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Purpose: To investigate the effect of changes in retinal illumination on Frequency Doubling Technology (FDT). Methods: Five eyes, of 5 adults who were free from identifiable ocular pathology, were examined using the Snellen chart and the Pelli-Robson chart, conventional automated perimetry, and the full threshold N-30 program of FDT. Each test was performed with and without a 0.9, 1.5, or 2.4 log unit neutral-density (ND) filter placed before the eye. Furthermore, the influence of pupil diameter on FDT test results was compared after treatment with pilocarpine or cyclopentolate with the influence of ND filters. Results: All tests showed a decrease in sensitivity with decreasing retinal illumination. Frequency Doubling Technology showed an especially pronounced and significant decrease in sensitivity. The maximum mean threshold difference in FDT results with ND filter was 31.2 dB while that with the Humphrey Field Analyzer and the Pelli-Robson chart were 13.3 dB and 0.66 log contrast, respectively. The mydriatic state of the pupil increased the sensitivity of FDT and the miotic state decreased it to about the same extent as the the 0.9 ND filter. Conclusion: The change in retinal illumination has more impact on FDT than on spatial contrast tests and conventional automated perimetry. It is important to take this into account in evaluating FDT results.