In contrast to soft lenses (see Chapter 7), rigid contact lenses are individually fitted and then lathe cut to meet particular optical, geometrical and material specifications. Although the manufacturing precision has improved significantly over the years, in particular with the introduction of computer numerical-controlled lathes, such improvements do not eliminate the need for careful lens inspection and measurements to ensure that only good-quality lenses are fitted on the eye. This quality assurance rests predominantly with the lens laboratories, but contact lens practitioners should also be able to confirm basic lens parameters and detect faulty lenses.
The more experienced contact lens fitters occasionally modify rigid lenses to adjust the optical power, remove surface scratches or smoothen the edge profiles. Subsequent inspection is recommended to ensure that the modification was successful. The identification of unknown materials might be a further reason why practitioners and laboratories need to measure material related properties. Most aspects of contact lens quality are covered by the International Standards Organization (ISO) four-part document ISO 18369:
Part 1: Vocabulary, classification system and recommendations for labelling specifications .
Part 2: Tolerances .
Part 3: Measurement methods .
Part 4: Physiochemical properties of contact lens materials .
The tolerances for measuring rigid contact lenses, as specified in ISO 18369, are listed in Appendix B.
Laboratories are obliged to deliver only lenses that do not exceed allowable tolerances.
Based on shape, rigid contact lenses can be divided into scleral lenses (including corneo-scleral and miniscleral lenses) and corneal lenses. Optical features can include aberration control, astigmatic correction, or translating or simultaneous add powers for treating presbyopia. Orthokeratology lenses are worn overnight and correct vision by reshaping the corneal surface. Each of these lenses has its own critical set of parameters and requires specific methods of inspection to ensure good vision and patient satisfaction.
The list of parameters that need to be checked on rigid lenses can be grouped into three main categories.
Lens and optic zone diameters;
Centre and edge thickness;
Back and front surface radius of curvature; and
Power and power profiles and
Optical quality and surface defects.
Luminance transmittance; and
Most rigid lens materials are relatively insensitive to ambient temperature and relative humidity and can be measured under normal laboratory or practice environment conditions.
Despite being called ‘rigid’, finished rigid lenses can be very flexible and can deform permanently or temporarily when put under stress. It is therefore important to keep the lens free from external forces when measuring geometrical parameters. There have been reports to suggest that uptake of water or disinfection solution can also alter the shape of rigid lenses ( ), but these effects appear to be minor and generally of no concern.
Lens and Optic Zone Diameters
The V-gauge is a simple and easy-to-use method with adequate accuracy to measure the outside diameter of rigid lenses. A tapered channel is cut into a strip of plastic with diameter markings along the channel. Fig. 14.1 illustrates the use of such a device. The lens is placed in the wider opening on the right hand side and then gently notched towards the left until it touches the opposite side of the channel. The diameter value of the precalibrated scale is read from the position at which the lens touches the channel.
Projection systems such as calibrated loupes or slit lamps fitted with measuring graticules can also be used to measure lens diameters. They are reported to achieve a tolerance limit of 0.1 mm ( ) and have the advantage of being able to measure the diameter across several meridians, identifying possible lens warpage.
With the correct illumination, distinct zones across the lens area can also be visualized and measured with projection systems. For more precise measurements, measuring microscopes or profile projectors can be used, providing a resolution of up to 1 µm.
Centre and Edge Thickness
The measurement of centre thickness provides key information on the rigidity of a lens and is also used to calculate oxygen transmissibility. Central, peripheral and edge thickness can be measured using dial or digital gauges ( Fig. 14.2 ). The gauges should have a readout resolution of 1 µm and a specified accuracy of ±3 µm. The anvil and the plunger need to be fitted with spherical ball tips with a radius between 1.2 and 5 mm. To minimize surface indentation and false thickness readings the measurement force should not exceed 1.4 N according to ISO 18369. By moving the lens away from its central position, radial thickness readings can be obtained at selected peripheral and edge locations.
Optical and acoustic methods have also been used for measuring lens thickness on eye or on the bench. Ultrasound pachymeters are commonly used to measure axial length and corneal thickness. However, these instruments are limited in their resolution and acoustical properties of lens materials need to be known to correct the measured thicknesses. Optical methods face a similar constraint, but the conversion is made easier as the refractive index of lens materials is generally known ( ). The two most commonly used optical methods are Scheimpflug photography and optical coherence tomography (OCT) imaging. For both methods, clinical instruments are commonly used for anterior segment imaging—for example the Pentacam (Oculus, Germany) and the OCT Spectralis (Heidelberg Engineering, Germany), respectively. Either instrument can be employed to measure cross-sectional thickness profiles as well as complete thickness maps. The built-in analysis software converts the measured optical thickness into physical thickness using the corneal refractive index of 1.377. To obtain the actual thickness for a rigid lens, the following equation is to be applied:
By obtaining thickness profiles across several meridians, the amount of unwanted lens prism can be determined. Lens prism is a common deficiency with lathe-cut lenses. This defect is caused by a decentration of the front surface relative to the back surface due to inaccurate re-blocking of the button on the lathe.
