Contrast and colour Flashcards
What degree of contrast is tested on visual acuity charts e.g. Snellen and how does this differ in real life?
they test high contrast (black letters on white background), most tasks require resolution of low/medium contrast
What are 3 examples of contrast sensitivity charts?
- Vistech chart
- Pelli-Robson chart
- Cambridge chart
What is the Vistech chart?
employs rows of broken circles which decrease in contrast across the row and dmiminish in size from row to row. identification of target orientation is plotted on a template to give a graph of contrast vs spatial frequency
At what 2 distances can the Vistech chart be used?
- 45cm
- 3m
What is the Pelli-Robson chart?
triplets of capital letters, usually read at 1m, until 2 or 3 mistakes in one triplet
What is the Cambridge chart?
square wave gratings, usually read at 6m, forced choice at to which of 2 luminance-matched pages the grating is on
At what distance is the Pelli-Robson chart performed?
1m
What what distance is the Cambridge chart performed?
6m
In addition to Vistech, Cambridge and Pelli-Robson charts how do other contrast assessment charts work?
vary the visual acuity of the task at a specific contrast such as the high (~90%) and low (~10%) logmar chart
How do newer tablet-based charts for contrast vision work?
patients can outline with their fingers where contrast gratings are visible to them, allowing information from multiple spatial frequencies to be quickly assessed
What are 6 tests of colour vision?
- Testing red desaturation
- Ishihara pseudo-isochromatic plates
- Hardy-Rand-Rittler plates
- Holmes Wright lantern
- Farnsworth-D15 test
- Farnsworth-Munsell 100-Hue test
How can red desaturation be tested?
comapre perception of redness (e.g. of a red pin) between eyes, occluding one at a time -for central or peripheral vision
What type of disease can cause central vision red desaturation?
optic neuropathy
What type of disease can cause peripheral field red desaturation?
bitemporally reduced in chiasmal lesion
How can red desaturation be graded?
approximate score can be assigned by the
patient to the ‘washed-out’ image in relation to the normal image, e.g. 5/10.
At what distance and under what conditions are Ishihara pseudo-isochromatic plates used?
2/3m under good illumination in patients with VA >6/18
Who can see the first Ishihara test plate?
even achromatis with sufficient acuity can see it
What type of colour blindness do Ishihara plates test?
red-green confusion
What can some Ishihara plates distinguish between?
some differentiate whethere defect of the protan (red) or deutan (green) system - doesn’t test tritan system
What type of mistakes are made in congenital red-green colour blindness bs acquired colour blindness with Ishihara plates?
congenital=predictable, acquired=do not follow specific pattern
What is the advantage of Hardy-Rand-Rittler plates over Ishihara plates?
test tritan as well as protan and deutan discrimination
What is the Holmes Wright lantern?
test of binary choice of two or three coloured spots of light viewed at 6m - colours are red, green and white
What is the function of the Holmes Wright lantern?
is a more practical assessment that aims to predict red-green
discrimination in a work situation, e.g. distinguishing red and green lights
on a runway
What is the Holmes Wright lantern used in conjunction with for testing colour vision in military personnel?
Ishihara plates
What is the Farnsworth-D15 test?
screening test for vision - colour tile ordering test of confusion
How much information does the Farnsworth-D15 test given on protan, deutan and tritan systems?
limited information
What does the Farnsworth-Munsell 100-Hue test involve?
time-consuming colour tile ordering test of discrimination where patient attempts to order 85 coloured caps by hue
How can the Farnsworth-Munsell 100 hue test provide information on protan, deutan and tritan systems?
when plotted onto a dedicated chart
What is often used as the final arbitrator for colour vision-requiring professions?
Farnsworth-Munsell 100-Hue test
