Glare Testing &

Dark Adaptation

Hira Nath Dahal

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

Classification

– Veiling or disability glare

– Discomfort glare

– Specular reflection glare

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.

Glare in Cataract

Normal

Cataract

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.

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.

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

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

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.

Glare Testers

– Brightness Acuity Tester (BAT)

– Optec 1500 Glare Tester

– Miller-Nadler Glare Tester

– Terry Vision Analyzer (TVA)

Assessment of Glare

Brightness Acuity Tester

Miller-Nadler Glare Tester

Vector vision CSV-1000HGT with 1 test face

Stereo optical company optec 6500P

Oculus, Inc. Mesotest II

Dark Adaptation

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

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

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

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’

Retinal Distribution

– Peak density of rods occurs 20o from fovea– 150,000 rods/mm2

– 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/mm2

– Only 4% of total cones are foveal– Total Number: 6 million

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

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).

– 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.

Fig. Dark adaptation curve.

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

– 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

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

Intensity and duration of pre-adapting light

– 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.

– 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.

Size and location of the retina used

– 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.

– 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.

Wavelength of the threshold light

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

– 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

– 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.

Rhodopsin regeneration

Log relative threshold as a function of the percentage of photopigment bleached

– 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.

Dark Adaptation Testing

The Goldmann-Weekers Machine

The Goldmann-Weekers Machine

– 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.

– 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.

Normal Dark Adaptation curve

Typical normal and abnormal clinical dark adaptation curve

Typical normal and abnormal clinical dark adaptation curve

Dark Adaptation curve of Retinitis Pigmentosa

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