Instrumentation 2 Chapter

centre. Zernicke analysis compares the image refracted through a real cornea to an ideal circular surface and analyses these aberrations. It can be very useful in determining the nature of corneal irregularities and quantifying them. Some manufacturers are already applying Zernicke polynomials in an attempt to correct some high order aberrations.

Applications

Topographers have become indispensable in the diagnosis and monitoring of corneal disease, especially keratoconus. They are routinely used before refractive surgery to screen for keratoconus and other corneal shape anomalies that may contraindicate the procedure. Postoperatively they are used to monitor wound healing11 and aid any decision for re-treatment. Corneal topography has also become an essential part of modern orthokeratology as it allows the practitioner to observe and document the changes in corneal shape as treatment proceeds (see Chapter 14).

It is now generally accepted that knowledge of the eccentricity value is superior to central corneal measurement alone.12 Topographic information therefore enables the practitioner to select the most appropriate back optic zone radius (BOZR) for the initial diagnostic lens to be used in rigid lens fitting.

Software programmes to design and simulate contact lens fluorescein patterns are incorporated into many topographical instruments: they use corneal eccentricity to calculate lens parameters and an optimal fluorescein pattern is achieved according to which lens parameters are chosen.

Aberrometry

Wavefront errors representing ametropia and other aberrations (e.g. spherical) are displayed as colour-coded maps similar to those found with corneal topography. The main system used is the Harkmann-Shack System which segments the wavefront using a microlens array. Any deviations from the reference image points are measured by a sensor.

Other methods for assessing aberrations are:

•Tscherning aberroscope.

•Laser ray tracing.

•Autorefraction.

•Psychophysical.

Evaluation of optical performance shows that reducing spherical aberration of the cornea does not maximize visual performance. Rigid gas-permeable lenses reduce asymmetric aberrations and spherical aberrations, while soft lenses give similar results to spectacles. This is expected as they mould to the shape of the cornea but it is claimed that some makes of soft lens reduce ocular aberrations.

2.4 Anterior segment photography

Due to the time delay and cost in processing film, digital imaging sensors are the main methods employed to capture images and video in contact lens practice.

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Section ONE Preliminaries

These can be used hand held in front of the eye, but benefit greatly from the variable illumination, magnification and stability of the patient achieved when mounted on a slit lamp, usually with a beam splitter. Imaging can, however, be directly through an eyepiece.

The two key aspects of a digital image are its resolution and format. Although a lot of emphasis is put on achieving the highest resolution possible, the quality of the image will depend on other aspects such as:

•The quality of the camera and slit-lamp optics.

•The light sensitivity of the chip.

•The size of the chip – as with small chips fewer photons will fall on each pixel.

•The screen resolution on which the images are viewed (compared to the resolution of the section of the digital image you wish to view).

Larger digital chips are now available which have a similar resolution­ to film, which is said to be equivalent to about 20 million pixels.

Compression algorithms can be used when storing images to save space but also to allow databases to recall images more efficiently. The most common formats for saving images are:

•JPEG (Joint Photographic Experts Group): a compression format which can result in the loss of some image integrity.

•TIFF (tagged information file format): a standard lossless format.

•RAW: stores all the data captured by the camera in raw form without any processing. Files are smaller than TIFF files but may only be viewed on the same camera type.13

Digital images can be captured with CCD (charge coupled device), CMOS (complementary metal-oxide-semiconductor) or foveon chip cameras. CCD is considered to be the highest quality as the ‘electronics’ are not on the imaging chip, so the maximum area can be used to collect photons of light. However, this results in the camera having additional processing chips, increasing the expense and power usage, and decreases the reliability compared to CMOS chips which do have all the processing on the imaging chip. The use of CMOS chips in phone technology has resulted in their rapid development, allowing similar performance to CCDs. Foveon chips are a new technology which uses the wavelength absorbing characteristics of silicon to extract red, green and blue light data from each pixel rather than just the intensity of the light. Colour information from CCD and CMOS chips can be gathered by using a colour filter matrix over the pixels (often in a Bayer pattern) using algorithms to interpolate colour (achieving about 96% spectral integrity but with 100% spatial resolution) or by splitting the light onto three chips. Three chip cameras can achieve 100% spectral fidelity but do not have a higher resolution than one chip versions and are more expensive, delicate and less light sensitive as they are splitting the light three ways.

