Glare testing and dark adaptation

description about glare testing and dark adaptation

1. Glare Testing
&
Dark
Adaptation
Hira Nath Dahal

2. Glare
–Refers to the presence of one or more
areas in the field of vision that are of
sufficient brightness to cause discomfort in
vision.
–Visual perception created by external light
–Glare source : Axial / Peripheral
–Reduces the quality of the image
– an unpleasant sensation
–a temporary blurring of vision
–a feeling of ocular fatigue

3. Classification
–Veiling or disability glare
–Discomfort glare
–Specular reflection glare

4. Veiling or disability galre
–Arises from stray light falling on the retina, usually
from scatter by the media of the eye.
–Scattered light falls as a patch of veiling illuminance
on the fovea and reduces the contrast of the retinal
image.
–Reduces the contrast of the retinal image.
–Reduces visibility and visual performance.
–E.g. sky, sand, brightly illuminated walls etc.- the
reflected images are large in angular subtense leading
to reduction in contrast observed in the visual field.

5. Glare in Cataract
Normal
Cataract

6. Discomfort glare
–Occurs when the illumination in a part of the
visual field is much greater than the level of
illumination for which the retina is adapted.
–Occurs when the ratio between the highest level
of illumination in the visual field and background
illumination exceeds a ratio of 3 to 1.
–An extreme case of glare often occurs during
night driving-causes extreme discomfort.
–Importance of having background illumination
while watching television.

7. Specular reflection glare
–Occurs when patches of bright light are
reflected form smooth, shiny surface into
the eye.
–Typical reflecting surfaces include
expanses of water, snowfields, roadways
etc.
–Reflections are not only annoying but
interfere with visibility, at times seriously.
–Can be well controlled by using polaroid
glasses.

8. Glare testing
–Objective :
– quantify the deleterious effects of light scatter on visual
performance
–Reduce the effect on impairment of vision
–When?
–Corneal opacities
–Corneal dystrophies/ Degeneration
–Cataract

9. Pre& post-operative
indications for glare testing
Pre-operative Post-operative
Cornea Cornea
–Infectious scarring-PK
–Traumatic scarring -Epikeratophakia
–Degenerative scarring-Keratomileusis
–Dystrophic scarring -Repaired laceration
Lens Lens
–Age-related cataract -PCO following ECCE,IOL
–Traumatic cataract
–Drug-induced cataract
–Disease-induced cataract

10. Glare Testers
–Instrumentation, theory &use
–A glare source when introduced in an eye with
media opacity causes some degree of visual
disability.
–Current glare testing devices gives this extent
of disability in the form of reduced contrast
sensitivity or visual acuity.

11. Glare Testers
–Brightness Acuity Tester (BAT)
–Optec 1500 Glare Tester
–Miller-Nadler Glare Tester
–Terry Vision Analyzer (TVA)

12. Assessment of Glare
Brightness Acuity Tester

13. Miller-Nadler Glare Tester

14. Vector vision CSV-1000HGT with 1 test face

15. Stereo optical company
optec 6500P

16. Oculus, Inc. Mesotest II

17. Dark Adaptation

18. Rods
–% Wavelengths of 507 nm are most readily
absorbed by rhodopsin
–When a molecule of rhodopsin absorbs one
quanta of light, it is ‘bleached’
–Bleached: the molecule is not capable of capturing
another quantum
–Spontaneously become ‘unbleached’
–50 recover within 5 minutes

19. Cone Photopigments
–There are 3 types of cone photopigments:
–Erythrolabe: maximum absorption at 565 nm
–‘Long wavelength cones’ (L-cones), red cones
–Chlorolabe: maximum absorption at 535 nm
–‘Middle wavelength cones’ (M-cones), green cones
–Cyanolabe: maximum absorption at 430 nm
–‘Short wavelength cones’ (S-cones), blue cones
–Recover from bleaching more rapidly than
rhodopsin
–50% of cones will recover within 1.5 minutes

20. Visual Thresholds
–The minimum amount of energy required
for a patient to detect a stimulus
–low threshold = high sensitivity
–Threshold = 1/Sensitivity
–Scotopic Threshold: threshold of a patient
measured in dim light conditions (night)
–Photopic Threshold: threshold of a
patient measured in bright light conditions
(sunny

21. Purkinje Shift
–Scotopic System: Stimuli of 507 nm are
perceived brighter than other stimuli
–Photopic System: Stimuli of 555 nm are
perceived brighter than other stimuli
–The difference in the peak sensitivity of
the 2 systems is the ‘Purkinje Shift’

22. Retinal Distribution
–Peak density of rods occurs 20
o
from fovea
–150,000 rods/mm
2
–No rods are present at the fovea: We are unable to
see small, dim objects when foveally fixated
–Total Number: 120 million
–Cones are most densely packed at the fovea
–150,000 cones/mm
2
–Only 4% of total cones are foveal
–Total Number: 6 million

23. Specific Cone Distribution
–Ratio of L-cones to M-cones = 2/1
–S-cones are less numerous than either L-cones or
M-cones
–No S-cones are at the fovea
–Peak distribution occurs just outside the fovea
–We are unable to see small blue objects when centrally
fixated

24. Introduction
–Sensitivity measured by determining the
absolute intensity threshold.
–Refers to how the eye recovers its
sensitivity in the dark following exposure
to bright lights.
–First person to estimate the threshold
stimulus in dark- Aubert(1865).

