The demographics and contact lens wearing history of two groups, each of 100 patients were examined, one grouphaving been assessed as suitable for photorefractive keratectomy (PRK) treatment but electing not to go ahead, the othergroup having proceeded with PRK treatment. Although only minor differences were found between the two groups in terms of demographics, the group that elected to be treated comprised a large number of contact lens-intolerant individuals. A breakdown is given of the contact lens-related reasons for opting to undergo PRK, emphasising the importance of the role of the contact lens practitioners in the preselection of patients for PRK.

A formula is developed to give the true thickness of a lens at various distances, measured radially, from the optical axiswhen the centre thickness, index of refraction and the power of the lens are known.

Infrared thermography was employed to measure the change in ocular surface temperature on eye opening within agroup of 11 dry-eye patients and seven control subjects. Qualitatively, the colour coded thermograms were found to be disorganised and irregular in the dry-eye group. Quantitatively, the decrease in temperature was found to be much more rapid in the dry-eye group (0.88 ± 0.23°C in 7s) compared with the control group (0.21 ± 0.27°C) (P<0.0001). This faster rate of cooling can be explained in terms of a greater evaporation rate and the increased ocular surface temperature on eye opening in a dry eye. This observation, if confirmed on a larger study group, potentially offers a new, objective, non-invasive system for analysis of the tear-film and dry-eye therapies.

Twenty-five Quantum II rigid gas permeable corneal lenses, in a range of five negative back vertex powers but otherwise of identical nominal specification, were immersed in distilled water for up to 6 weeks and the back optic zone radius of each lens was measured periodically to determine the magnitude and time course of curvature changes during hydration. Changes in back optic zone radius were of small magnitude and were less than the tolerance limits of ±0.05mm listed in British Standard BS7208: Part 1: 1992 (‘Specification for Rigid Corneal and Scleral Contact Lenses’) for rigid gas-permeable corneal lenses.

The Nidek KM-500 is a recently introduced hand-held automatic keratometer. If the accuracy and reliability of thisinstrument are good then it offers a number of benefits to contact lens practitioners and other clinicians. This study investigated the repeatability and reproducibility of readings taken using the hand-held Nidek KM-500, the Topcon RK-3000A and the Bausch & Lomb keratometer. The Nidek KM-500 was found to be reliable when used according to the manufacturer's instructions and the readings equated well with those obtained with the table-mounted Topcon KR-3000A. The repeatability of the Bausch & Lomb keratometer can be improved by taking the mean of three readings. Clinicians should consider conducting a simple repeatability study of the keratometer used in their own practice. The results may help to pinpoint a weakness in technique when using a non-automated instrument, and will establish the criterion for a true change in corneal radius.

Tear volumes of 15 normal Hong Kong (HK-Chinese were measured using a cotton thread test (CM and the phenol red thread (PRT) test on 6 different days. The results showed that both CTT and PRT tests performed on normal subjects were reliable over time. The mean CTT and PRT values were 11.8 (±4.4)mm/15s and 15.4 (±4.9)mm/15s respectively. There is a significant difference between the CTT values and the PRT values so the two tests cannot be used interchangeably. There is a day-to-day variation in both tests which may be clinically significant for some subjects and we suggested that more than one measurement be taken when an accurate tear volume measurement is important for the management of the patient. We also compared our PRT results with those of previous investigators and found that the mean PRT value is significantly lower in HK-Chinese subjects than in Caucasian subjects, while no difference was found between HK-Chinese and Japanese subjects.

Two separate international ring tests were carried out to assess the measurement capability of the focimeter and the Moiré deflectometer as applied to the power-related parameters of toric soft contact lenses. The results show that in order to measure the sphere and cylinder powers and axis direction using the focimeter a high number of replicate measurements are required to be confident of the test to within reasonable tolerance limits. The availability of the deflectometer provides lens manufacturers with measurement technology which is objective and reproducible and can be used to measure the power-related parameters of toric soft lenses with increased reproducibility using relatively few replicate measurements.