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In the simulation of Fig. 13.8(b), the Navarro eye model is used as the base model, and a ZERNIKE phase plate is utilized on either the surfaces of the anterior cornea or the lens to produce an aberration of the desired magnitude. The vision simulation of the letter chart at near and far distances describes the effect of pseudo-accommodatrion. The simulation with the eye model allows investigation on how various degrees of aberration can produce visible clinical signs and allow scientists to examine the consequent patient vision due to the degree and type of ocular aberration without using real human subjects.

13.4.2. Simulation of PR

With some similarity to retinoscopy, stationary PR was introduced during the 1970s and 1980s. Kaakinen,27 Howland,28 and Bobier and Braddick29 replaced the observer with a camera. The stationary PR is classed into two types: the coaxial PR (CPR) and the eccentric PR (EPR). CPR has the light source positioned in front and at the center of the camera lens.29,30 The retinal reflex coming from the eye to the camera is defocused into a blur pattern, which changes with the eye’s defocus magnitude. CPR bases the judgment of the refractive error on the extent of the defocused retinal reflex. In contrast, EPR places a light source eccentric from the camera lens aperture, and the camera is focused on the examined eye to form a sharp pupil image of reflex. Like the spot retinoscope, the EPR measurement is objective, and the light source and camera are placed away from the examinee. EPR calculates the state of refractive error from the size of the bright crescent that appears in the focused pupil image. EPR is currently the most used PR method to screen for binocular refractive errors in children and to detect accommodation in lab animals.

The commercial PR instruments for pediatric vision screening include the iScreen Photoscreener,32 the MTI Photoscreener,33 and the PediaVision Vision Screener34 (previously Power Refractor II35). Shown in the upper portion of Fig. 13.9 is a photograph using the iScreen Photoscreener. Contact lenses were worn on the two eyes to produce refractive errors of +4 and −4 diopters. For comparison, the simulated pupil reflex images using model eyes of various degrees of refractive errors are illustrated at the bottom of Fig. 13.9.36 The Navarro eye models are used with additional thin lenses at

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Fig. 13.9. PR image of refractive eyes acquired with aid of contact lenses (upper) and simulation images of +10 to −10 D refractive eye models (lower).

Fig. 13.10. Gazing angle effect on the EPR measurement of the same eye using eye model.

the corneal plane. Simulation of the measurement permits the investigation of various human factors and environmental conditions. For example, the simulation result in Fig. 13.10 predicts the tilting and size changes of the reflex due to the alignment of ocular orientation or gazing angle. The effects of pupil size, light source location and dimension, and the wavelength of light source can be evaluated through the computation with eye models. Further demonstrated in Figs. 13.11 and 13.12 are the investigation of the sensitivity in detecting high-order ocular aberrations with a new PR set-up.37

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Fig. 13.11. (a) Five measured infrared PR KC pupil reflex images (pupil only), (b) the lowest of the five original reflex images, unprocessed digital photograph, and (c) simulated images using the personalized model of the patient.

Fig. 13.12. (a) 5 measured infrared PR myopic pupil reflex images (pupil only). (b) The lower one of the 5 original, unprocessed digital photograph. (c) Simulated images using the personalized model of the patient.

An infrared camera with five NIR LEDs that are placed on center, above, below, and on both sides of the camera aperture is assembled. Photographs of one CPR and four EPR images are acquired in sub-second. One raw PR photograph image of a KC eye with a high-order aberration is illustrated in Fig. 13.11(b). The pupil reflexes are identified and cropped from the five raw images of the KC eye with computer image analysis program and shown in Fig. 13.11(a). Figure 13.11(c) shows the corresponding simulation images using the personalized eye model that is constructed with the patient’s topography and WFA. The simulation result clearly predicts the measurement. Figure 13.12 illustrates the same investigation with a myopic eye. The

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