9.3 stenopaeic slit Flashcards
what is width of stenopaeic slit ?
. 1mm width central slit in an opaque disc
what is the function if the stenopaeic disc ?
. reduces pupillary aperture along one meridian only
. the stenopaeic slit minimises astigmatic blur when the slit axis is parallel to the correcting cylinder axis
what happens when you put stenopaeic slit in front
Rx 0.00/-6.00 x 180 ?
. 0.00 x 180
. -6.00 x 90
if we locate stenopaeic slit along 90 meridian , all rays that go through slit are blurred by 6D worth of blur and as results lines on fan chart along 90 are clear but lines become increasingly blurred as we go along fan chart
. if we rotate slit along 180 meridian , the fan chart looks very clear
why are lines more clear along 180 meridian ?
. blur is caused by power along meridian perpendicular to cylinder axis
. for 180 axis cyl , objects are blurred along the vertical direction , so verticals stay sharp
. a horizontal stenopaeic slit will reduce the blurring in the vertical direction and all lines will appear sharp
. for a slit orientated along 90, there is no reduction of aperture along the blurring meridian
what happens as a stenopaeic slit is rotated along RX: 0.00/-3.00 x 45 ?
. we got simple myopic astigmatism with an axis of 45
. we are considering a subjects view here and hence the axis are reversed the fan chart Looks clearest where the stenopaeic slit angled along 45 deg, the 45 deg axis now that looks like 135 but of course we are now behind the trial frame in subjects position and therefore that orientations stenopeic slit acts to reduce the blurring which is caused by the Rays at the 135 Meridian .
what happens as we rotate the stenopaeic slit ?
. you could see that the appearance of the fan chart changes little bit as we rotate the slit different
. positions including some slight changes in the appearance of the clearest direction as we rotate it around
.but what we’re most interested in doing is finding this orientation when all of the lines in the fan chart appear clearest that helps find 1 principle gradient once we found one principle meridian then of course we know that the other principal meridian is at right angle .
what is the routine for stenopaeic slit ?
. used to locate astigmatic axis
. possible to carry out subjective refraction along each meridian using sphere , as slit isolates one meridian
. method same as subjective determination of refractive error in spherical ametropia
when can we use stenopaeic slit ?
can be used in practice if irregular cornea and the best sph/cyl prescription is required
why might we not use stenopaeic slit ?
. works better for high levels of astigmatism
. it’s not sensitive , even though we can see the blurring quite clearly in the higher levels of astigmatism , we need to be accurate in terms of axis and that doesn’t work well with stenopaeic slit
. JCC or fan and block its superior to it
