This document discusses light and dark adaptation in the eye. It describes how the eye adjusts to bright light through light adaptation over 1-10 minutes as cones take over from rods. Dark adaptation is the eye's ability to recover sensitivity in low light after exposure to bright light, which can take rods up to 30-35 minutes. The mechanisms that allow vision over a wide range of light levels include pupil changes, rods and cones in the duplex retina, dark adaptation, and light adaptation of the visual system. Dark adaptometry is used clinically to diagnose retinal disorders by measuring dark adaptation recovery curves.

1. Light & Dark Adaptation
Trisruta Deb
M.Optom (P.G Scholar)
BV(DU) MC School of Optometry, Pune

2. Light Adaptation
• Light adaptation is the ability of the eye to adjust in bright
light.
• Within about one minute the cones are sufficiently excited by the
bright light to take over.
• Visual accuracy and colour vision continue to improve over the next
ten minutes. During light adaptation rod sensitivity is lost.
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3. Dark Adaptation
• Ability of the eye to adapt itself in low illumination after
immediate light exposure.
Or,
• Dark Adaptation is the ability of the eye to recover its
sensitivity in the dark after being exposed to bright light,
making vision possible in relative darkness.
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4. Let’s take an example
• Most of us have had the experience of entering a dark movie
theater on a sunny afternoon. Immediately on entering the
theater, we are virtually blind. Yet, after several minutes,
vision recovers to the point where we can walk down and find
an empty seat. This gradual improvement in vision, after
exposure to a bright adapting light (in this case the sun), is
referred to as dark adaptation.
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5. Trisruta Deb, M.Optom 5
After 35 minInitial phase

6. Responsible retinal cells for vision
• Rods & Cones
• Bipolar cells
• Ganglionic cells
Retinal rods are responsible for dark adaptation & cones are
responsible for light adaptation.
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7. Rod & cone operating ranges
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cone vision
day
LUMINANCE RANGE
rod vision

8. Key points about Rods
• The human retina contains approximately 120 million rods.
• Rhodopsin (opsin+ retinal) are the photo pigments present in
the rod cells.
• There are no rods in the fovea.
• Rods are responsible for vision at low illumination (scotopic
vision).
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9. Key points about Cones
• The human retina contains approximately 6.5 million
cones.
• Photopsins (protein opsin)are the photo pigments
present in the cone cells.
• Cones are most densely packed at fovea.
• Cones are much more sensitive in well lit condition, thus
cones are used in light adaptation.
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10. Structure of Rods & Cones
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11. Pupil Changes during bright/dim light
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12. The sensitivity curve
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12
• Rods take up to 30-35
min to fully recover
sensitivity.
• Rods are much more
sensitive in low
illumination than
cones so rods are used
more in dark
adaptation.
Low Threshold = High sensitivity
Low Threshold
High Threshold
100000
10000
1000
100
10
0

13. Dark Adaptation
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14. Mechanisms that enable us to see over a
wide range of light intensities:
• Pupil changes
• Duplex retina: rods & cones
• Dark adaptation & pigment bleaching
• Light adaptation of the visual system
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15. Duplex function of retina
1. Scotopic vision, occurs under dim (night-time) lighting conditions.
2. Photopic vision, which occurs under bright (daytime) lighting
conditions, cone cells shows poor sensitivity to dim lights; however,
it is characterized by both excellent visual acuity (20/20) and colour
discrimination. Cones mediate Photopic vision.
The existence of two classes of photoreceptors, each operating under
different lighting conditions, leads to what has been referred to as a
duplex retina.
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16. Purkinje Shift
• Rods are relatively more sensitive to the short wavelengths (507
nm), whereas the cones are relatively more sensitive to the longer
wavelengths (555nm). This difference in spectral sensitivity is
named the Purkinje Shift.
• A shift in the color appearance at dusk.
• Reds look darker (during low illumination),
• Blues look brighter ( during low illumination)
• At a time in low illumination all color appear as grey
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17. Trisruta Deb, M.Optom 17

18. Blue hour- Mesopic vision (Rod & Cone mediated vision)
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19. Clinical significance
Dark adaptation has proven useful in the clinical diagnosis of various retinal
disorders.
Disease affecting dark adaptation curve:
1.Oguchi’s disease (congenital stationary blindness)
2. Choroideremia (an X-linked chorioretinal dystrophy)
3. Diabetic retinopathy
4. Retinitis pigmentosa
5. Glaucoma
Refractive error dark adaptation:
1. Myopia
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20. Factors affecting dark adaptation related to individual
• Vitamin A deficiency
• Effects of anoxia
• Effects of tobacco inhalation
• Effects of opacities in ocular media
• Effect of anaesthesia (halothane anaesthesia)
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21. Dark Adaptometry
• A type of retinal testing done to diagnose and manage retinal
degeneration by measuring the length of time it takes for the
retina to regain maximal sensitivity to low amounts of light
after it has been exposed to bright light then returned to
darkness.
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22. Measurement
Fig: Goldmann Weekes
Dark Adaptometer
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23. Trisruta Deb, M.Optom 23
High
Low
Low Threshold = High sensitivity

24. Technique of Goldmann Weekes Adaptometry
1
• Subject is exposed to an intense light
• Which bleaches the photoreceptors
2
• Then suddenly placed in the dark
• The threshold of light which the subject can perceives is plotted
3
• Flashes are repeated at regular intervals;
• Sensitivity of eye to light gradually increases.
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25. Trisruta Deb, M.Optom 25
High
Low
Low Threshold = High sensitivity

26. The sensitivity curve
• First section(CONE):
-rapid threshold reduction(5 min)
-this represents the cone thresholds
• Cone rod break:
- break in curves that occur after 7-10 min of
adaptation, when cones achieve their maximum
sensitivity & rods become noticeably sensitive
• Second section:
-reduction in threshold that extends to 35 min
-this represents the rod thresholds
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27. References
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