The Influence of Race, Age, and Pupil Size on the Measurement of a Photorefraction Device

The PR optical theory is described previously ^{2, 3, 4, 5, 6, 17, 18}. Figure 1 briefly summarizes the difference of DOES and other PR systems in the optics (upper figure) and the refraction analysis algorithm (lower figure). The DOES system uses a near-infrared (810 nm) source, multi-eccentricity-meridian video PR method ^{17, 18} to acquire a sequence of retinal reflex PS images. Using a beam splitter, the camera and light-source array were collimated (as illustrated at the middle right). This allowed a co-axial PS image to be captured and used as a calibration baseline for individual’s retinal reflectance. Each PS image included both eyes. Each pupil was identified from the photograph using a customized image analysis code, and the background (offset) intensity was removed mathematically to reduce the scattering variation. The remaining signal intensities render a two-dimensional Point Spread Function (PSF) of the eye (illustrated at lower right of Figure 1). Without the beam splitter and the resulting co-axial PR intensity, this double-pass PSF would not be accessible. Finally, the astigmatism amplitude and angle were derived using the 2-Dimensional elliptical fitting and the principle axes of the PSF.

Figure 1.Optical arrangement and refraction analysis process. Upper portion shows the optical arrangement of the PR detection. The related light source and camera position are illustrated at the middle right. The lower left-image shows an example of pupil images corresponding to the illumination sources. The lower right-plot shows the double-pass point-spread function (PSF) that is calculated from the relative intensities of the pupil images. The waist of PSF increases with refractive error and is proportional to the pupil size. The fitted principle axes indicate astigmatic axes.

155 patients with an age range 4 to 81 years were recruited during their visits for comprehensive eye examinations for eye glasses or contact lens in Walmart Vision Center in Tullahoma, Tennessee. After obtaining informed consent, the volunteers were brought to a testing room to view through a binocular eye-piece a 3D cartoon that is located on a screen at 75 cm. Four refraction measurements were acquired using infrared DOES while the subjects viewed a fixation cartoon figure at 4 different stereo distances that ranged from 75-cm to infinity. To investigate the influences of environment lighting and the consequent pupil-size on the PR measurement, the test was performed twice under the conditions of normal room-light and lights-off. After the cartoon viewing, the patients were returned to their original eye exam schedule. Each eye would generate 8 sets of PR refraction data points under the two environment lighting and four visual stimuli conditions. For comparison, the clinical refraction data were collected. These include data from an autorefractor, which was an average result of three measurements, from the phoropter and trial lenses performed by the optometrist, and from cycloplegic retinoscopy for young children from whom subjective refraction were not feasible.

To prevent error from accommodation, eye data from 56 hyperopic or mildly myopic (Spherical Equivalent > -1 diopter) subjects who were younger than 50 years old were omitted in this refraction evaluation, leaving 99 subjects included in the refraction analysis.

In the PR measurements, 18.7% of tests were identified to contain images with poor quality due to eye movements or obstruction from the eyelids or eyelashes. Images were filtered with a quality control code based on the circularity, edge sharpness, and significant location-shifting. Since four detections were attempted in each measurement, the rejection rate of refraction measurements is 0.12% = (18.7%)4. In fact, all 198 eyes from the included 99 subject yielded qualified refraction results. In summary, the comparison was able to be made from the 99 subjects, 198 eyes, 1791 data points (OD= 888, OS=903; total n=1791) measured under 4 visual stimuli distances and 2 environment lighting conditions.

Age range: 7-81 years old.

Pupil range: 2.48 to 8.05 mm.

Refraction range: -13.5 to +6.1 diopter

We used both the correlation and the Bland-Altman statistical methods to evaluate agreement between DOES and the clinical gold-standard measurements that were obtained from the optometrist.

The correlation plots of spherical equivalent (SE) and crossed cylinders (J0 and J45) between the clinical gold standard (x-axis) and the DOES data (y-axis) are shown in the upper three panels of Figure 2. The dashed lines mark the perfect correlation (1:1) and the deviations of ±1 diopter in SE plot and ±0.5 diopter in J0 and J45 plots. The standard deviation of SE and (J0 and J45) within the +4 to-4 diopters region (shaded area in the plots) for vision screening are 0.45, 0.3 and 0.18 diopter, respectively (n=1337). The inaccuracy increases at large refractive error over 4D, which is limited by the eccentricity coverage of this PR system.

The Bland-Altman analysis is shown in the three lower panels of Figure 2. The biases (mean differences) between the binocular DOES and the subjective clinical refraction testing were minimal at 0.00, 0.06, and 0.02 diopter in SE, J0, and J45, respectively.

