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A portable optical instrument for the measurement of macular pigment optical density


Purpose : To design and develop a compact, portable, stand-alone optical instrument, which can reliably measure the macular pigment optical density (MPOD). The instrument operates by measuring the reflectance of the ocular fundus at two different wavelengths at two retinal locations: the fovea and the perifovea. We validated the instrument by measuring the MPOD in-vivo in two human subjects.

Methods : The instrument uses light emitting diodes (LEDs) to project disks of different wavelengths on the ocular fundus on the fovea and the perifovea. The LEDs are driven by an Arduino 32-bit board producing frequency-division multiplexed pulses at the order of 1ms each. A diaphragm conjugated to the pupil plane allows illumination through a rectangular slit located off-axis to avoid specular reflections from the cornea. Four LED sources, two at 450nm and two at 550nm, are used to project sequential disks of angular size of 3 degrees at the fovea and at 8.1 degrees at the perifovea. Each LED produced a pulse of 1ms and the total time of the signal acquisition was 4.5ms. The light reflected at the fundus is collected by a high-speed photodetector and analyzed on the instrument’s microcontroller. In order to improve the signal to noise ratio, a total of 30 pulse trains are produced and the amplitudes of each pulse are averaged. The measurement is completed in 200 msec. Pupil alignment is achieved by a CMOS camera and a 3.2” TFT screen that is also used to display the results of the measurement. Τhe power of each LED reaching the cornea of the subject was 22μW, more than an order of magnitude below the maximum permissible exposure for the specific wavelength.

Results : The recorded signals had amplitudes between 0.1 and 1.0 Volts. The signal corresponding to the reflection of blue light from the macula was decreased. The resulting macular pigment optical density was 0.32 and 0.4 D.U. for the two subjects respectively. The instrument is already available for data collection in a larger populations of normal subjects and patients.

Conclusions : We have designed and built an optical, non-midriatic, portable instrument to measure MPOD by analyzing the spectral reflectance of the ocular fundus in 2 wavelengths in the fovea and the perifovea. As an initial validation the instrument was used to measure macular pigment in two subjects. This instrument may be a tool for MPOD screening in large populations.

This is a 2020 Imaging in the Eye Conference abstract.

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