purpose. To measure and quantify effects of variation in retinal illuminance on frequency doubling technology (FDT) perimetry.

methods. A Zeiss-Humphrey/Welch Allyn FDT perimeter was used with the threshold N-30 strategy. Study 1, quantifying adaptation: 11 eyes of 11 subjects (24–46 years old) were tested with natural pupils, and then retested after stable pupillary dilation with neutral density filters of 0.0, 0.6, 1.2, and 1.6 log unit in front of the subject’s eye. Study 2, predicting effect of reduced illuminance: 17 eyes of 17 subjects (26–61 years old) were tested with natural pupils, and then retested after stable pupillary miosis (assessed with an infrared camera). A quantitative adaptation model was fit to results of Study 1; the mean adaptation parameter was used to predict change in Study 2.

results. Study 1: Mean defect (MD) decreased by 10 dB over a 1.6 log unit range of retinal illuminances; model fits for all subjects had *r* ^{2} > 95%. Study 2: Change in MD (ΔMD) ranged from −7.3 dB to +0.8 dB. The mean adaptation parameter from Study 1 accounted for 69% of the variance in ΔMD (*P* < 0.0005), and accuracy of the model was independent of the magnitude of ΔMD (*r* ^{2} < 1%, *P* > 0.75).

conclusions. The results confirmed previous findings that FDT perimetry can be dramatically affected by variations in retinal illuminance. Application of a quantitative adaptation model provided guidelines for estimating effects of pupil diameter and lens density on FDT perimetry.

^{ 1 }Goldmann’s standardization

^{ 2 }of kinetic perimetry reduces the effects of prereceptoral factors by requiring proper refraction and by using a background luminance for which sensitivity should adhere to Weber’s law for most subjects with clear ocular media and pupil diameters of 3 mm or greater. Adherence to Weber’s law ensures that sensitivity will be relatively unaffected by change in retinal illuminance.

^{ 1 }Ability to monitor change over time is greater when account is taken of prereceptoral factors.

^{ 3 }

^{ 4 }although correction of refractive error is still required for optimal performance.

^{ 5 }FDT perimetry uses high-frequency flicker,

^{ 6 }for which high mean luminances are required to reach the Weber region.

^{ 7 }Previous studies

^{ 8 }

^{ 9 }

^{ 10 }have demonstrated that results of FDT perimetry can be dramatically affected by changes in retinal illuminance (e.g., lenticular density or pupil diameter) and optical scatter (e.g., cataract).

^{2}mean luminance of the FDT perimeter, pupil diameter in a normal population

^{ 11 }varies from 2 mm to 7 mm. The present study varied retinal illuminance in normal eyes over the range of retinal illuminances expected in clinical populations (normal variations in pupil diameter and lenticular density, as well as pharmacologic mydriasis). The data were analyzed using a quantitative two-parameter adaptation model for the effects of retinal illuminance. A single adaptation parameter was estimated by analyzing effects of mean luminance on FDT perimetry for volunteers with healthy eyes and clear ocular media (Study 1); then the model was used to predict effects of change in retinal illuminance (Study 2). The analysis provided clinical guidelines for estimating effects of prereceptoral factors on FDT perimetry.

^{2}yields an effective mean retinal illuminance of 3.4 log td. The FDT perimeter sends an error message when mean luminance drops below 85 cd/ m

^{2}. Because this message did not appear during any of the tests, the nominal mean luminance was considered to be within 0.1 log unit of actual luminance. Neutral density filters of 1.6, 1.2, and 0.6 log unit were used to obtain mean retinal illuminances of 1.8, 2.2, and 2.8 log td, respectively.

^{ 12 }to correct for the Stiles-Crawford effect. The effect of lens density on effective retinal illuminance was computed using the spectrum of the FDT phosphor and a set of standard

^{ 13 }cone fundamentals with macular pigment removed, for age-related

^{ 14 }and cataract-related

^{ 15 }increases in lens density. Since illuminance varies with the square of pupil diameter, the effect of change in diameter is more dramatic for smaller pupils: a 2 mm increase in pupil diameter causes retinal illuminance to increase by 0.55 log unit from 2 mm to 4 mm, but by only 0.05 log unit from 8 mm to 10 mm. For all subjects pupil diameter was judged to be at least 8 mm by direct measurement with a pupil gauge; a value of 8 mm (3.4 log td) was used for all subjects.

