Age Related Macular Degeneration
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Fosse - (Fiber Optic Spectral Sensitivity Equipment)
Carden D, Simpson J, Parry NRA, and Murray IJ. A portable fibre optic device for measuring spectral sensitivities. Perception 19, 359, (1990).
This system, designed in the University in Manchester, allows spectral sensitivity of visual mechanisms to be measured in a free viewing situation. The unit is compact and portable and ideal for use in a clinical environment with untrained subjects.
By using selected spatial and temporal stimulus characteristics it is possible to examine, virtually independently, the sensitivity of the parvo- and magno-cellular visual pathways.
By applying a simple modification using LEDs the device also measured Macular Pigment, but still using the older technique first formulated by Frederick Ives in the early 1900s. (see Beatty et al 1999)
Heterochromatic Flicker Photometry (HFP)
In the early 1900s Frederick Ives developed the method of heterochromatic flicker photometry which proved to be more reliable than brightness matching and has become one of the standard photometric techniques for measuring properties of the eye.
In simple terms, two lights of different colours alternate at a reasonably fast rate. If they are of different luminance then flicker is seen. When the two lights are of equal luminance flicker is minimised.
Using HFP it is possible to assess the amount of Macular Pigment (MP) in the eye. The subject (person being tested) first looks directly at the centre of the flickering light and adjusts the ratio of blue and green intensities until minimum flicker is percieved. They then look eccentrically (eg at the X) so that the image of the flickering spot lies outside the central macula area were there is minimal Macular Pigment and once again adjusts the ratio of the Blue and Green lights until minimum flicker is again achieved. The ratio of the two settings gives an indication of the amount of Macular Pigment present. (See bottom of page for a simple example - Press Start)
This is the basis of most modern HFP devices currently available for measuring MP. Unfortunately it suffers a number of problems, not least being the problem of fading. If you look at a rapidly flickering small light the flicker soon fades and dissapears. This is known as the Troxler effect.
The new device, known as the Elektron Technology MPS II® (and QuantifEYE® in the USA), avoids these and many other major problems encountered using older HFP techniques.
This makes it a much more user friendly instrument and also makes it far easier even for the more elderly subject. So much so that many subjects are able to carry out their own measurement without the need of trained operators.