross cylinder, the trial lens axis is correct. If however the vision is better in one position, the trial lens is moved in this direction. When using plus cylinder, the trial lens axis is moved toward the cross cylinder plus axis. When using minus cylinder, the trial lens is moved the cross cylinder minus axis. Next the cross cylinder is repositioned again to straddle the trial lens axis, and the procedure is repeated until the patient reports equally blurred vision in both positions of the cross cylinder.

The cross cylinder may also be used to refine cylindrical power. Let us assume a patient's eye has 1.00 D of astigmatism, and the appropriate sphere is placed before the patient's eye to position the circle of least confusion on the retina. One focal line, such as the horizontal will be 0.50 D anterior to the retina, and the follow focal line, the vertical, will be 0.50 D behind the retina. If we put a 0.25 D cross cylinder in front of the aligned as +0.25X90 and -0.25 X180, each focal line will be move 0.25 a way from the circle of least confusion. Thus the total astigmatism will now measure 1.50 D, and the circle of least confusion enlarges, and the patient will report an increased blur. If however, we now flip the cross cylinder so that it is aligned as +0.25X90 and -0.25 X180, each focal line will move inward 0.25 D. The astigmatic error now measure only 0.50, and since the circle of least confusion decreased in size, the patient reports improved vision.

Accommodation It is the ability of the crystalline lens to change its shape (the converging power) to focus objects of any distance upon the retina.

Mechanism of accommodation Contraction of the ciliary muscle Relaxation of zonule of Zinn Relaxation of tension on the lens capsule Increased curvature of the anterior lens surface Crystalline lens becomes more convex Near objects are focused on the retina

Unaccommodating Eye

Three aging factors of accommodation in presbyopia:- Decrease in the elastic modules of the lens capsule. Increase in the lens substance (size+ density). Flattening of the lens as whole.