^{ 7 }

^{ 16 }which has a fixed template on these axes. The two parameters scale the template vertically and horizontally. For a given retinal illuminance,

*I*, the corresponding MD is

_{max}, the asymptotic maximum MD at high retinal illuminances, and will be referred to as the

*sensitivity parameter*. Since MD is relative to mean values for age-similar controls, in the middle panel of Figure 1MD

_{max}is adjusted for each age so that MD = 0 for the mean pupil diameter, using the equations of Winn et al.

^{ 11 }for pupil diameter versus age, and of Pokorny et al.

^{ 14 }for lens density versus age. The horizontal scaling factor is

*K*, the mean retinal illuminance at which MD drops to 6 dB below MD

_{max}; log

*K*will be referred to as the

*adaptation parameter*.

^{ 11 }

^{ 17 }; accuracy of the prediction (difference between measured and predicted ΔMD) was plotted against the mean ΔMD (average of measured and predicted ΔMD), and linear regression was used to determine whether accuracy varied across values for mean ΔMD.

^{ 18 }In conventional perimetry, sensitivity is reported in terms of the log ratio between the maximum stimulus and the threshold stimulus, with 1 dB equal to 0.10 log unit change in Weber contrast. The version of the FDT perimeter that we chose used a proprietary method to compute “dB” from stimulus contrast, for which a change by 1 dB is usually near an 0.05 log unit change in contrast, but can approach 0.10 log unit.

^{ 19 }For comparisons across stimuli we used the definition of 1 dB equals 0.05 log unit of Michelson contrast. For the luminance increments used in conventional perimetry, Weber contrast equals Michelson contrast, and the “25 dB” stimulus is 100% Weber contrast. Sensitivities were converted from conventional perimetric units to 0.05 log Michelson contrast by first subtracting 25 dB from the instrument printout, then doubling the remainder (for 20 dB per log unit contrast).

^{ 9 }who computed their own index for FDT sensitivity by converting dB values into luminance values using equations provided by the manufacturer.

*K*), and 1.6 ± 2.1 dB (range, −2.7 to +5.6 dB) for the sensitivity parameter (MD

_{max}); for all subjects the predictions accounted for at least 95% of the variance in the data (solid circles). Data for the initial MD with natural pupil (open circles) are plotted for a retinal illuminance of 3.25 log td, corresponding to a 5.7 mm diameter at the group’s mean age of 33 years.

^{ 11 }For all subjects, the initial MD with natural pupil was within 3 dB of the predicted value (mean difference 0.1 ± 1.4 dB,

*t*= 0.27,

*P*= 0.79).

^{ 9 }The adaptation parameter (±1 SEM) was 2.53 (±0.24) log td for the mean FDT CS data of Membrey et al., and 2.32 (± 0.16) log td for our mean FDT MD data; this 0.19 log unit difference did not reach statistical significance (

*t*= 0.78,

*P*> 0.4). By comparison, adaptation parameters for fits to the Membrey et al.

^{ 9 }data for the standard size III perimetric stimulus and for letter contrast sensitivity (0.95 and 0.47 log td, respectively) were significantly lower than for FDT perimetry (

*t*> 3.9,

*P*< 0.005).

*K*= 2.32 log td), assuming natural pupil diameters from 2 mm to 7 mm and a standard 32-year-old lens density. All subjects had ΔMD within 1 dB of this predicted range. Note that only the adaptation parameter (log

*K*) and natural pupil diameter affect the predictions, because the sensitivity parameter (MD

_{max}) does not affect ΔMD.

^{ 11 }predicted ΔMD accounted for 69% of the variance in measured ΔMD. The right panel shows analysis of agreement

^{ 17 }: measured ΔMD averaged slightly greater than predicted (measured − predicted = 0.9 ± 1.5 dB,

*t*=2.41,

*P*= 0.028) and was independent of ΔMD (

*r*

^{2}< 2%,

*P*> 0.60).

