Week 6: Contrast Sensitivity & Colour Vision Flashcards
Describe Contrast Sensitivity
- Ability to detect differences in luminance between regions that are not separated by physical borders
List 5 Clinical Reasons to Test Contrast Sensitivity
- Ocular disease
- Low vision
- Contact lenses
- Dry eye
- Glare issues
Define Contrast (Luminance Contrast) and what is its formula?
- Is the relationship between the luminance of a brighter area of interest & that of an adjacent darker area
Weber Contrast – most commonly useful one in the context of lighting:
C = (Lmax - Lmin) / Lmin
Define Simple Contrastand what is its formula?
- Values are often used in photography, to specify the difference between bright & dark parts of the picture
Csimple = Lmax / Lmin
Define Peak to Peak Contrast (Michelson Contrast, Modulation) what is its formula?
- Measures the relation between the spread and the sum of the two luminances
To determine the quality of a signal relative to its noise level:
Modulation = (Lmax - Lmin) / (Lmax + Lmin)
Define Fourier theory for Sine Wave Gratings
- Any pattern in the visual field can be derived from the summation of a series of sinusoidal patterns of specific frequencies, amplitudes, orientations and phases
What is the formula for Spatial Period ?
Width of one cycle = wavelength
What is the formula for Spatial Frequency?
Cycles/degree
What is the formula for Mean Luminance
Average of maximum & minimum luminance = (Lmax + Lmin / 2)
What is the formula for Modulation Amplitude
Difference between maximum & mean luminance = (Lmax - Lmin) /(Lmax +Lmin)= contrast
What happens when low modulation occurs?
- Low modulation = low contrast
What is the Non-repetitive backgrounds/background formula that remains constant?
- Weber contrast: Brightness difference/background
CW = (Lmax – Lmin)/Lmin
What is the formula for repetitive/changing patterns?
- Average brightness/brightness amplitude
- Modulation CM= (Lmax – Lmin)/ (Lmax +Lmin)
What charts would suit variable size / variable contrast?
- Vistech Chart
- Functional Acuity Contrast Test (FACT)
What charts would suit variable size/fixed contrast?
- Bailey-Lovie low contrast chart
What charts would suit fixed size / variable contrast
- Pelli-Robson Chart
- Melbourne Edge Test
Describe Vistech Chart
- No literacy required
- 5 rows with gratings (1-24 cycles/degree)
- 9 contrast levels with an average step size of 0.25 log units
- Patient indicates whether the patch is blank or which direction the grating is tilted
Describe Functional Acuity Contrast Test (FACT) Chart
- Similar to Vistech but has smaller contrast steps of 0.15 log units
- Size of the gratings are larger
List the Grating Method Testing
- Place chart at eye level 3 m from patient
- 60 to 120 cd/m2 even lighting illumination
- Demonstrate high contrast samples at bottom of chart to teach
- Test monocularly
- Ask patient to read from top & across each line left to right and identify orientation of lines
- Ask patient to guess at the orientation of the lines until they can no longer see the gratings
- Record the patch with the lowest contrast correctly identified & compare with normal range
What are the benefits of Grating Chart?
- Assessing functional vision
- Clinical research
- Illiterate patients or children
Describe High & Low Contrast Bailey-Lovie Charts
- Fixed number of letters per line
- Logarithmic progression of letter sizes and spaces
- Standardised letter set & scoring system
Describe Pelli-Robinson Chart
- Uses letter targets
- Letters composed of complex mixture of tests
- Involves recognition task
- The contrast of each successive group decreases by 0.15 log units
- Score 0.05 log for every letter read correctly
Describe Melbourne Edge Test
- Non-grating CS test
- Test arranged in 4 rows each with 5 disks
- Each disk contains an edge, which decreases in contrast from top to bottom of chart
- Patient must indicate direction of edge in each disk
What is used due to glare sensitivity?
- Measurement of effect of glare on patients’ visual function using CS chart and brightness acuity tester (BAT)
What are the types of glares? (hint 2)
- Disability glare: causing reduction of visual performance
- Discomfort glare: causing discomfort without any
Causes of Sensitivity to Glare? List 5
- Patients with cataracts
- Intraocular lenses
- Contact lenses
- Ocular disease
- Older age
What is Brightness Acuity Tester (BAT)?
• Hand-held instrument with two functions
- Brightness acuity test
- Macular photostress test
What are the three conditions Brightness Acuity Tester (BAT) can simulate?
- High – direct overhead sunlight,
- Medium/partly cloudy day
- Low – bright overhead commercial lighting
List the steps on how to use a Brightness Acuity Tester (BAT)
- Test monocularly
- Should be used at all 3 settings if possible
- Patient seated at standard distance from letter chart
- Undilated pupils in dark examination room
- Test and record visual acuity at each of the three settings
Describe Berkeley Glare Acuity Tester (BGAT)
- Reduced high and low contrast Bailey Lovie letter chart
- Chart is front illuminated and glare source provided by transillumination of plexiglass panel
- High and low contrast VA measured at 1 m with & without glare source
What is colour?
