E C B TZ FCLBO toebhfc ZEBHCLFOB VISUS , =°.5 = 0.67 cbfzetfbocze # = 1 b EZCOBFCH ELBZ Ü=1,2 Visual Acuity • Definition: ability of the visual system to detect spatial changes • Performed at viewing distance of 5 or 6 m and 40 cm for near Minimum separabile • Emetropic eye: we can only distinguish two points from one another if there is between two cones irritated to the retina one cone not irritated • These two cones are observed at an angle of one minute Minimum separabile • The average length of a cone is approximately 0,005 mm and the retinal distance from image modal point of the eye is approximately 17 mm. The angular distance still distinguishable poinds are: • tga=0,005/17 -> a~l angular minute 17 mm tga = 0,005 / 17i=>a= 0,0167°= 1 ühlovä minuta Evaluation of visual acuity ■ used to measure the visual acuity during clinical refraction-optotypes ■ Snellen optotypes: letters, numbers, pictures, symbols ■ Each symbol is inscribed in a square visible from a specified distance below the viewing angle of 5' ■ The symbol thickness (not size) is equal to one fifth of the square and corresponds to angle of 1' ■ Examination distance: 5 m or 6 m (the eye is in the rest, the acommodation is less then 0,25 D - the eye is looking to infinity, it does not acommodate) ■ We measure first the right eye and then the left eye Optotypes ■ According to the design: printed panels, light panels, projection charts and LCD optotypes, 3 D charts ■ Snellen, Pfluger, Landolt, pictures, LEA, ETDRS optotypes ■ The most of optotypes chats have the range of visual acuity - 1,6 - 0,1 Projection optotype with mirror image Optotyp zrcadlo Optotype symbols are turned over, optotype - placed over the patient's head, opposite the patient is the mirror at eye level Snellen optotypes • Herman Snellen (1834-1908) - a Dutch ophthalmologist • 1862 - Snellen optotypes • 6/60, 6/36, 6/24, 6/18, 6/12, 6/8, 6/6, 6/5, 6/4 • The signs: C, D, E, F, L, N, O, P, T, Z • The patient have to identify minimum 60% of the optotype sign at the line Landolt C optotype is recommended by the ICO is the standard optotype ( C - a standardized symbol) for acuity measurement in most European countries. It was standardized. The Landolt C consists of a ring that has a gap, thus looking similar to the letter C ( The width of the gap and the thicknes of the line of the symbol is V5 of the diameter, and the gap width is the same) 8 different positions of the gap (comparison to the positions of a clock face) j Pfluger E (hooks) = (Tumbling E test) • Can be performed on children or patients who do not speak the same language as the practitioner • The directions of E in 4 ways (up, down, right or left side) • higher probability of guessing • The aid: the child's hand or E in (rotation of the direction the letter E) —. •••• ®ss 3 • e lu m m 3 W E 3 in 3 e n 3 epibe« ji 3 m e hi m - LEA SYMBOLS • Homework - please write me, what are Lea symbols, why and at whom we use them for, you can attach the pictures of Lea symbols Optotypes for children 5 LU S 4 d o 3 LU 2 3 S H * □ E III 3 9 2 7 O a 2 e e s O □ ö h a t a ? r Iii m e 3 kl 3 E PI 11 ■ a ■ a ? ■ ? a * ■■ a- -i ■ th □ n * * a ■ ¥ n * Cr tr ri y a e m ] e