The following example essay on “Contrast Sensitivity Measurements” talks about an experiment that would help was to find if comparable contrast sensitiveness measurements could be taken utilizing different psychophysical methods.
Contrast Sensitivity is defined as the ability of the oculus to observe luminosity contrast ” . ( Millodot, M, 2009 ) It is of import as it makes objects stand out from one another. Contrast Sensitivity proving provides a better apprehension of the patients public presentation of real-world undertakings than ocular sharp-sightedness measurings. ( Lay et al, 2009 )
It is besides a more dependable manner of observing ocular tract alterations than VA.
VA detects alterations caused by spherical fuzz, nevertheless non all abnormalcies are caused by this, such as, cataracts, glaucoma or diabetic retinopathy ( Vectorvision.com 2010 ) . On the other manus, Contrast Sensitivity trials are sensitive plenty to place elusive alterations in patients. ( Lay et al, 2009 )
Poor contrast sensitiveness may do troubles while utilizing stepss or stepping off kerbs as patients may happen it hard to see the borders due to them being of the same contrast.
Night clip drive may besides be hard as they would hold trouble seeing walkers in ailing illuminated countries and may hold trouble with traffic visible radiations. ( Stephenson, M, 2010 )
We tested the Contrast Sensitivity Function which is defined as the graphical representation of contrast sensitiveness as a map of spacial frequence ” . Spatial Frequency is defined as rate of change of the luminosity in a ocular stimulation as a map of length, expressed in rhythms per grade ” . ( Millodot, M, 2009 ) There are a figure of ways to mensurate threshold ; Method of Ascending Limits, Method of Descending Limits, Method of changeless stimulations, Method of accommodation and Forced pick method.
In the Method of Ascending Limits a stimulation is presented to the perceiver at an strength lower than their threshold and this strength is increased until the perceiver can see it. The Method of Descending Limits is the opposite. The stimulation is presented at a degree above threshold until the perceiver can no longer see it. The stairway method uses both these rules. The strength of the stimulation is increased until the perceiver can see it and recorded. It is so decreased until the perceiver can non see it. An norm of these reversals is taken as the threshold. ( Kalloniatis et al, 2010
Staircase method. Y = Yes, stimulation can be seen and N = No, stimulation can non be seen.
In the Method of Constant stimuli the stimulation is presented within a scope of values and the perceiver responds whether “ Yes ” they can see it or “ No ” they ca n’t. Accurate replies range from 0 % to 100 % . ( Kalloniatis et al, 2010 )
Psychometric map for a YES-NO paradigm.
The topic changes the stimulation strength until it is merely seeable in the Method of Adjustment. This allows for a speedy and efficient acquisition of contrast threshold. ( Kalloniatis et al, 2010 )
The Forced Choice Method gives the topic a limited figure of picks, one of which is the stimulation. The greater the figure of picks the less likely the topic will be able to think the correct reply and will do the consequence more dependable. In a two option forced pick, the topic already has a 50 % opportunity of acquiring it right and the threshold is hence considered 75 % as seen in the diagram below. ( Scwartz, S, 1999 ) ( Kalloniatis et al, 2010 )
Psychometric map for 2AFC. Threshold is taken at the 75 % seen degree.
The trials we used for our experiment were the Pelli-Robson chart, the Freiburg Landolt-C Contrast Sensitivity Test and The Gabori Contrast Sensitivity Test. The Pelli-Robson Chart is a 26-alternative forced-choice method, as any of the 26 letters of the alphabet can be used. ( Lay et al, 2009 ) .The Freiburg Landolt C Contrast Sensitivity Test is an adaptable stairway, 8- alternate forced- pick method due to the 8 orientations of the spread. The Gabori uses the method of go uping bounds. The stimulation is presented to the perceiver at an strength below their threshold and the strength increases until the perceiver can see it. We set out to find whether these trials gave similar consequences despite utilizing different psychophysical methods.
The trials were carried out on five female patients aged 20, with refractile mistakes runing from -2.75D to plano. A comprehensive medical history of the patients showed that they were all in good optic wellness, with no systemic conditions and were non taking any medicines which are known to impact the oculus. There was no history of optic pathology or surgery which is known to impact contrast sensitiveness. The trials were performed monocularly in both the topics ‘ right and left oculus. Each of the topics right and left eyes were tested one time for each of the three trials. Refractive rectification was worn when required.
Pelli-Robson Chart is a 26-alternative forced-choice method, as any of the 26 letters of the alphabet can be used. It is 86x63cm and consists of 8 lines of letters, each missive of the same size. Each line consists of two subsets of three letters, with each subset of diminishing contrast sensitiveness. Contrast Sensitivity ranges from high to moo, down the chart. Log Contrast sensitiveness tonss range from 0.00 to 2.25. The capable stood at a distance of 1m from the chart with her left oculus occluded. The topics mark is allocated based on the faintest three where 2 of 3 letters are right named. The trial was so repeated one time for the topics left oculus. ( Lay et al, 2009 )
The Freiburg Landolt-C Contrast Sensitivity Test uses Landolt-C optotypes to cipher the contrast threshold of the topic. There are 8 gap waies and the topic has to observe the correct one, entering it utilizing the corresponding figure on the keyboard as seen in the diagram. The consequence is displayed onscreen. The screen size was 49cm2.