^{ 8 }

^{ 9 }that sensitivity measured with FDT perimetry can be dramatically affected by change in retinal illuminance. The range of retinal illuminances we used was similar to that encountered clinically: for a population ranging from patients with small (2 mm) pupils and dense lenses to patients with dilated pupils and clear lenses, the expected clinical range of retinal illuminances is 2.1–3.4 log td. The range of mean retinal illuminances used with our subjects was 1.8–3.4 log td for Study 1 and 2.0–3.3 log td for Study 2.

^{2}.

^{ 11 }This normal variability represents a 0.9 log unit variation in retinal illuminance across observers; for the adaptation model this range of retinal illuminances corresponds to a 3.8 dB range in FDT sensitivity for subjects with clear lenses and a 5.7 dB range for subjects with dense lenses. When pupil diameter is <4 mm, and/or lens density is high, reduced retinal illuminance may produce moderate defects (4–6 dB). Between-subjects variability in pupil size presumably influenced the age norms used by the FDT perimeter; for the examples in Figure 1we set MD = 0 for the mean pupil diameters based on age norms,

^{ 11 }using mean lens density for each age.

^{ 14 }

^{2}mean luminance, FDT sensitivity should increase by no more than 1 dB under pharmacological mydriasis, even in eyes with relatively dense lenses. When the natural pupil diameter is small, effects of mydriasis on FDT MD should be substantial: 2–4 dB at 3 mm and 4–7 dB at 2 mm. By comparison, effects of normal moment-to-moment fluctuations in natural pupil size should have minimal effect on within-test variability: ±0.2 mm fluctuations around a 4 mm mean diameter should vary FDT sensitivity by no more than 0.5 dB, even for relatively dense lenses.

^{ 9 }who used equations provided by the manufacturer and computed average contrast sensitivity by converting dB values into luminance values. The adaptation parameter we derived by fitting the adaptation model to their group mean sensitivities was slightly higher than for our mean MD data, but this did not reach statistical significance. By comparison, the adaptation parameters for the fits to the data of Membrey et al.

^{ 9 }for conventional perimetry and for contrast sensitivity were much lower than for FDT perimetry. The clinically available MD index was sufficiently accurate for results of Study 1 to be consistent with Membrey et al.’s direct calculation of sensitivity, and for predicted ΔMD to account for most of the variance in measured ΔMD in Study 2.

^{2}) is imprecise. The FDT perimeter provides a warning if mean luminance drops 0.1 log unit below nominal luminance, and this never occurred during the experimental period. Variability in mean luminance during the course of the experiment was expected to be lower than the normal SD for the adaptation parameter, given the consistency between values of the adaptation parameters for our data and those of Membrey et al.

^{ 7 }concerning effects of temporal and spatial frequency on adaptation to mean retinal illuminance. They analyzed data on the mean retinal illuminance required for Weber’s law

^{ 20 }to hold, demonstrating a systematic increase in the adaptation parameter with spatial and temporal frequency. Temporal modulation for FDT perimetry is restricted to a high temporal frequency, so a high value for the adaptation parameter is expected. Conventional perimetry measures sensitivity to increments which include lower temporal frequencies in their amplitude spectra, consistent with a lower value for the adaptation parameter. Contrast sensitivity measured with letter charts corresponds to lower spatial and temporal frequencies than conventional perimetry, consistent with an even lower value for the adaptation parameter. At 25 Hz, the temporal frequency used by our FDT perimeter, Figure 6of Graham and Hood,

^{ 7 }indicates that a retinal illuminance of 3.1 log td is required for Weber’s law to hold. Our mean value of the adaptation parameter is consistent with their analysis, since a retinal illuminance of 3.1 log td gives sensitivity within 0.6 dB of maximum (MD

_{max}).

^{ 21 }The original motivation for use of a high temporal frequency for FDT stimuli was to tap nonlinear ganglion cells, but recent work has shown that nonlinear ganglion cells cannot mediate frequency doubling.

^{ 22 }We concur with the assessment of previous studies

^{ 8 }

^{ 9 }that use of lower temporal frequencies is advisable, and provide a method to quantify effects of retinal illuminance for new perimetric stimuli.

^{ 23 }When FDT is used in screening, it is important to recognize that MD values of −4 dB or worse may be caused by pupillary miosis and/or a dense lens. Furthermore, pupillary dilation can be expected to improve sensitivity by 2 dB or more.

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