Conversion factor for comparison of data from Humphrey and Medmont automated perimeters

Background: Clinical experience has shown that the sensitivity indices reported by the Humphrey Field Analyser (HFA) are generally higher than those given by the Medmont Automated Perimeter (M600). It is the purpose of this paper to determine a conversion factor for the two perimeters and to confirm this prediction using clinical data. Theory predicted that HFA(sensitivity) - 5 dB = M600(sensitivity). Methods: Sensitivity versus eccentricity profiles were measured over the central visual field on 10 young subjects using both perimeters. Results: Both the HFA and the M600 operate within the realms of the Weber law and measure similar Weber fractions. The sensitivity profiles had similar slopes (about -0.2 dB/degree) and were separated by about six decibels with the HFA reporting higher sensitivity values. This result confirmed the theoretical prediction. Conclusion: The difference in threshold sensitivities between the two perimeters is a result of differences in scaling factors and instrument luminances. A suggested clinical conversion factor is to subtract 5 dB from the HFA data to approximate those of the M600.