A distorting mirror, funhouse mirror or carnival mirror is a popular attraction at fairs and carnivals. Instead of a normal plane mirror that reflects a perfect mirror image, distorting mirrors are curved and often use convex and concave sections to achieve the distorted effect.
When a light ray hits the surface of the mirror it reflects back to you as though you were looking at the object in front of it. Those reflections are distorted because the reflected light waves don’t travel in the exact same location as the incident rays.
Concave
Concave distortion mirrors curve inwards, reducing their reflective surface and squashing the light that passes through them. This creates a very slim, lanky, reflection that reminds us of Slenderman.
They also produce short, squat, twisted, and double reflected images which are fun for a sensory room! In addition, concave mirrors can be made into a variety of different shapes and sizes.
For instance, a concave mirror could have a round shape which would show a broad (though distorted) image of a shop aisle. This type of mirror is often found in security cameras at amusement parks, to show traffic at a blind intersection or to show a hidden store shelf in a shop window.
Another popular use of concave distortion mirrors is in astronomical telescopes to help viewers to see objects in space. The light rays entering the reflector are directed to a focal point inside the mirror, which causes them to become parallel light beams that travel a large distance.
In an ophthalmoscope, a doctor focuses through a small hole in the centre of a concave mirror and then a beam of light is directed into the pupil to examine the eye’s retina. This is done to check for any abnormalities in the eye.
Similarly, concave mirrors are used in solar furnaces to focus the Sun’s rays and in car headlights to form a beam of light from a single source of light. They are also used in satellite dishes and electronic microscopes.
A concave image of an object appears closer to the mirror and smaller than it is, while a convex image of an object appears further away from the mirror and is the right way up. This is a very strange phenomenon.
It’s easy to understand this behavior using a simple equation. It is based on the distance between the object and the mirror, called the object distance, and the size of the image formed by the reflected light rays, called the image distance.
Convex
In some countries, convex mirrors on the passenger-side of a car are labeled with a warning to warn drivers that their distance perception is distorted by the distorting effect of the mirror. This distortion can lead to double vision, eyestrain, and nausea.
In order to understand why reflections in a curved mirror look distorted, we need to look at how light rays reflect off an object. These rays travel in all directions and bounce back like a ball would on a smooth surface. The location where they meet is where the brain thinks the object is, so that is where we see it when we look at it in the mirror.
When you hold a mirror up to the light, you will notice that it reflects almost all of the light coming from an object. That’s because it has a flat surface and is shiny. The light that reaches the mirror is reflected, and if the mirror is flat, it will reflect an image on your retina that looks very good.
What happens if you push the sides of your mirror back so that the middle bulges out? You will notice that it also reflects your body in a very different way. You will probably look thinner and taller in this distorted mirror than you did when you held the standard flat mirror.
To make sure you can understand this, take a regular mirror and turn it on its side so that the long sides are facing you. Now, place a small object in front of the mirrored side.
Normally, light rays that reach the mirror make a perfectly symmetrical V shape with the normal as a line of symmetry. If the mirror is flat, this line of symmetry is parallel to the rays.
If the mirror is curved, however, the normals become angled toward one another. This is because the rays are diverging toward the mirror instead of converging. The resulting image of the rays is very distorted and will not match up with where they actually went.
As you can see, there is a lot of confusion with this type of mirror and it may not be the best choice for some applications. That’s why many people opt for a dome style mirror, which offers a wider angle of view and less distortion. For most applications, a quarter dome or half dome mirror will suffice for viewing objects at various distances. For areas that require visibility at blind corners and intersections, a 360 degree “full dome” mirror is the ideal choice.
Curved
A curved distortion mirror has an optical surface that is either convex (bulging outward) or concave (recessed inward). The reflected light rays from the curved surface are distorted and may appear to come from different points on the curved surface. This type of distortion is often used in funhouse style mirrors for entertainment, but can also be used to enhance motor skills and visual development.
Normally, flat mirrors reflect almost all of the light hitting their surfaces. This is because the skewers that represent the normal to the surface are parallel to each other. This causes reflected rays to meet at a point behind the mirror so that an image appears at the other side of the mirror. This is the way that our eyes locate where things are.
But if the rays don’t cross at a point, they will form a virtual image. This is called a coma. It can happen when the rays are reflected from a curved mirror that is too small compared with its radius of curvature. The resulting coma is a blurred image of an object that extends beyond the focal length of the reflected rays.
This is what happens when a broad beam of parallel rays impinges on a spherical mirror, as shown in Figure 2.3.2. The rays strike the mirror at different angles, as the rays are traveling through different parts of the spherical mirror. As a result, the rays do not cross at a point. This is a problem called spherical aberration.
Therefore, spherical mirrors usually need to be made larger than their radii of curvature to eliminate spherical aberration. Spherical mirrors are useful for reflection telescopes that need to see distant objects, but they are less useful for imaging objects that are very close to the horizon.
To determine whether the image formed by a curved mirror is real, we use an equation, as discussed earlier in this section. The formula, given in Equation 2.3.6, gives the distance of an object from the focal point and the distance of an image from the focal point as a pair of positive numbers. This is consistent with the sign convention that we have been using throughout this lesson and the rest of this chapter. If the 1 / f
Inverted
The lateral view mirrors in your car use convex surfaces to improve the field of view and the distance perception of drivers. However, this characteristic can result in a degree of distortion that can cause the image to be distorted or inverted; the result could pose a safety risk for drivers.
The JIS-D-5705 standard specifies that a convex mirror must have a distortion factor lower than the one obtained by analyzing a pattern of concentric circles reflected on it. To perform the calculation of the distortion factor, a radial line pattern is placed at a distance of 300 mm in front of the mirror and an image of it is captured.
As a result, the mirror’s reflection of the pattern will be analyzed and its distortion will be calculated; this method is useful to evaluate the degree of distortion of the mirror in order to detect any alterations or deviations from the ideal circle. As the number of points used in the standard measurement is small, it would be possible to accept mirrors with a small deformation and/or distortion that do not reach these points.
This paper proposes a new distortion calculation method that is based on image processing (DCMIP). Its main objective is to increase the robustness and precision of the quality control of the mirrors, by avoiding that changes in the scale, resolution and rotation of the reflected image affect the measured value, as it does with the standard.
To validate the proposed method, a series of experiments were performed using five commercial lateral-view mirrors, each one representing a different manufacturer. The measurement results were performed according to the JIS-D-5705 standard, and it was found that all of them presented a distortion factor lower than 5%; thus, they passed the test.
A similar test was also performed with the DCMIP to assess its performance and find out that it was able to detect that one of the mirrors presented an important distortion, which did not pass the quality criteria. This fact suggests that the proposed method has higher robustness and precision than the standard.