- A perceptual construct from retinal inputs to the primary visual cortex
What is Colour Good For?
- Originally evolutionary advantage
- Segmenting the visual world
- Signalling
- Traffic lights
- Ripeness of fruits
- Complex colour codes
What is the spectrum measurement for visible colour?
380 – 780 nm
What are the perceptual dimensions of colour?
- Hue
- Saturation/chroma
- Brightness/value
Describe Hue
Colour (based on wavelength)
Describe Saturation/Chroma
- Intensity of hue
- Quality of a colour in terms of purity/intensity/saturation
Describe Brightness/Value
- How much is reflected and/or emitted that enters the eye
- Darker colours have a lower value
Describe the young-Helmholtz Trichromatic Theory
• Humans have 3 cone types
- 3 different peak λ absorption
- different spectral sensitivities
• Cones are colour blind
What are the limitation for Trichromatic Theory?
- Fails to account for the four unique colours; red, green, yellow and blue
Describe Hering’s Colour Opponent Theory
- Colour is processed by bipolar hue channels – red/green or blue/yellow
- Brightness is coded by a white/black channel
- The theory explains how we see yellow even though there is no yellow cone
What are the wavelengths for rods, S, M & L peak sensitivity?
- Rods 493 nm
- S peak sensitivity = 430 nm
- M peak sensitivity = 530 nm
- L peak sensitivity = 560 nm
Describe trichromacy
- Individual cones do not transmit wavelength information
- Photoreceptors are blind & tend to have different sensitivity levels
- These three colours can then be combined to form any visible colour in the spectrum
Describe what is in photopigments?
- Photopigment consists of a chromophore chemical and an opsin (protein)
What are the different opsins?
- S-cone opsin
- M-cone opsin
- L-cone opsin
- Rhodopsin
Explain the CIE Chromaticity Diagram
- Created in 1931, a map that shows what colours can be perceived by the human eye
- Each colour has an x, y, z coordinate
- CIE primaries are imaginary
Briefly describe the statistics of colour vision anomalies
- Approx 4.5% of people have colour vision anomalies
- 8% males, 0.5% females
- Most are inherited, some acquired
Describe what is missing from Anomalous Trichromacy Cone Pigment Anomaly
Refer to week 5 table
What are two categories of colour vision anomalies?
- Dichromacy (missing)
- Anomalous Trichromacy (displacement)
Describe Anomalous trichromats
- Have 3 photopigments, but their colour vision is abnormal
- The greater the displacement of the photopigment the worse the colour vision problem
What are the general features of Anomalous trichromats?
- Can see colours, just perceived differently
- Problem with mixture of red and green
- May be unaware of any colour deficiency
Describe dichromatic
Have only two photopigments, 1 missing
List the colour deficiencies for dichromatic
- Deuteranopia
- Protanopia
- Tritanopia
Describe Deuteranopia
- Red-Green deficiency
- Chlorolabe is missing (Green cone opsin) – missing green pigment
- No confusion with yellow and blue
- Sees green objects black
Describe Protanopia
- Red-Green deficiency
- Erythrolabe is missing – missing red pigment
- No confusion with yellow and blue
Describe what Protans & Deutans have in common
- Red-green dichromats are monochromatic for wavelengths beyond 545 nm
- However they can label colours very well (context of object or brightness of object)
Describe Tritanopia
- Blue-yellow deficiency (Cyanolabe) – s cone pigment
- Very rare
- Cannot distinguish blue and yellow
- No issue with red or green
Describe Monochromacy
- Only one photopigment
- Rod monochromacy or cone monochromacy
Describe Strategies to Overcome Monochromacy
- Labelling based on brightness
- Intensity of image used to discriminate colour
Describe achromatopsia
- Rare condition
- Patients will have monochromatic vision
- Commonly Autosomal recessive
- Complete achromatopsia – only rods present; see in Black and white
- Incomplete achromatopsia – only Long or/& Medium cone function
What are the Symptoms of Achromatopsia?
- No/poor colour discrimination
- Photophobia
- Poor vision (6/60)
Describe Treatment for Achromatopsia
- Dark red lenses can be prescribed to minimise bleaching of rhodopsin & allow for rods to function in brighter lighting conditions
Describe chromatopsia
- Patients complain of distortion of colour vision with a coloured tinge/halo
- Often can occur following removal of cataracts
Inherited vs Acquired Colour Vision Defect
- Inherited anomalies are bilateral and symmetric
- Acquired anomalies can be unilateral or asymmetric
- Acquired anomaly = if there is a colour vision difference between the pair of eyes
Describe Kollner’s Rule
- Outer retinal disease and changes in media causes blue-yellow colour vision anomalies
- Red-green anomalies
- Disease of inner retina, optic nerve, visual pathway & visual cortex causes
List three colour vision tests
- Pseudoisochromatic tests (e.g. Farnsworth D-15)
- Hue discrimination (e.g. Farnworth Munsell)
- Colour matching
What does the Ishihara detect?
- Screening for protan and deutan defects only