The monocular autorefractor is currently the most reliable objective refraction tool in the clinical setting. We also compared monocular autorefractor measurements to the clinical subjective refraction results in the same group of 99 patients (198 eyes). The standard errors of the SE were 0.36 diopter and crossed cylinders J₀ and J₄₅ were 0.14 and 0.18 diopters, respectively. The mean differences were also minimal (-0.09, 0.03, and -0.04, respectively). The 0.36 D error with 0.09 D mean error is a decent reference. It should be mentioned that each autorefractor data is an average of 3 consequent measurements, which automatically reduces the measurement inaccuracy in each individual. The DOES analysis in this section, however, includes every applicable measurement without taking the average.

Figure 2.Overall binocular DOES refraction test analysis in comparison to clinical gold standard eye exam result. 

The intraocular scattering normally increases with age and as cataracts develop. The onset age of age-related cataract is typically in the 40s. Intraocular scatteringis not only an important problem that reduces contrast of vision, it could also decrease the reliability of ophthalmic measurements. This is especially significant to PR since the measurement relies on the double pass light signal. To examine the influence of intraocular scattering on DOES accuracy, analysis was carried out in three age groups with similar distribution of refraction and pupil size. The details of these 3 age groups are:

1. 20 children from 7 to 15 years old with the total of 38 eyes and 377 tests. The distribution of refractive errors is SE= -1.8 ± 1.0 D. Pupil-size distribution is 5.8 ± 0.9 mm (range from 3.4 to 8.1 mm);

2. 24 young adults from 17 to 38 years old with total of 45 eyes and 434 tests. The distribution of refractive errors is SE= -1.9 ± 0.9 D. Pupil size distribution is 5.2 ± 1.3 mm (range from 2.9 to 7.2 mm);

3. 33 older adults from 41 to 81years old with total of 65 eyes, 525 tests. Distribution of refractive error is SE= +0.3 ± 1.8 D. Pupil size distribution is 4.7 ± 1.0 mm (range from 2.5 to 7.2 mm).

The result in SE, J₀, J₄₅, and the total focus errors from the standard exam were shown in Figure 3. The total focus error of a single measurement is defined as the root-mean-square of the three errors (SE, J₀ and J₄₅). Judging from the total focus error, the group of young adults was measured by DOES with the greatest accuracy of 0.42 D. The measurements in the group of older adults, had an increased inaccuracy amount of 0.09 D. This result seems to support intraocular scattering influence in older subjects. However, group of children also yielded similar amount of errors at 0.52 D. The result indicates that the small amount of error increased (0.09D) does not provide sufficient evidence to conclude contribution from intraocular scattering to DOES measurement.

Figure 3.Comparison of DOES accuracy in three age groups. The total focus error of measurement is defined as the root mean square of the three errors (SE, J0 and J45)

Pupil size variation has been one of the major difficulties in traditional PR or PS devices. We acquired DOES data in bright and dark environment settings to study the effect of pupil size in measurement accuracy. The older adults’ pupils did not undergo a large enough size change in response to room light conditions and could not be included in the analysis. Therefore, only children and young adult groups were considered in pupil size analyses. The subjects’ distributions in different pupil groups and the results are shown in Figure 4.

Total focus error was 0.47D in small pupils and 0.57D in large pupils in children. In adults, total focus error was 0.45D in small pupils and 0.40D in large pupils. There was no discernable effect of pupil size on accuracy in either age group. The difference of focus error was 0.1D or less between pupil sizes and the tendency was toward opposite directions. Thus, pupil size does not appear to be a significant factor in the DOES refraction measurement.

Figure 4.Comparison of DOES accuracy in smaller and larger pupil groups.

We compared measurement accuracy across races to investigate the influence of retinal reflectance on the double-pass PR technique. The group of 13 non-white subjects included 3 Asians, 3 Hispanics, and 7 blacks. The 42 white subjects were selected for the equivalent age, pupil size, and refraction distribution. The sample conditions of each group and the error analysis results are shown in Figure 5. The result shows that the difference of accuracy between the two groups was merely 0.04D. This investigation of race dependence also showed no significant change in the two groups’ measurement accuracies.

Figure 5.Comparison of DOES accuracy between white and non-white groups.

The authors would like to acknowledge the UTRF maturation funding support, the support and consultation from Dr. Palmer and Walmart Vision Center at Tullahoma, Tennessee, and the support of Center for Laser Applications of the University of Tennessee Space Institute.