hello and welcome to insight of thermology this is dr amrit welcoming you to the series on refraction today we are studying terms conoroid and spherical equivalent we will be discussing the concepts right from the basics and going to the advanced [Music] before we go into the details of this term schonoid it is very important for us to know what is meant by a tauric surface a perfect spherical surface as shown over here will have equal refractive powers along all the meridian that means the 90 degrees meridian and even the 180 degree meridian or any other meridian if you take all of them will be equally curved and all of them will have equal refractive power however if we take a surface like this which is shown over here it does not look like a perfect sphere however it is it is looking as if the perfect sphere has been compressed from the either side in such a way that the vertical meridian has become now more curved compared to the horizontal meridian such a surface in which the meridians which are aligned at 90 degrees to each other are not equal and one of the meridian is having more power compared to the other meridian or what i mean to say is one meridian is more curved compared to the other meridian such a surface is called a toric surface when the light rays pass through a perfect spherical surface as shown over here what happens is that all the rays will actually come to a point focus because the power of all the meridians is same and all the meridians are equally curved and therefore they will equally refract and bring the rays to a focus to a single focal point and this is called a point focus however the refraction of the light rays is totally different in case of a toric surface in case of a toric surface as shown over here the vertical meridian is having more power compared to the horizontal meridian therefore the rays of light will get convergent or they will get to a focus sooner come from the vertical meridian compared to the horizontal meridian that means if the rays are passing like this the vertical meridian will focus them at a point say a the horizontal meridian however are going to focus them at a point b which is far from the point a right and why does this happen because the horizontal meridian is having less power compared to the vertical meridian so whenever there's a difference in the powers of the meridian there will not be a single focus as what we saw in a spherical surface instead we are actually having two focus now if you draw two planes one passing through a and another passing through b okay so a and b will be two focal planes so what do we get to know now that in a torque surface two meridians are not having equal power one meridian is having greater power compared to the other meridian because of which we have two focal point and we have two focal planes so when we are studying the configuration of the rays as they are passing through this storage surface okay so as we are studying how these rays when they pass through a toric surface how will they get refracted how will they form two foci at every point of their journey what will be the configuration of the rays as they are passing through the tauric surface we are actually studying nothing but the storms conor so this is the definition of terms canoid that's terms canoid is nothing but it is the configuration of rays refracted through a toric surface consider this to be our toric surface and in the story surface the vertical meridian is more curved compared to the horizontal meridian so we can see that the vertical marine is getting focused at a point much earlier compared to the horizontal meridian next what we'll do is we'll pass a ray of light or a point source of light through the storage surface and now we are going to observe what will happen when the ray of light will pass through this surface at every individual point now let us imagine that we are standing behind the storage surface at a point a so what will be the shape of that point source of light that we have projected into the storage surface the shape will be actually an oblate ellipse and why is that so at this point a the vertical rays are actually more converging compared to the horizontal rays right so the vertical rays are more converging compared to these horizontal rays now because of that the the same point source of light it will look as if it is compressed it is more converged from the vertical side and from the horizontal side it is actually elongated that is the reason we are seeing this shape and the shape is an oblate ellipse now let us imagine that we are standing at a point b okay now at this point b the vertical rays have totally converge however the horizontal rays are still converging so what is happening to this oblate ellipse that means the vertical it is as if you are compressing it further but horizontally it is not yet that much compress so we are going to get is we are going to get a horizontal line at the point b because the vertical rays have totally converged at point b now imagine that we are standing at this point c now what is happening at point c at point c the vertical lines which actually converge at point b now they have started to diverge okay that means this horizontal line will now start opening however what is happening to the horizontal line the horizontal lines are still converging but they are little bit better converging compared to the point b so what are we going to get at this point we will get an oval okay and since this oval is more horizontal in shape this is called horizontal oval now let us imagine that we are standing at a point d now at this point what happens is the divergence of this vertical rays will actually become equal to the convergence of the horizontal meridian so what we are going to get is we are going to get a perfect circle and this perfect circle where the convergence and divergence of both the meridian is equal is called the circle of least now what i mean to tell you is that in an astigmatist surface we actually do not get a single point focus so the crisp and clarity of a image will not be present in case of a toric surface however there will be a point that is a point d represented in this diagram where this uh confusion will be least okay where we can actually get the maximum clarity although it is not perfect there will be maximum clarity because of the circle of least confusion because these meridians will not although they are not equal in power at a particular point that is d over here their convergence and divergence will become equal and