Development of a Novel
Portable Instrument for Measuring Macular Pigment
Hammond, Wooten & Snodderly
(1997) suggested that Age Related Macular Degeneration (ARMD) might be
correlated with the Macular Pigment Optical Density (MPOD). Thus it may
be clinically useful to know a person's MPOD in order to prognosticate
maintenance of visual function. Macular Pigment (MP), which is yellow,
is a mixture of carotenoids only obtainable from the diet. Knowledge of
a patient's MPOD could be used to advise about diet in order to improve
protection against long-term oxidative light damage. A practical
screening method for determining MPOD in a clinician's office is
therefore desirable.
Heterochromatic Flicker Photometry
(HCFP) is a well established technique for measuring MPOD (Werner
& Wooten, 1979). A flickering green light, which is not
absorbed by MP, is matched in a test field with an adjustable
flickering blue, which is absorbed by MP (the green and blue lights
flicker in counter phase). The matches are made either with the retinal
image of the test field on the fovea (Case 1) or on the parafovea (Case
2) outside the pigmented area. At the match point of minimum perceived
flicker the brightness of the blue and the green will be the same. The
blue light is absorbed by the yellow MP so in Case 1 the subject needs
more blue than in Case 2. Provided that the green is constant it
follows that:
MPOD = log B1 - log B2
where B1 and B2 are the mean
blue luminance values.
The flicker frequency must be high
enough not to stimulate the rods - above about 12 Hz. The blue-green
match is made with the medium and long wave cones which are assumed to
be evenly distributed across the central areas of the retina. Any
contribution of the short wave cones and the rods to the matches is
prevented by adapting them with a blue background
A novel method of obtaining the
stimuli and background is to use Light Emitting Diodes (LED's) as
sources. We have demonstrated that a small portable device based on
this principle can successfully measure MP density.
The measures of MPOD show distinct
differences between individuals which are consistent from day to day: a
few subjects have also been measured by an objective method which
yielded values similar to those from the LED device.
By using a novel combination of
cheap, commercially available LED's and suitable optical techniques we
have made a practical system for measuring MPOD which is small and
portable which would be suited to automated clinical screening in a
wide range of environments. Several of these small screening
"maculometers" have been used in London, across Europe and in Austalia
and have measured the MPOD in many hundreds of eyes. The
instrument was described at severasl meetings from 1998 and
described fully in 2002 (Mellerio et al) but was
not commercially available.
More information about the maculometer may be seen
on my macular
pigment pages.
Currently, a different approach has been adopted, using apparent motion
photometry (AMP) and a CRT monitor driven by 14 bit resolution
software: this is commercially available and is described at http://www.crsltd.com/research-topics/macular-pigment/index.html.
References
Mellerio, J., Ahamdi-LarI, S., Van
Kuijk, F.J.G.M., Pauleikhoff, D., Bird, A.C. & Marshall, J. A portable instrument for measuring
macular pigment with central fixation Current Eye Research,
25:37-47, 2002
Hammond, BR, et al. Individual variations in spatial profilr of human
macular pigment. J. Opt. Soc. Am. A., 14:1187-1196,
1997
Werner, JS & Wooten, BR. Opponent chromatic mechanisms etc. J.
Opt. Soc. Am., 69:422-434, 1979
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