The trial makes 24 presentations of each way. The Contrast C diameter is 60 mins of discharge. The topic is at a sing distance of 3m and had one oculus occluded. The trial was done one time for both eyes.
The Gabori Test is used to prove for contrast sensitiveness map. It has a screen size of 49cm2. The capable sits 57cm from the screen with their left oculus occluded. The topic is required to look straight at the Centre and to respond every bit rapidly as possible to grates that easy come into position from the Grey background by snaping on them with the computing machine mouse. The consequence is so plotted on a graph at a figure of spacial frequences with their corresponding contrast sensitiveness as shown in the appendices.
The spacial frequences were given for Gabori on the graph but had to be calculated for the Pelli-Robson and Freiburg Trials by mensurating the breadth of the letters on the Pelli-Robson Chart and the size of the spread of the Landolt C in the Freiburg.
The topics pupil sizes were besides recorded for each trial utilizing a palladium regulation and luminosity degrees were measured utilizing the Spectra Scan PR650. These are shown in the Appendixs.
I recorded the consequences for each person ‘s right and left eyes.
The contrast sensitiveness for the Pelli-Robson Chart was recorded in Log Contrast Sensitivity. To change over it to contrast sensitiveness I had to acquire the anti-log of the mark.
For illustration, Subject A ‘s consequence for her right oculus was 1.5 Log Contrast Sensitivity. To acquire the contrast sensitiveness we got the antilog of 1.5, which gives us a contrast sensitiveness of 31.6227766.
The contrast sensitiveness for the Freiburg Landolt-C Contrast Sensitivity Test was expressed as threshold contrast in % . To cipher the contrast sensitiveness we divided 100 by the mark.
For illustration, subject A ‘s consequence for her right oculus was 0.58 % . To acquire the contrast sensitiveness we divided 100 by 0.58 % , giving us a contrast sensitiveness of 172.41.
The Gabori Test gave us a figure of contrast sensitiveness consequences for the topics ‘ right oculus, but at different spacial frequences. See Appendices for a tabular array of consequences for the Pelli-Robson chart and the Freiburg Test and a graph for the Gabori Test.
Once we had converted all the consequences so that they were in the same units, we were able to cipher the average contrast sensitiveness of all five topics for each of the three trials and roll up a graph as seen below.
The standard divergence of the contrast sensitiveness of the five topics was besides calculated and the standard mistake of the mean calculated utilizing standard divergence divided by v5. Once this has been determined, an mistake saloon can be inserted into the graph. This mistake saloon shows the scope for which I am 95 % confident that if this trial were carried out on the whole population, their mean contrast sensitiveness at each of these spacial frequences would lie within.
The spacial frequences had to be calculated by change overing the size, S ( units of ocular angle, ) into spacial frequence, F ( cycles/degree ) . This was done utilizing the expression, a/b = sunburn a.
For the Pelli-Robson chart, a is the breadth of the letters, while for the Freiburg trial a is the size of the spread in the Landolt C. B is the proving distance. Spatial frequence is equal to the opposite of the spacial rhythm, T ( deg ) . Spatial rhythm, T, equates to two times the size, S. Thus Spatial Frequency, F, is equal to half the Size, S. These computations can be seen in the Appendixs.
It was found that the Pelli-Robson Trials and the Gabori Test gave similar consequences while the Freiburg Landolt-C Test gave a significantly different consequence. It gave a important overestimate of the contrast sensitiveness when compared to the other two trials. Besides when taking into history the uncertainness of the place of the stimulation presented and therefore the hold in response clip, we were anticipating an underestimate of the topics contrast sensitiveness. However, this was non the instance. At a similar spacial frequence to the Pelli-Robson Chart it gave similar contrast sensitiveness.
This caused us to travel hold a expression at the other measurings we had taken, in peculiar the luminosity and pupil size of each topic for each trial.
It was found that the luminosity of the Pelli-Robson trial was greatest and the Gabori was least and hence pupil size was smallest with Pelli-Robson and biggest with Gabori, which is what is to be expected. This can therefore propose a ground for our findings. Alfonso et Al ( 2007 ) did a survey on the Correlation of student size with ocular sharp-sightedness and contrast sensitiveness after nidation of an apodized diffractive intraocular lens ” and found that contrast sensitiveness was better with larger students than smaller student. This would propose that contrast sensitiveness would be greatest with the Gabori trial and explains why it had a similar consequence to the Pelli-Robson.
The Gabori trial may besides hold had a higher consequence than expected due to the mistake of habitation. The topic may hold developed a wont of reacting to the stimulation and respond to a stimulation above their threshold.
The Freiburg may hold had a significantly larger consequence due to the luminosity being lower than the luminosity for the Pelli-Robson Chart. To find whether the Freiburg trial can be compared to the other trials it would necessitate to be repeated at a luminosity degree similar to the Pelli-Robson.
Besides each trial was merely carried out one time for each topic and would necessitate to be carried out a figure of times so an norm may be taken to acquire a more accurate consequence.
It was found that the Freiburg Landolt-C Contrast Sensitivity Test gave a significantly different consequence than the Pelli-Robson Chart and the Gabori Test. However it would non be justifiable to state that it is hence non comparable to the Pelli-Robson and Gabori as all three trials were carried out under different luminosities. As old surveies have shown there is a correlativity between luminosity and contrast sensitiveness and this experiment would necessitate to be carried out once more with the same luminosity for each trial.