they might act as if they are similar although they are not similar so such a point is called a circle of least confusion and if this circle of least confusion is present on the retina the patient will still have a better vision compared to the points a b c okay so what i mean to say is if this a b c were present on the retina the patient will still have a bad vision however if somehow this point d that the circle of least confusion comes on to the retina of the patient the astigmatic patient also will have good vision now let us see what happens if our retina was present at point e now at this point e we can see that the convergence has increased and the divergence has increased so what i mean to say is that the conv these horizontal meridian is actually now coming close together but the vertical meridian is now going far together so what we are going to get now is actually a horizontal oval okay so we are getting a horizontal oval y because the vertical meridian is now diverging and the horizontal meridian is now converging so we are going to get a vertical oval now what will happen if we actually stand at a point f now at the point f what is happening to the horizontal meridian lines the horizontal meridian lines are totally meeting with each other whereas the vertical meridians lines are still diverging so what are we going to get we will get a straight vertical line at the point f okay because the vertical rays are diverged now let us stand at this point g over here so what is happening at point g both the horizontal and the vertical lines are now diverging so finally what we are going to get is we are going to get actually a prolate ellipse okay so just remember that in the beginning we were actually getting a oblate ellipse and then slowly slowly it kept on changing from two horizontal line horizontal oval circle of least confusion vertical oval vertical line and finally we got the prolate ellipse so this was the journey of our point light source passing through the tauric surface becoming an oblate ellipse at point a horizontal line at point b horizontal oval at point c perfect circle at point uh d and then vertically oval vertical line and then prolapsed ellipse at point g so there are two points at which there was actually convergence of the rays so the first point was at b where the vertical rays were converging and then there was a point f where the horizontal rays were converging so from the point b we got a horizontal line and from the point f we got a vertical line so if you observe carefully nowhere we got any point focus instead we are getting focal lines and these focal lines are our horizontal focal line and the vertical focal lines right and it can also be called the anterior focal line and the posterior focal line so the focal line at b will be your anterior focal line okay which is being formed from the vertical meridian because it is converging before and the f will actually be forming the vertical focal uh the second focal line or the posterior focal line because of the convergence of the horizontal rays now this is actually showing you the exact configuration of that journey that the light rays were taking and that journey is nothing but this terms conoy so we got and we were getting horizontal spheres like this and then we got the anterior focal line then we got the circle of least confusion and from there it started becoming more vertical we got the posterior vertical line or the posterior focal line and finally we got the prolate ellipse right so now the difference or the distance between the anterior focal line and the posterior focal line is called this terms interval right and this entire thing is called the sturm's canal now based on this terms canoid can we tell about what type of astigmatism is present or not yes we can okay and this concept is very important now let us imagine an eye where the retina is actually present at a point a okay and if you remember at point a we were actually getting our prolate eclipse right so now what type of astigmatism this patient has okay the type of astigmatism depends upon where the images are actually formed from both the meridians right so here we saw that the vertical meridian was forming the image at the point b and horizontal was actually converging at a point f now if we are having the retina at the point a where are these focal lines present b and f b and f both are present behind the retina and we know that the hypermetropes will form the images behind the retina right and since both the meridians are forming the images behind the retina this is nothing but this is a case of compound hypermetropic astigmatism now if the retina was actually present at the point say b okay so if the retina is actually present at our point b so here now where are the focal lines present and what type of astigmatism it is we can see that one focal line is directly present on the retina and the other is present behind this retina so what type of astigmatism astigmatism is this this is simple hypermetropic astigmatic now what if the retina was actually present at a point say c or at a point say d okay any of these points now where are the focal points which are present one focal point is in front of the retina that means it is myopic and the other focal point is actually behind the retina that means it is hypermetropic so such a type of astigmatism it is called mixed astigmatism similarly if the retina is present at a point e at this point also we are going to get mixed acid matter now if the retina was actually present at the point f now what type of astigmatism it is in this astigmatism one focal line is directly present on the retina and where is the other focal line that the focal line is actually in front of the retina that means the other focal line is myopic so what type of astigmatism will this be this is simple myopic astigmatism in the end let us now place the retina at this point that is g now where are the focal points both the focal points are present in front of the retina so this is the case of compound myopic astigmatism so this is why storms canoid is so important the configuration of these focal points where these focal points are present and what is the configuration of the bundle of light which is passing through the storage surface is important because through this we can actually know what are the types of astigmatism and the concept behind the types of astigmatism and through this we also know the concept of spherical equivalent okay so let us see what is meant by spherical equivalent [Music] you