In my April Custom Soft Corner column, clinical considerations associated with pupil-related measurements during custom soft multifocal lens fitting visits were discussed. This column will address acquisition of this pupil-related information.
Methods of Testing
A basic pupillary distance (PD) ruler or metric stick is a rudimentary form of assessment, but it can be used to subjectively determine pupil diameter. Hand-held pupilometers, corneal topographers, and various digital devices can all provide pupil size measurements via objective means. Additionally, corneal topography provides information on pupil location, and its output maps can be manipulated.
Assessing pupil size during photopic and scotopic light settings is necessary to determine the lower and upper limits of pupil size, respectively. This information is critical to estimate the size of concentric power zones in multifocal lenses.
According to the International Commission on Illumination, scotopic condition illuminance levels are below 0.05 lux, and photopic conditions are above 50 lux (Rosen, 2002). To put this in perspective, an exam lane in our clinic uses a Testo 540 Lux Meter to measure the overall ambient or existing room illumination that falls directly in front of a patient’s eye as 191 lux (±3 lux), as in when measuring pupils subjectively (Figure 1). The same measuring device was used to measure light levels that patients encounter in testing rooms outside of the exam lane—for example, where corneal topography is carried out.
Figure 2 shows a set-up of two separate monocular field tests to acquire pupil measurements. In this case, while the left eye is being tested via infrared illuminated mires, the right eye is exposed to a certain amount of ambient light. Conversely, in Figure 3, although testing is also carried out one eye at a time, both eyes are exposed to the same illuminated field. Alternating cycles of dark and bright lights in this single field can produce different results for pupil size compared to the setup in Figure 2. All three setups in these figures have a patient exposed to a different light level.
Although it is atypical for today’s contact lens fitter to measure the lux values of a testing room, a variation in light level can affect pupil size and visual performance. Future advancements in multifocal contact lens optics may warrant more specific reporting of the lighting conditions during acquisition of pupil-related data. CLS
For references, please visit www.clspectrum.com/references and click on document #265.