Innovative Troxler-free measurement of macular pigment and lens density with correction of the former for the aging lens

Abstract

Simplified measurement of macular pigment optical density (MPOD) is important because of the ocular health benefits that are attributed to these retinal carotenoids. Here, we describe a novel instrument designed for this purpose, based on heterochromatic flicker photometry (HFP), which removes a number of difficulties that subjects often experience with traditional HFP. The instrument generates 1.5- and 15-deg diameter, centrally viewed stimuli that alternate between blue and green colors generated by light emitting diodes (LED). The 15 deg stimulus replaces the small, eccentrically viewed stimulus used in traditional HFP. Subjects adjust the blue LED intensity until flicker is eliminated in the case of the 1.5 deg stimulus and eliminated around the periphery in the case of the 15 deg stimulus. A microprocessor computes the subject's MPOD, in addition to the lens OD, and uses the latter to correct the MPOD. Good repeatability was confirmed through test-retest measurements on 52 subjects. The overwhelming majority of them stated that they found the test easy. The importance of the lens correction on MPOD measurements was confirmed in a simulation study. The study showed that, without the correction, MPOD would show an apparent age-related decline in a population for whom there was no real age dependence.

Macular Pigment and Lens Optical Density Measurements-Evaluating a Flicker Machine with Novel Features

“…The difference between the intensity settings for the foveal and the perifoveal flicker null settings can hence be considered to be a direct consequence of the presence of MP. 43 The other occurs if the subject, during the perifoveal testing, inadvertently looks directly at the flickering stimulus instead of the eccentric fixation mark while reporting the flicker null condition. In such cases, he/she would merely be repeating the foveal phase of the testing procedure.…”

“…However some instruments have an alternative approach to the above mentioned procedure. The alternative approach associated with perifoveal testing is minimizing or adjusting for a flicker null condition in the periphery of a larger centrally viewed stimulus either in the form of a full circle as used in the instrument described by Bone et al 43 or an annulus as described in Wooten et al 41 which replaces the previously tested smaller stimulus. The large stimulus replacing the smaller stimulus is typically of the order of 15º diameter in size.…”

“…Regardless of the type of stimulus used, the flicker null settings reported can be attributed to the presence of pre-receptoral filters aside from MP, primarily the lens. 43 Similar to the foveal measurements, the measurements are repeated at least three times. The difference between the intensity settings for the foveal and the perifoveal flicker null settings can hence be considered to be a direct consequence of the presence of MP.…”

“…Through instrumentation techniques, the frequency of flicker can be controlled by adjusting the angular frequency of a rotating chopper as described in Bone and Landrum 32 or electronically as described in Bone and Mukherjee. 43 To facilitate measurement, one of the component light sources of the stimulus has a peak wavelength close to the peak absorbance of the MP and the other with a peak at a region of the spectrum where the absorbance of MP is almost zero. 44 Such sources can be derived from the light emitted by a single Halogen lamp and passing the light through interference filters to obtain the desired wavelengths, 35 or from independent LED sources 43 .…”

“…43 To facilitate measurement, one of the component light sources of the stimulus has a peak wavelength close to the peak absorbance of the MP and the other with a peak at a region of the spectrum where the absorbance of MP is almost zero. 44 Such sources can be derived from the light emitted by a single Halogen lamp and passing the light through interference filters to obtain the desired wavelengths, 35 or from independent LED sources 43 . Heterochromatic Flicker Photometers that derive light solely from LED sources are termed as LED-based machines.…”