Monday, May 22, 2006



a. Lens Clock = Lens Gauge = Geneva Lens Measure (Figure 41)

Click on image to enlarge.

The lens measure, lens clock, or lens gauge has two fixed pins on the outside and in the center, a spring-loaded, movable pin. This device physically measures the sagital depth of a refracting surface and calculates the refracting power of the surface. A pointer that is activated by a system of gears indicates the position of the movable pin in relation to the fixed pins. If the instrument is placed on a flat surface, the protrusion of the central pin is equal to that of the fixed pins, with the result that the scale reading is zero. If placed on a convex surface, the protrusion of the central pin is less than that of the fixed pins, but if placed on a concave surface, the protrusion of the central pin is greater. Because the chord length (the distance between the two outer pins) has a constant value for the instrument, the position of the central pin, indicates the sagitta of the surface, which provides a direct reading of diopters of refracting power of a surface of the lens.

  • The lens clock physically measures the sagital height/depth.
  • The reading is in power (diopters)
  • The lens clock assumes that n is in air and n’ = 1.53 (crown glass)

To calculate for the lens radius (assumes that s is very small) r = y2/2s (see diagram)

To calculate true power of a single refracting surface (SRS)

F true = F lens clock (n’ true – n)/(n’ lens clock – n)

Question: What is a Geneva lens clock?

Answer: A device used to determine the base curve of the back surface of a spectacle lens. It is often used clinically to detect plus cylinder spectacle lenses in an individual who is use to minus cylinder lenses. It is specifically calibrated for the refractive index of crown glass (n = 1.53). Special lens clocks are available for plastic lenses.

Question: A lens clock measures the power of a high index plastic lens (n=-1.66) to be –5.00 diopters. Has the lens clock overestimated, underestimated or accurately determined the power of the lens?

Answer: The lens clock has underestimated the power of the surface.

Question: A lens clock is used to measure the power of a SRS where n = 1.00 and n’ = 1.60.

  • What is the true power of the SRS if the lens clock reads –10.00D?

F true = F lens clock (n’ true – n)/(n’ lens clock – n) = -10.00D (1.6-1)/(1.53-1) = -10.00 (.6/.53) = -10.00D (1.132) = -11.32D

  • How much error was induced by the lens clock? -11.32 – (-10.00) = -1.32D

Question: A lens clock is used to measure the power of a SRS where n = 1.00 and n’ = 1.498.

  • What is the true power of the SRS if the lens clock reads –10.00D?

F true = F lens clock (n’ true – n)/(n’ lens clock – n) = -10.00D (1.498-1)/(1.53-1) = -10.00 (.498/.53) = -10.00D (0.939) = -9.39D

  • How much error was induced by the lens clock? -9.39 – (-10.00) = +0.61D

From these examples, you see that for lenses made with indexes of refraction greater than crown glass, the lens clock will underestimate the true lens power and for those lenses with indexes of refraction less than crown glass, the lens clock will overestimate the true lens power.

b. Lensometer

The lensometer measures the vertex power of the lens. The vertex power is the reciprocal of the distance between the back surface of the lens and its secondary focal point. This is also known as the back focal length. For this reason, a lensometer does not really measure the focal length of a lens. The true focal lengths are measured from the principal planes, not from the lens surface. The lensometer works on the Badel principle with the addition of an astronomical telescope for precise detection of parallel rays at neutralization. The Badel principal is Knapp’s law applied to lensometers.

A lensometer is really an optical bench consisting of an illuminated moveable target, a powerful fixed lens, and a telescopic eyepiece focused at infinity. The key element is the field lens that is fixed in place so that its focal point is on the back surface of the lens being analyzed. A lensometer measures the back vertex power of the spectacle lens. However, when measuring a bifocal addition, the spectacles must be turned around in the lensometer so that the front vertex power is measured. This is because the distance portion of the spectacle lenses is designed to deal with essentially parallel light. However, the bifocal addition is designed to work on diverging light, originating from a standard working distance of 40 centimeters. This diverging light from the near object is made parallel by the bifocal lens. The parallel light then enters the distance lens where it is refracted with the expected optical affect to give the patient clear vision. In this way, the bifocal exerts its effect on the light from the object before it passes through the rest of the lens. For strong bifocal corrections, there would be a significant difference in the bifocal strength measurement when using the front versus back vertex measurement.

Question: To measure the power of spectacles in a lensometer, when do you want the temples towards you and when do you want them away from you?

