There are two methods of evaluating the refractive status of an eye: 1. This process relies on the cooperation of the patient. An autorefractor can also be used to obtain an objective refraction accessing tool. Subjective refraction: Subjective refraction consists of three distinct phases.

Author:Nikokinos Gardazragore
Language:English (Spanish)
Published (Last):20 December 2014
PDF File Size:20.91 Mb
ePub File Size:2.25 Mb
Price:Free* [*Free Regsitration Required]

There are two methods of evaluating the refractive status of an eye: 1. This process relies on the cooperation of the patient. An autorefractor can also be used to obtain an objective refraction accessing tool. Subjective refraction: Subjective refraction consists of three distinct phases.

The First Phase: This is designed to correct the spherical element of the refractive error in such a way as to facilitate the accurate determination of any astigmatic element present. The Second Phase: This phase is to determine the astigmatic error. It should be remembered that, although astigmatism is often present, a refractive error may be entirely spherical. Objective Refraction: Objective refraction retinoscopy is often used to determine the initial spherical element of refraction.

The purpose of the first phase of a subjective refraction is to determine the best vision sphere BVS. This can be defined as the most positive or least negative spherical lens that provides best visual acuity. During a subjective refraction, the accommodation must not be allowed to fluctuate randomly.

The eye should be as relaxed as possible so that changes in the accommodative state do not influence the end-result. The ability to discriminate and communicate will of course vary widely from person to person but, in general, the simpler the task, the more likely it will be performed well. Determination of the Best Vision sphere: The following discussion takes place in two parts: the first assumes that retinoscopy has not been performed whereas, in the second, the determination of the BVS follows retinoscopy.

If a good retinoscopy has been performed, the technique of finding the BVS in isolation should, in theory, be redundant. However, it can prove useful when retinoscopy is difficult small pupils or media opacities.

It is usual to start with the right eye, the left being occluded. This is called a monocular refraction. The procedure is repeated on the left eye with the right occluded. However, it is possible and often preferable to refract under binocular conditions. Traditionally, the right eye is usually refracted first, because it is the nearest one to the practitioner in most consulting rooms.

However, when the left eye has significantly worse acuity as a result of amblyopia or pathology, or if the right eye is markedly dominant, the left eye should be refracted first. When using the best sphere technique without the aid of retinoscopy, the practitioner must find the maximum amount of positive power or the minimum amount of negative power that can be tolerated by the eye, without causing blurring of the retinal image.

After occlusion, the first task is to measure the unaided vision. This is useful because unaided vision can give a reasonable estimate of the magnitude of any uncorrected myopia or manifest hypermetropia. The questioning technique is very important throughout subjective refraction because the use of appropriately phrased questions can make the difference between a quick, precise refraction and a long-winded, potentially inaccurate refraction. Whenever a positive lens is held before the eye, the question to the patient should take the following format: Is the target better with or without this lens, or is there no real difference?

Positive lenses either blur the retinal image, indicating that the maximum amount of positive power is already in place, or relax accommodation where it is in use. When the total power of the trial lenses in the trial frame is close to the endpoint; the practitioner should add spherical lens power in 0.

The lens must be held in the plane of the trial frame and along the visual axis to avoid inducing off-axis aberrations. It should also be moved quickly and precisely, allowing enough time in each position for the patient to make a decision. Some patients require longer than others.

The question required when adding negative power should be altered to: Is the target clearer or just darker with this lens? Should the target appear darker but not clearer, extra minus power should not be added because this just stimulates accommodation. Also, extra minus power should not be added if the target appears smaller but not clearer.

A negative lens should be added to the trial frame only if the patient can resolve a greater number of letters on the letter chart. The results are often rechecked and confirmed throughout the test using the same or a different technique, e. When the BVS has been reached the point focus in the case of spherical ametropia or disc of least confusion in the case of astigmatic ametropia should be on or very close to the retina.

The distance vision with this correction should be measured and recorded because it is useful for estimating the magnitude of any uncorrected astigmatic error. Remember that the endpoint is the maximum plus or minimum minus that the patient will tolerate without causing blurring of the retinal image.

Summary of the procedure to find the BVS without the use of retinoscopy 1. Occlude the left eye. Measure the unaided vision. If possible, estimate the ametropia. This is particularly helpful in the case of uncorrected myopia. Also in myopia, the position of the true far point can be used to estimate the refractive error , e. Is the vision worse?

No: add more plus spherical power until the vision blurs. The BVS should be the maximum plus that the eye can tolerate without causing blur on a letter chart. Yes: add minus spherical power until the best line can be resolved. Make sure that each addition actually increases VA and does not just make the letters smaller.

Record the VA. Occlude the right eye and repeat the procedure for the left eye. After retinoscopy, the procedure is as follows: 1.

