A manual lensometer, also known as a focimeter, is an optical instrument used to measure the precise characteristics of spectacle lenses. Its primary function is to determine the refractive power, the axis of astigmatism correction, and the location of the optical center of a lens. This device works on the principle of neutralization, where the unknown lens power is counteracted by an adjustable lens system until the target image is brought into sharp focus. The measurements, expressed in diopters, are used by eye care professionals to verify that a lens meets the prescribed specifications.
Instrument Preparation and Eyepiece Focusing
Before measuring any lens, the manual lensometer must be properly prepared and calibrated for the user’s eye to ensure accurate readings. The first step involves adjusting the eyepiece to eliminate the user’s own refractive error, preventing accommodative errors. To begin, the power drum should be set to a high plus power, or the eyepiece is rotated fully counter-clockwise until the internal reticle is blurred, or “fogged.” The reticle is a set of crosshairs and concentric circles that serves as the visual reference point.
The user then slowly rotates the eyepiece clockwise, bringing the reticle into sharp focus and stopping immediately at the point of maximum clarity. This ensures the user’s eye is relaxed and not actively focusing, which is crucial for objective measurement. After focusing the eyepiece, the power drum is turned to the zero or plano setting, and the illuminated mires—the target lines—should appear sharp and centered, confirming the instrument’s zero calibration. If the mires do not focus precisely at zero, the reading is noted as an instrument error and must be compensated for in all subsequent measurements.
The lens should be clean and dry before being placed on the lens stage, with the convex surface resting against the lens stop to measure the back vertex power. This placement standardizes the measurement for consistency across different lens designs. The lens holder is then secured to stabilize the lens against the lens stop, preventing movement that could introduce errors. The prism compensating device, if present, must also be confirmed to be set at zero before beginning any power determination.
Measuring Spherical and Cylindrical Power
The core function of the lensometer is to determine the spherical and cylindrical power of a lens by bringing the internal target mires into focus. The mires consist of two sets of lines: a single line (sphere) and three parallel lines (cylinder), oriented 90 degrees to each other. The lens is placed on the stage, and the operator rotates the power drum until the first set of mires comes into sharp focus; this first reading represents the power of the first principal meridian.
For a lens with astigmatism, the two sets of mires will not focus simultaneously, indicating a difference in power between the two meridians. To measure the cylinder, the operator rotates the axis wheel until one set of mires appears continuous and unbroken, signifying alignment with a principal meridian. The power drum reading at this point is the spherical component, or the weakest power, which is the sphere power in the plus-cylinder notation system.
The power drum is then rotated until the second set of mires comes into sharp focus. The difference between the first and second power drum readings quantifies the cylindrical power of the lens. The axis is read directly from the axis wheel where the first set of mires was straightened. This difference represents the total cylinder correction, which is added to the sphere reading to formulate the final spectacle prescription (e.g., +2.00 Sphere +1.00 Cylinder x 90 Axis).
Locating Optical Center and Determining Prism
After measuring the refractive power, the next step involves locating the lens’s optical center and measuring any prescribed or induced prism. The optical center is the point on the lens where there is no prismatic effect, and light passes through undeviated. To find this point, the operator moves the lens horizontally and vertically on the stage until the intersection of the mires is perfectly centered on the reticle’s crosshairs.
Once the mires are centered, the integrated lens marker applies three ink dots onto the lens surface, with the center dot marking the optical center. If the mires cannot be centered, it indicates the presence of a prismatic effect. The amount of prism is quantified by counting the number of concentric circles, or prism diopter rings, the mire intersection is displaced. Each ring represents one prism diopter.
The direction of the prism base is determined by the direction of the mire displacement. For example, if the mires are displaced upward from the center, the lens contains Base Up prism. Similarly, displacement toward the operator’s nose is Base In, and displacement toward the temple is Base Out. Some lensometers include a prism compensating device, a knob that brings the mires back to the center, with the rotation indicating the total prism power and base direction.
Recording and Verifying Lens Specifications
The final stage is the recording and verification of all measured parameters. Every value—spherical power, cylindrical power, cylinder axis, and prismatic power—must be documented in a standardized format. For multifocal lenses, the additional power (Add power) must be measured by taking the difference between the distance and near sphere readings.
Verification involves comparing the measured values against the original prescription or industry standards. The American National Standards Institute (ANSI) specifies acceptable tolerances for power and axis measurements, ensuring the lens provides the intended visual correction. Measurements outside these tolerances indicate a potential error in lens manufacturing or mounting. This documentation and verification process confirms the lens is fabricated correctly before it is dispensed.