25. –forms the basis of the Duplicity Theory which states
that above a certain luminance level (about 0.03
cd/m2), the cone mechanism is involved in
mediating vision; photopic vision.
–Below this level, the rod mechanism comes into play
providing scotopic (night) vision.
–The range where two mechanisms are working
together is called the mesopic range, as there is not
an abrupt transition between the two mechanism.
–dark adaptation curve depicts this duplex nature of
our visual system.

26. Fig. Dark adaptation curve.

27. Dark adaptation curve
–Dark adaptation curve shows this duplex nature of
our visual system.
–One way to demonstrate that the rod mechanism
takes over at low luminance level, is to observe the
color of the stimuli.
–The first curve reflects the cone mechanism.
–The sensitivity of the rod pathway improves
considerably after 5-10 minutes in the dark and is
reflected by the second part of the dark adaptation
curve

28. –From the above curve, it can be seen that initially
there is a rapid decrease in threshold, then it declines
slowly.
–After 5 to 8 minutes, a second mechanism of vision
comes into play, where there is another rapid
decrease in threshold, then an even slower decline.
–The curve asymptotes to a minimum (absolute
threshold) at about 10
-5
cd/m2 after about forty
minutes in the dark

29. Factors Affecting Dark
Adaptation.
–Intensity and duration of the pre-adapting light

–Size and position of the retinal are used in
measuring dark adaptation
–Wavelength distribution of the light used
–Rhodopsin regeneration

30. Intensity and duration of
pre-adapting light

31. –Dark adaptation depends upon differing intensities
and duration of pre-adapting light.
–With increasing levels of pre-adapting luminances,
the cone branch becomes longer while the rod
branch becomes more delayed.
–Absolute threshold takes longer time to reach.
–At low levels of pre-adapting luminances, rod
threshold drops quickly to reach absolute
threshold.

33. –The shorter the duration of the pre-adapting
light, the more rapid the decrease in dark
adaptation.
–For extremely short pre-adaptation periods, a
single rod curve is obtained.
– It is only after long pre-adaptation that a bi-
phasic, cone and rod branches are obtained.

34. Size and location of the
retina used

35. –The retinal location used to register the test spot
during dark adaptation will affect the dark
adaptation curve due to the distribution of the
rod and cones in the retina.
–When a small test spot is located at the fovea
(eccentricity of 0o), only one branch is seen with
a higher threshold compared to the rod branch.
–When the same size test spot is used in the
peripheral retina during dark adaptation, the
typical break appears in the curve representing
the cone branch and the rod branch.

37. –When a small test spot is used during dark
adaptation, a single branch is found as only cones
are present at the fovea.
– When a larger test spot is used during dark
adaptation, a rod-cone break would be present
since the test spot stimulates both cones and rods.
–As the test spot becomes even larger, incorporating
more rods, the sensitivity of the eye in the dark is
even greater, reflecting the larger spatial
summation characteristics of the rod pathway.

38. Wavelength of the threshold light
–When stimuli of different wavelengths are used,
the dark adaptation curve is affected.

39. –From figure, a rod-cone break is not seen when using light of
long wavelengths such as extreme red.
–This occurs due to rods and cones having similar sensitivities
to light of long wavelengths

40. –This curve depicts the photopic and scotopic
spectral sensitivity functions to illustrate the point
that the rod and cone sensitivity difference is
dependent upon test wavelength (although
normalization of spatial, temporal and equivalent
adaptation level for the rod and cones is not present
in this figure).
–On the other hand, when light of short wavelength
is used, the rod-cone break is most prominent as
the rods are much more sensitive than the cones to
short wavelengths once the rods have dark adapted.

41. Rhodopsin regeneration
Log relative threshold as a function of the percentage of photopigment bleached

42. –Dark adaptation also depends upon photopigment
bleaching
–Using retinal densitometry, it was found that the
time course for dark adaptation and rhodopsin
regeneration was the same.
–Bleaching rhodopsin by 1% raises threshold by 10
(decreases sensitivity by 10)
–Bleaching of cone photopigment has a smaller
effect on cone thresholds.

43. Dark Adaptation Testing

44. The Goldmann-Weekers
Machine

45. The Goldmann-Weekers
Machine

46. –Depends on the increase in visual sensitivity
occurring in the eye when it goes from the
light adapted state to dark adapted state.
–Pre-adaptation is important for normalisation
and to ensure a bi-phasic curve is obtained.
–Subjects gaze at a pre-adapting light for 5-
10mins and then absolute threshold is
measured.
–Wavelength of light is 420nm.

47. –At intervals of 30secs, a measurement of light threshold is
made in one area of the VF by presenting a gradually
increasing light stimulus until it is barely visible to eye.
–Graph of decreasing retinal thresholds against time shows
and initial steep slope denoting cone adaptation and a
subsequent gradual slope due to dark adaptation.
–Depression of the curve occurs in conditions affecting the
outer retina and RPE; such as Retinitis Pigmentosa.

48. Normal Dark Adaptation
curve

49. Typical normal and abnormal
clinical dark adaptation curve

50. Typical normal and abnormal
clinical dark adaptation curve

51. Dark Adaptation curve of
Retinitis Pigmentosa

52. Management
–Absorptive glasses-
–Fixed tint
–Wear before you expose to bright light
and take out just entering inside room or
shaded area- to protect the rods getting
bleached