However, in a patient with small pupils the acuity may be rather better. An occluder is placed before the left eye. The acuity of the right eye may now be checked with the left eye occluded and the right working distance lens still in place.

In older patients the effect of the overcorrection may be less, but it is also less likely that retinoscopy under-plussed the correction if accommodation is inactive. Refinement of the BVS after retinoscopy After retinoscopy the spherical correction may be refined by use of the duochrome or by using plus and minus spheres of equal but opposite power.

The two methods give statistically similar results Jennings and Charman , although this does not mean that they necessarily agree on every patient. This method has been commonly employed in optometers and some autorefractors. It is also employed as the focusing mechanism in the one-position keratometer. Using this technique, the plus lens must be presented first for at least 1 second to relax accommodation.

The minus lens should not be held for more than 1 second, which is the reaction time plus response time for accommodation. If this time is exceeded, it is likely that the patient will accommodate.

The initial comparison should be between more plus and more minus. The third option should be offered only if the patient could not differentiate between the first two. The end-point is the most plus or least minus that does not blur the VA.

If acuity improves, add further minus lenses in 0. The rapidity with which the minus lens must be withdrawn can cause problems when a patient is slow to react. For this reason, many practitioners have modified the Simultan technique to eliminate the minus lens completely. Adding plus only. After initially determining that the sphere is a little not more than 0. Are the letters the same with the lens or worse? The first variant has the disadvantage of being a compound question.

It is necessary to pick the question to suit the patient, and it is sometimes necessary to change the question once the practitioner has got used to the patient. If in doubt, try both variations in succession. With this method, accommodation may be induced when minus power is added to the sphere in the trial frame, but we are always adding plus, and therefore relaxing accommodation, immediately before the comparison is made. Both this technique and the unmodified Simultan method are repeated until the patient accepts no more plus without losing clarity.

At this point the investigation of the astigmatic element of the refractive error can proceed. The use of the pinhole disc: Where there is uncorrected ametropia, a distance point source of light produces a blurred image on the retina composed of a series of blurred discs.

A pinhole may be employed to reduce the diameter of these burred discs and thus improve the VA. The pinhole disc is an opaque disc with a central circular aperture of about 1 mm in diameter. A pinhole with an aperture smaller than 1 mm would cause diffraction effects and also a reduction in retinal illumination. This would result in a dim, unfocused image. An aperture larger than 2 mm approaches the size of some human pupils and so might not significantly reduce the blur circle produced by an uncorrected refractive error.

A diameter of 1. If the pinhole is placed before an uncorrected ametropic eye, the VA should increase. Normally correction of the refractive error should improve the VA by at least as much as that produced by the pinhole.

The pinhole disc can therefore be used to estimate the maximum VA that the eye would obtain if the refractive error were to be corrected. If acuity does not improve through the pinhole, it is unlikely that reduced acuity is caused by an uncorrected refractive error and pathology is suspected, e.

However, if the patient has an irregular cornea or peripheral media opacities, the pinhole may give a better result than can be achieved by refraction. If the pinhole fails to improve VA, the reason for the reduced acuity is unlikely to be purely refractive. In practice, the pinhole disc test can prove very useful, especially if subjective techniques are unsuccessful and VA does not improve with the addition of lenses.

The duochrome test The eye, in common with most optical systems, displays a certain amount of axial chromatic aberration ACA. The refractive indices of the various optical components of the eye vary with the wavelength of the incident light, with light of longer wavelength i. The total amount of ocular chromatic aberration present has been estimated as approximately 2. With white light, this should cause some defocus, although placing an achromatic doublet before the eye does not appear to improve VA significantly.

Chromatic aberration appears to be slightly reduced about 0.


Duochrome test

Add to Wishlist Install This eye test is a light form of Visual Acuity Test and is designed to reveal shortsightedness and long sightedness hereinafter referred to as myopia and hyperopia in the most pleasant, intuitive and stress free way for FREE. Because this test is based on chromatic aberration and not on color discrimination, it is used even with color-blind patients. Description: Myopia and hyperopia are common problems. It can be hereditary, but may also occur because of eye, nerve or brain damage or due to exposure of certain chemicals. But we can usually come across with myopia and hyperopia caused by exhaustion, reading or working from wrong distance, etc. The Duochrome Test is made with the best of intentions. The intuitive and flawless design allows you to navigate easily through the application.


Duochrome tests

Duochrome Purpose: Initial Duochrome test, also known as Bichrome test, consists of two sides of different colours, red and green. Chromatic aberration, the basis of the test, occurs because different wavelengths of light are bent to a different extent. The longer wavelength red is refracted less than the shorter green. Test tries to determine the correcting spherical lens power monocularly first. The duochrome test should be used after monocular refraction and it represents the endpoint procedure of the tested eye.

Related Articles