How does the dentist to don’t cast a shadow with your hand inside the patient’s mouth? The answer is the special lamp he uses, the surgical lampthe same one that is also used in operating rooms. It is a lamp capable of neutralize the shadow of the obstacle placed between the light source and the surgical field.
But how does a lamp not cast a shadow? Let’s see the basic idea behind its operation: it is a geometric question of superposition of light beams and ellipsoids.
The basic principle of the surgical lamp is simple: instead of illuminating the operating field (the area where the operation is to be performed) with a single light source, it is illuminated with many beams of light coming from different directions. Let’s explain better, let’s imagine illuminating the affected area with a single lamp, in this case the dentist’s hand (or any other obstacle), which must approach to operate, would create a shadow making the operation difficult.
Now let’s imagine adding other lamps that illuminate the area affected by multiple directions bypassing, in a certain sense, the obstacle: the shadow area created by the obstacle with respect to the first lamp is illuminated by the beams of light produced by the other lamps and the shadow disappears.
In current surgical lamps the individual light beams are generally made with many small ones LEDs arranged on a portion of the cap, however in the past, instead of many small light sources, a single lamp (usually halogen) was used which pointed towards a cap which reflected the light towards the operating field. But how can you control the light of a single lamp by concentrating it in a single point and eliminate the shadows?
The secret lies in the use of curved mirrorsgenerally shaped ellipsoid, a particular geometric figure that allows you to control the direction of the light rays. An ellipsoid is nothing more than an elongated sphere (or flattened depending on your point of view) and is characterized by two points inside it, called fires which enjoy the following special property:
if a mirror has the shape of an ellipsoid, and we light a light bulb placed in one of the two focuses, all the light rays that come out of the bulb end up in the other focus.
We have represented this property in the image below by drawing a section of an ellipsoid: the rays that start from the focus at the top are reflexes and they all concentrate in the fire below.
The property also works if you take a portion of an ellipsoid, and this is precisely the idea of the first surgical lamps: if we place a lamp in a focus of an ellipsoid-shaped mirror the reflected rays of light are all concentrated in the second focus which can be positioned, by moving and reorienting the lamp, precisely in the area where action needs to be taken.
In this way we obtain the same shadow cancellation effect that we saw previously: if a obstacle intercepts some rays of light coming from the mirror, there will still be all the other rays that will cancel out the shadow, just as if the mirror were made up of many light sources all directed towards a single point, the focus of the ellipse, which must be placed right in the field operative.
