Visual Acuity [ Sub – ENG ] – Visual Acuity part 1


Hi to everybody and welcome to OptoTubeChannel, the first Italian YouTubeChannel dedicated to Optometry Today let’s talk about visual acuity, which from a perceptive point of view can be considerated as the skill of identify a visual stimulus in the space. There are many type of visual acuity There’s the acuteness of vision which is the capability of identify a stimulus an so it matches to the minimum angle subtended from a target to have stimulus detention or better to have the perception of that stimulus, so is the smaller target dimension which make us percept that target presence compared to a ground. This kind of acuity can be positive or negative: positive for example if we take as target a point and this is black on a white ground, so let’s change the spot dimension, or negative if we take a white spot and a black ground. Then we have acuteness of resolution which is the minimum angle of resolution named MAR which is the minimum angle subtended from the mutual distance of two stimuli to have their distinct perception, so in this way we have their discrimination. In according to Helmoltz this angle has to be about 60 arc seconds because he considered a cone diameter which is 20 arc seconds so it said that the vision of a target needs a cone, the space in between needs another, and for the other target needs another cone, so 20 arc seconds x 3=60 arc seconds. But today we know that we need less we can see also just with a MAR of 30 arc seconds. This kind of acuteness can be evaluateb using grates or gabor patch ( gabor patch are better ) or Albini’s E, or Landolt’s C Then we have Vernier or Nonio Acuity which is named also acuteness of localization and it is the minumum angle which make us percept the misalignment between two stimuli. Usually this kind of acuity is very high also of 4 – 5 arc seconds and of this kind is part also the tilt, which is the ability of recognize the verticality of a visual stimulus, and in that case we can evaluate it as the angle that separate the middle target line from the normal line, so that angle becomes the acuteness of localization for the tilt. The last kind of acuity is acuteness of recognition or morphoscopic acuity which consist in the ability of recognize determinated simbols which could be drawing, letters, numbers or other types. From a functional point of view is very interesting, more usefull than others also from a cognitive point of view. From a clinical point of view this kind of acuity give us many different informations about visual status and binocular status if there’s a different perception between the two visual channel or a perception difference compared to target distance. Visual acuity is usually write down as fraction form commonly known as Snellen fraction Snellen was one of the first to give a sistematical approach to visual acuity studing So what he did? He created charts, and the letters in his chart was the composition of 25 little square 5 x 5 and each square subtends a MAR of 1 arc minute, so the entire letters subtends 5 arc minutes. Infact according with Snellen an healthy eye have to subtends almost 5 arc minutes=MAR So how his fraction works? It works changing target distance and it is compared to the distance at which that target subtends 5 arc minutes, for example : in Snellen fraction the numerator represents the higher distance at which the target can be disciminated and the denominator represents the distance at which it subtends 5 arc minutes. So if I would see 10/10 it means that at 10 meters I have have a clear vision of a target which subtends 5 arc minutes as MAR. If I saw 12/10 I would see clear at 12 meters. Today there are surely more precise techniques to evaluate visual acuity, and they don’t change target distance along sagittal plan, but is changed the size so in this way we can get a better result. So this is the video about visual acuity, if you liked leave a like and subscribe this channel! Let’s see at the next opto-video! OPTO-STILE!!!

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