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William E. Smollon, Jr., Billy R. Wooten, Billy R. Hammond, Jr.; The Validity of Heterochromatic Flicker Photometry as a Measure of Macular Pigment Optical Density: Effects of Varying Stimulus Conditions and Additional Short-wave Absorbing Pigments. Invest. Ophthalmol. Vis. Sci. 2011;52(14):4899.
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Heterochromatic Flicker Photometry (HFP) is perhaps the most commonly utilized method for measuring the in vivo macular pigment (MP) absorbance. Validity of the method rests primarily upon the similarity of the HFP-derived relative spectral absorbance curve and those derived from ex vivo procedures. Below about 440 nm, however, the HFP data slightly underestimate the ex vivo measurements. This study examined possible sources for this discrepancy.
MP density was measured using the standard HFP method while (1) varying the intensity of the measuring wavelengths (2) varying the intensity of the normalizing wavelength and (3) varying the intensity of the background. Detailed spectral absorbance curves were also obtained and then differences with the ex vivo spectrum were analyzed.
Derived MP density across subjects did not change as a function of the intensity of the normalizing wavelength or background. Changes in the intensity of the measuring wavelength produced concomitant linear changes in MP density (e.g., MP adds linearly to other filters). Deviations from the ex vivo curve at the shorter wavelengths (405-440 nm) is well predicted by the existence of the P410 and P435 pigments originally postulated by Snodderly et al. (1984).
The HFP method of measuring MP optical density is robust to changes in stimulus conditions (most of which are obviated by the fact that the method relies on an eccentric reference). The discrepancy in the spectrum of macular pigment from the extinction spectrum of lutein and zeaxanthin may be due to the presence of additional short-wave absorbing retinal pigments (e.g., oxidized and reduced hemoglobin).
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