Lecture 9 - Lecture 9 Lecture 9 Sensitivity of the Eye The...

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Lecture 9 1 © Jeffrey Bokor, 2000, all rights reserved Lecture 9 Sensitivity of the Eye The eye is capable of “dark adaptation.” This comes about by opening of the iris, as well as a change in rod cell photochemistry Min detectable flash:outside fovea 50-150 photons inside fovea ~150,000 photons Accommodation – Ability of eye to focus (automatically) The relaxed lens focuses far (infinity). The lens “accommodates” to focus near. Amount of accommodation:10 diopters at age 20 ~2 diopters at age 60 Myopia (nearsightedness) – lens power too large, or eyeball too long The myopic eye can only accommodate between a far point and the near point. This can be corrected by a negative lens, chosen so that an object at infinity has a virtual image at the far point. 10 5 10 20 30 t (min) least perceptible 10 2 10 3 10 4 brightness 10 from fovea fovea only in the dark, the fovea becomes a blind spot at maximum power of the eye, the closest image plane occurs at the “near point” near point far point
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Lecture 9 2 © Jeffrey Bokor, 2000, all rights reserved Hyperopia (farsightedness) – too little power in lens, or the eyeball is too short In this case, the near point is too far for comfort. It is corrected with a positive lens. Presbyopia As we age, the eye loses the ability to accommodate. This is why “reading glasses” are used. Astigmatism – Shape of cornea is not radially symmetric. – Focal power is different along 2 orthogonal axes.
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Lecture 9 - Lecture 9 Lecture 9 Sensitivity of the Eye The...

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