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Factors of Dark Adaptation Essay

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Dark adaptation: why can’t I see straight away when I turn the lights off?

‘When the subject/patient is exposed to a bright adapting light’1 causing the photo pigments to appear bleached by a biological pigment called Rhodopsin, this causes light to then turn off.
There are two types of photo receptors present in the Retina; cones and rods.
The rods are greater in quantity and are about 120 million. Rods are responsible for vision at low light levels, and aren’t sensitive to colour this is known as scotopic vision. The peak density of rods occurs about 20 degrees from the fovea. Rods usually take about 35 minutes to fully recover from sensitivity
‘Cones are active at higher light levels’, this is called photopic vision. There are about 6-7 million cones, they are split into three categories; Erythralobes, red (64%) that have maximum absorption at 565nm and are also known as long wavelength cones (l-cones). Chlorolabe, green (32%) have a maximum absorption at 535nm are also known as middle wavelength cones (m-cones). Cyanolabe, blue (2%) have a maximum absorption at 430nm, also called short wavelength (s-cones) that enable the eyes colour sensitivity. The red and green cones are mainly concentrated in the Fovea centralis which is located in centre of the macula of the retina and responsible for sharpening the central vision, this area consists of very thin densely packed cones and is a rod free region. The blue cones have maximum sensitivity and can be found mainly outside the fovea, which may lead to a few distinctions in the eye’s blue perception. Cones usually only take five minutes to recover from sensitivity.

There are four factors affecting dark adaptation;

Intensity and Duration of the pre-adapting light

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...ast and results in dimness of vision. One of the most prominent problems in assessing cataract vision loss is that a large number of cataract patients maintain good acuity, however, complain about their ocular ability. The true ‘real world’ functional vision of cataract patients can be established as a functional acuity score using contrast sensitivity and glare testing.

Contrast sensitivity is known to be very helpful in two areas of glaucoma evaluation. Research now shows that before treatment, glaucoma patients show signs of abnormal contrast sensitivity and this malfunction can then assess the presence or development of the disease. Post treatment, about 60-70% of the patients demonstrate clinically significant progress in Contrast sensitivity.
Contrast sensitivity can be used to assess the patient response to the beginning or to the change of therapy.







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