LUMINOUS RETINA

Luminous Retina

As we have seen in the previous material, we will illuminate the fundus with the retinoscope and observe rays coming from the retina, as if it were luminous. When light leaves the retina, the optical system of the eye applies vergence to the rays. If we illuminate the retina with parallel rays (plane mirror), the reflected rays leave the eye according to the refractive error.* That is: In emmetropia, rays leave parallel. In hyperopia, rays leave diverging. In myopia, rays leave converging. This backward approach may seem confusing at first, but simplifies understanding of the retinoscopic reflex. Things happen differently when we illuminate the retina with rays that are not parallel, but we will ignore these for now. In visualizing the situation, we will use a graphic presentation to illustrate luminous retina optics in three basic situations: emmetropia (plano), hyperopia (+1 D), and myopia (–1 D). Since the rays entering the eye remain parallel in all cases, we will ignore them entirely and simply look at what emerges. This is a very graphic presentation, so study and compare the diagrams carefully. Figure 4-1 offers one more way of looking at the FP optics . We see the emmetropic FP at infinity, the hyperope’s FP beyond infinity, and the myope’s FP at less than infinity. Figure 4-1. Rays from the illuminated retina. Correction refers to the lens desired to correct the refractive error. Now, picture yourself sitting before each of the eyes in Figure 4-2. Looking through the peephole in your retinoscope, you see these emerging rays as a red reflex in the patient’s pupil. If you sweep the streak across the eye, the reflex you see will also move. If the emerging rays have not converged to a point (the FP), the retinal reflex will move in the same direction as you move the streak; this is called the with motion reflex (WITH). If the rays have come to the FP and diverged, the reflex will move opposite to your movements; this is the against motion reflex (AGAINST) (see Figure 4-2).† Figure 4-2. Retinal reflex movement. Note movement of the streak from face and from retina in WITH versus AGAINST motion. Now picture yourself sitting almost at infinity looking through your retinoscope. This is what you would see in each of the three cases (Figure 4-3). In the emmetrope and hyperope, the emerging rays have not converged to the FP, so you see WITH motion. In the –1 myope, the rays have come to a focus at the FP (1 meter) and have diverged; thus, you would see AGAINST motion. Figure 4-3. Retinoscopy at infinity. Note that within the FP, you see WITH motion; beyond the FP, you see AGAINST motion. Consider this situation another way: if you see AGAINST, you are beyond the FP; if you see WITH, the FP is beyond you! So much for what you would see if you sat at infinity. Optical infinity is anywhere beyond 6 meters (20 feet), but you cannot reasonably sit that far away: the reflex would be too dull, and you cannot place correcting lenses before the eye. But if you sit at 1 meter, the reflex appears brighter, and you can (almost) reach the patient conveniently (Figure 4-4).  Retinoscopy at one meter. Note that the FP of the 1 D myope (–1) is at 1 meter. With your scope 1 meter from the patient, you would still see WITH, but in the case of the emmetrope and hyperope, their FPs are beyond you. However, in the case of the 1 D myope (FP at 1 meter), you would see a different reflex: if you leaned forward, you would now see WITH; if you tilted backward, you would see AGAINST. But when you sit with your retinoscope right at the FP of the eye, you see the neutrality reflex (Figure 4-5). . Neutrality reflex (NEUT) FP conjugate with the peephole of the retinoscope. When you are at the FP, the pupil floods with light. There is no streak reflex and no movement WITH or AGAINST. The retina of the eye is conjugate with the peephole of the retinoscope. Since the reflex reverses itself (ie, changes from WITH to AGAINST motion) at the FP, some call neutrality the reversal point.