The image is usually transmitted directly to a computer via either a USB or a firewire cable; wireless transmission is also possible. Although not as convenient, images can be downloaded from a media storage card such as compact flash. Subsequent image enhancement is possible with a variety of software applications.

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Advantages of image capture

•Images are captured in real time, ensuring that the desired picture is recorded.

•Unsatisfactory images can be deleted and retaken.

•Where necessary, results can be demonstrated immediately to the patient.

•Digital images can be transmitted to other locations within or outside of the practice.

•There is no degradation of digital images over time.

•Images can be time or date stamped to provide an audit trail for legal protection.

•Efficient storage of images permits their easy retrieval, analysis and display.

Contrast sensitivity (CS)

Visual acuity (VA) is usually measured using high contrast letter charts and measures the resolution of the smallest detail that can be resolved by the human visual system in a high level of contrast. Some patients are able to achieve good VA under these conditions but still complain of a poor quality of vision. This may be because the problem only occurs under conditions where the contrast is lower than the 95% found on a typical letter chart (conditions which are abundant in the real world). Occasionally, it is due to a selective loss of sensitivity to low spatial frequencies (gradual changes in luminance across the visual field) which will not be detected by tests of visual acuity. These patients will show deficits in their contrast sensitivity functions.

The contrast sensitivity function (CSF) is obtained by measuring the threshold contrast for sinusoidal luminance gratings of different spatial frequencies. Sine gratings are described by their spatial frequency, contrast and orientation. Contrast sensitivity provides an alternative test of vision and describes the quality of vision better as it accounts for aberrations in low, medium and high contrast levels. It is defined as the reciprocal of the contrast threshold for any particular spatial frequency. Contrast threshold is the point at which the gratings can just be perceived on the screen.

2.5 Specialist instruments for higher magnification

Specular microscope

The specular microscope is used to give a highly magnified image (in excess of ×100) of both the corneal epithelium and, more particularly, the endothelium. These structures would otherwise be difficult to observe because of their normal transparency. Light is focused near to the focal plane of the objective close to a point of reflection. This gives a specular image to permit an assessment of features such as cell density, polymegathism and corneal thickness.

Confocal microscope

The confocal microscope focuses the illumination system at exactly the same point as the focal plane of the microscope objective. Scanning through the struc-

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Section ONE Preliminaries

ture under examination at different depths allows the instrument to reconstruct the image to give three-dimensional observation, sacrificing field of view for resolution and magnification. Instruments typically give a magnification of 680× with a field of view of 300 µm × 220 µm, a depth of field of 10 µm and lateral resolution of 1 µm.

2.6 Other instruments

Burton lamp

The Burton lamp uses ultraviolet light at a safe wavelength of about 400 nm. The lamp is also referred to as a blue light or black light and is used in conjunction with fluorescein (see Section 11.1.2).

Most Burton lamps are combined with a low-power magnifier, large enough to permit binocular viewing. Many models combine white light tubes and some incorporate a yellow filter to intensify the fluorescence.

Pachymeters

Corneal thickness measurements are made with a pachymeter and are useful:

•To assess or monitor corneal oedema.

•In the management of keratoconus.

•During orthokeratology.

•Before and after refractive surgery.

•For compensation after tonometry.

Pachymeters in their original and simplest form are slit lamp attachments fitted to the instrument post with a separate image splitting eyepiece (Haag Streit). The increasing requirement to measure corneal thickness has resulted in much more sophistic devices such as:

•Contact ultrasound probes (Tomey), which are considered ‘gold standard’.

•Scanning slit topography (Orbscan).

•Non-contact specular microscopy (Topcon).

•The Schleimpflug camera system, which can also be used for corneal topography.

Placido disc

The Placido disc is a flat, circular disc with alternate black and white concentric rings. The width and separation of the rings increase towards the periphery and are designed so that the reflections appear the same width when reflected from an average cornea. It gives a qualitative assessment of the regularity of the cornea itself. The eye is viewed through a convex lens in the centre of the disc. An internally illuminated version of the Placido disc is known as the Klein keratoscope.

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