The lensmeter measures the power of a spectacle or contact lens. It uses a fixed lens positioned a focal length away from the spectacle plane at the tip of the nose cone (Fig 8-1), by which the vergence induced in the spectacle plane becomes linearly related to moving a target.
In the lensmeter, an illuminated target is moved, varying the vergence at the tip of the nose cone, until it becomes focused on the reticle of a telescope. The target is focused only when parallel rays are entering the telescope (ie, when the light entering has zero vergence), which indicates that the “unknown” lens is exactly neutralizing the vergence that is coming out of the nose cone. The actual power of the lens can then be read from a dioptric scale, which is opposite of the power emerging from the nose cone.
The addition of the telescope facilitates the detection of zero vergence. This is because it magnifies vergence by the square of the power of the telescope, which enables the examiner to detect very small deviations from zero vergence without having his or her uncorrected refractive error cause significant error in the measurement. For best accuracy, the eyepiece can be adjusted beforehand, by focusing it on a reticle located inside the telescope, which eliminates any residual effect from the examiner’s refractive error.
Figure 8-1 The lensmeter. An illuminated target is moved, varying the vergence at the tip of the nose cone, until it becomes focused on the reticle of a telescope. This is the case when the “unknown” spectacle lens is exactly neutralizing the vergence that is coming out of the nose cone. The light will then emerge from the spectacle lens with zero vergence (ie, collimated), and the target will be seen in sharp focus by the observer. The telescope magnifies vergence by the square of its power and thus facilitates the detection of zero vergence, increasing the precision of the measurement and reducing the effect of the observer’s accommodation or refractive error.
(Reproduced from Guyton DL, et al. Ophthalmic Optics and Clinical Refraction. Baltimore: Prism Press; 1999. Illustration modified by Kristina Irsch, PhD.)
Note that we cannot measure the true power of a lens with the lensmeter, as this would require us to measure from the lens’s principal planes rather than from the lens’s surface. To measure the distance correction using a lensmeter, we place the spectacle lens on the nose cone with the temples turned away from us, so the nose cone is against the back surface, because spectacle lenses are designated by their back vertex power (ie, the reciprocal of their back focal length).
The target usually has a set of lines (eg, the American cross-line target as depicted in Fig 8-2) that permits the observer to determine whether the lens has cylindrical power. In the measurement of cylindrical power, as illustrated in Figure 8-2A, the cross-line target is first rotated, as well as moved forward or backward, until one set of lines is sharp. Then, it is moved forward or backward until the perpendicular set of lines is sharp. The difference in target settings is the cylindrical power. The cylindrical axis is read from a scale indicating the orientation of the cross-line target in degrees.
Figure 8-2 Illustration of view inside lensmeter. A, In the measurement of cylindrical power, the cross-line target is rotated, via the axis wheel, so that the lines are continuous and do not appear “broken,” indicating the correct axis of the cylinder. The cross-line target is also moved forward or backward, via the power wheel, until 1 set of lines is clear (left). Then, it is moved forward or backward until the perpendicular set of lines is clear (right). The difference in target settings is the cylindrical power of the lens. B, Upward displacement of the target pattern indicates the presence of base-up prism.
(Modified from Gutmark R, Guyton DL, Irsch K. Prescribing Prisms. Focal Points: Clinical Modules for Ophthalmologists. San Francisco: American Academy of Ophthalmology; 2011, module 10:p4.)
Prism in spectacles can also be detected by means of a lensmeter. If there is no prism present in the lens, the center of the cross-line target will be in the center of the lensmeter reticule. If, however, the intersection of the target lines is off-center (Fig 8-2B), and the lens must be shifted away from the patient’s normal viewing position to recenter it in the reticule, then one knows that prism is present in the spectacle lens. If the intersection of the target lines is decentered horizontally, horizontal prismatic power is present, whereas a vertical shift indicates the presence of vertical prism power, with the shift in the direction of the prism base. In Figure 8-2B, the target lines are vertically displaced upward from the center, as is the case when vertical prism power, base-up, is present.
When determining a patient’s reading add, the examiner places the spectacles on the nose cone with the temples turned around, toward the examiner, so that the front of the glasses rests on the nose cone. The front vertex power of the distance portion is measured, which will be significantly different from the back vertex power for high plus lenses (in cases other than a distance lens with strong plus power, there is usually little or no clinically significant difference in the measurements), and the front vertex power of the near portion is measured. The difference in power between the distance and near portions of the lens specifies the reading-add power.