Answer: The distance correction is measured with the temples facing away from you (back/posterior vertex power). The bifocal power is measured with the temples pointing towards you (front/anterior with the vertex power). This is particularly important when checking the prescription of an individual with corrections > +4.00. Between 4.00D and 8.00D, there is approximately 0.25D difference between the fabricated add and the effective add. For lenses between 8.00D and 12.00D, the disparity is approximately 0.50D. For plus lenses, the effective add is always greater than the fabricated add, and for minus lenses it is just the opposite. In this case, you must measure the top of the lens with the temples away from you, then the bifocal segment with the temples towards you, in the opposite direction), to get an accurate reading of the bifocal power.

c. Ophthalmoscopes

Direct – image is upright. Magnification is based on the total refractive power of the eye. Using the basic magnification formula of M = F/4, an emmetropic eye of +60.00D would provide +60/4 = +15X. An aphakic eye of +40.00D would provide +40/4 = +10X.

Indirect – the image of the fundus becomes the object of the condensing lens, which then forms an aerial image that is larger and inverted. The two plus lenses (the eye and the condensing lens) determine the magnification of the aerial image. For the emmetropic eye, using the formula MA = (-)DEye/Condensing lens= (-)60/D(condensing lens), we find that a 20D condensing lens results in (-)60/20 = -3X.

As the power of the condensing lens decreases, the magnification increases. Axial magnification increases exponentially, based on the formula Axial magnification: MA = (M)2.

d. Keratometer

Instrument used to measure the curvature/refractive power of the cornea. It accomplishes this by measuring the radius of curvature of the central cornea. The central cornea can be thought of as a high powered (~-250D) convex spherical mirror.

Question: How do you compute the anticipated astigmatic correction based on K-readings?

Answer: Take the amount of with the rule astigmatism noted by keratometry readings, multiply that by 1.25, and then subtract that number from 0.75 diopters (lenticular astigmatism).

Example: 1.00 diopter of with the rule corneal astigmatism would result in an expected refractive astigmatism of 0.50 with the rule. (1.00D x 1.25 = 1.25D - 0.75D = 0.50D)

Question: What instrument uses the reflecting power of the cornea to determine its readings?

Answer: The keratometer uses the reflecting power of the cornea to determine the corneal curvatures. The formula is: D = (n-1)/r. Where D is the reflecting power of the cornea and n is the standardize refractive index of the cornea (1.3375).

Question: How much of the cornea is measured with a keratometer?

Answer: Only the central 3-mm. For this reason, using a keratometer instead of a corneal mapping device may miss peripheral corneal scar defect.

e. Gonioscope

Total Internal Reflection (TIR) makes it impossible to view the anterior chamber angle without the use of a gonioscopic contact lens. Normally light from the angle undergoes TIR at the air-tear film interface. As result of this, the light from the angle is not able to escape from the eye making the angle impossible to visualize. This problem is overcome by the gonioscopic contact lens which sits on the cornea. In this way, the air at the surface of the cornea is eliminated. Total internal reflection occurs when light is trapped in the incident medium. Because TIR never occurs when light travels from a lower to a higher index, light is able to enter the gonioscopic contact lens where it is reflected by the gonioscopic mirror. This allows the angle of the anterior chamber to be visualized by the examiner.

Gonioscopic tilt angle should be approximately 7.5 degrees to the visual axis. This minimizes reflections and image distortion.

f. Retinoscopy

A retinoscope allows the clinician to objectively determine the spherocylindrical refractive error; irregular astigmatism, and also evaluate opacities and irregularities of the cornea and lens.

Most retinoscope today use a streak projection system. This streak of light is reflected from a mirror. Additionally, the streak can be moved in relation to a convex lens in the device by way of the sleeve. This allows the light to leave the device as if it were coming from a point behind the retinoscope ( plano mirror setting) or as if it were coming from a point between the examiner and the patient (concave mirror setting). For Copeland retinoscopes, the plano position is with the sleeve up, while the Welch Allyn retinoscope is in the plano position with the sleeve down.

Normally, the examiner will use their right eye to perform retinoscopy on the patient's right eye and their left eye for the patient's left eye. The examiner should align themselves just off-center to minimize lens reflections and to allow the patient to visualize the distance target to relax their accommodation. The patient should be instructed to look at a distance target such as a large Snellen letter (20/200-20/400).

  • When doing retinoscopy, the examiner is attempting to put the far point of the patient’s eye at the plane of the examiner’s pupil.
    • When the reflex shows “against” motion, the far point plane lies between the patient’s eye and the examiner’s eye, indicating myopia.
    • When the reflex shows “with” motion, the far point lies outside the interval between the patient’s eye and the observer’s eye, indicating hyperopia, emmetropia or mild myopia.

Question: If you obtain “with motion” during retinoscopy, is the far point of the patient in front of the peep hole, at the peep hole, or beyond the peep hole?

Answer: Beyond the peephole. The goal of neutralization is to have the light reflex of the patient’s far point at the peephole. The light at the patient’s pupil fills the entire space at once when neutrality is reached. “With” motion requires more plus to be added to the prescription to move the far point to neutralization. “Against” motion means that the far point is in front of the peephole. Therefore, more minus must be added to move the far point to neutralization.


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