Back and Front Surface Radius of Curvature
The standard instrument with which to measure the radius of concave or convex surfaces on rigid lenses is the radiuscope. The operating principle of the radiuscope is explained in Fig. 14.3 . Rays from the illuminated target are projected onto the surface via a beam splitter. At the in-focus position A, the reflected target is seen through the eyepiece as a clear, sharp image. In position B, the rays are reflected perpendicularly to the lens surface and also generate a sharp image. The relative distance between A and B corresponds to the radius of curvature. This operating principle applies to convex as well as concave surfaces.
The steep and flat radii of toric surfaces can also be measured by orienting the respective meridians with the vertical and horizontal line of the target and bringing each one in focus alternately. As many surfaces are now manufactured as aspheric ellipsoids or paraboloids, the target images in position B won’t be in sharp focus and only an average radius can be determined. Should more detailed surface information be required, radii analyzers such as the Brass 2000 (Rotlex, Israel) that utilize Moiré deflectometry can generate topographic maps and extract surface profiles with fitted mathematical functions. This methodology is particularly useful for manufacturers of orthokeratology lenses. The precise cutting of the complex back surface profile is critical to achieve the targeted corneal reshaping.
The keratometer is a clinical instrument that can also be used to measure front and back radii of rigid lenses. To minimize interfering reflections, it is best to use a folding mirror and place the rigid lens horizontally, with a drop of water on the surface facing downwards ( Fig. 14.4 ). Keratometers measure the curvature at only two defined points, usually about 3 mm apart. Any asphericity cannot be captured, but toric surfaces can be measured by rotating either the instrument or the lens to determine the steep and flat meridians. showed that keratometer measurements agree well with results obtained by a radiuscope, with the mean difference being less than 0.03 mm.
Corneal topographers like the Humphrey Atlas (Carl Zeiss Meditec, Germany) have successfully been used to measure the curvatures of rigid lenses, including aspheric and toric surfaces. found that this instrument measures within the base curve tolerance of 0.05 mm. However, not all topographer instruments are suitable to measure concave surfaces. When an appropriate instrument is lacking, an impression can be taken from the back surface and then measured as a convex surface.
Complete cross-sectional surface profiles can also be obtained by Scheimpflug and OCT-based imaging instruments (see the ‘Centre and Edge Thickness’ section). Some instruments have built-in curve fitting algorithms or provide data export options for external analysis. The sharpness of junctions between different zones on the lens can also be determined using the appropriate analysis tools. To avoid optical distortions, the surface to be measured should face the instrument. Measuring the lens shape while fitted to an eye provides the advantage that fitting characteristics and interaction with the cornea can also be assessed ( ).
A dedicated OCT-based instrument has recently been introduced to measure not only the lens curvatures, but also the thickness profiles and to a limited extent also the edge profiles across several meridians. The is830 (Optimec Systems, United Kingdom) is mainly used by manufacturers and research institutions to obtain complete lens geometry measurements by cross-sectional scanning across a number of selected meridians. With its integrated curve fitting functions, central and peripheral radii of curvature can be obtained, which is of particular relevance when evaluating the back surface shape of orthokeratology lenses.
The edge shape can be of critical importance for rigid lenses with respect to on-eye comfort ( ). The relative position of the apex, the sharpness of the anterior and posterior edge radii and edge thickness are features that can be inspected by practitioners using readily available equipment. used a simple projection magnifier with graticule to check the edge profile. To better visualize the posterior edge shape, a slit lamp can be used to illuminate and observe a narrow section of the entire edge profile. A lens-holding device as used by assists with the rotation of the lens to observe profiles around the circumference of the lens ( Fig. 14.5 ). This ability to rotate the lens is a critical aspect, as most lenses are made with some degree of unwanted prism, which has a major impact on edge thickness and edge shape. Full inspection is also required to detect any chipping and dents or other localized defects.
also recognized the importance of checking edge shape at different meridians when she developed a moulding technique to obtain impressions of edge profiles. The entire edge of the rigid lens was pressed into a blob of quick-setting dental mould material. After setting, the lens was popped out and the mould sliced in four quadrants with two razor blades pressed against each other. The slices obtained could be viewed under a microscope and images taken for further analysis and documentation. classified edges as sharp, blunt, round or square.
For full-edge shape quantification, developed an optical profiler that measures complete cross-sectional lens profiles along several meridians, including high-resolution edge profiles. The self-guided optical sensor moves along the lens profile, recording data points with micron precision. Mathematical algorithms are then used to analyse the profiles and extract numerical parameters for quality assessment and classification.
By measuring edge profile and back surface with this instrument, the axial and radial edge lift can also be obtained from the collected data sets. As shown in Fig. 14.6 , edge lift is the distance between the extended back optic zone radius and the apex of the edge, in either radial or axial direction, and can be calculated using measured parameters of lens diameter, back sag and back optic radius: