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“Lens flares”, those spectacular flashes

publicado a la‎(s)‎ 5 nov 2018, 11:40 por Plataforma Sites Dgac   [ actualizado el 7 oct 2020, 7:26 ]
The English word “flare” can be defined as “a sudden, brief burst of bright light.” It is also used in the sense of fire burst or flash. A large percentage of reports that CEFAA receives include photographs that show them. We’ll briefly go over their nature and how to discover them before rushing to label them as unidentified flying objects.

Figure  1. Aerial phenomenon reported over Temuco.

Figure  2. What produces a “flare”?

Figure  3. Schematics of a photo lens.

Figure  4. Geometric analysis of a parasite flash.

Figure  5. “Flare” during a sunset.

Figure  6. Histogram of a sunset with a “flare”.

What’s their origin? 

Under optimal conditions, almost all of the light that passes through a photographic lens reaches the focal plane (*) and forms the image. However, sometimes some of this light is not properly refracted inward and is reflected off the surface of lenses and other items inside lenses or mounts. And while they do not form an image in the focal plane, they still pass through it and are then imprinted on the image and alter it. (Figure 2).

If we take into account that the interior of a current zoom lens can be composed of ten or more lenses and individual elements, there are often light beams that are reflected, scattered and re-reflected inside it. (Figure 3). 

Do they depend on the camera that is used? 

Reflections on cameras are corrected in the photographic lens design and manufacturing process, using interior coatings and specially designed lens combinations. Solutions do exist, but since they make the lenses more expensive, higher flare rates can be expected in more entry-level digital reflex camera kits. 

Reflections have always been present in the lenses of photographic equipment. In analog cameras, they result in irregular markings on the negative or their general degradation when photographed with the light source in front. In the case of digital cameras, the existence of these parasite lights causes the sensor to receive “different” information from that seen through the equipment’s viewfinder. 

When to suspect that there’s a lens flare? 

Depending on the intensity of the light that hits the lenses, the “flare” effect that alters the photograph will change. A constant is their appearance in the final images without having been seen in the photographed scene. 

When it comes to diffuse lights, these can be manifested in the images as veils, similar to illuminated fogs that fade the colors, impoverishing their contrasts and their intensity. If the intensity of the light increases, star-like figures, regular geometric shapes or ring-like shapes of light can be generated.

When the photographs are taken facing intense light sources, such as the Sun, the Moon, backlit spotlights and strong glares, then the “flares” can be superimposed on the images forming different types of artifacts (**) or light spots considered abnormal. Sometimes they may be reflections of some part of the interior structure of the photographic lenses.

The “flares” that CEFAA receives 

When analyzing the photographs of the cases, we investigate them by complementing two procedures: looking for symmetries and reviewing the tone histogram (***) of the images. We will now show examples of these methods: symmetries in the images and traces of the flashes in the histograms.

a) Symmetries in the images 

In January 2018, CEFAA received a series of photographs of what was classified as an unidentified flying object associated with a meteorological condition. The object in Figure 4 had not been seen other than in the shots. To confirm their origin, we proceeded to analyze them geometrically, drawing the diagonals that joined the vertices of the image and located its point of symmetry. Thus, the relationship between the reported object and the Sun, which was the main source of light in the scene, was verified. 

b) “Flares” in the histograms

Even when we have very dark elements in the photographed scene (Figure 5), an image with “flare” produces a histogram in which there are few values associated with the dark tones area, which is found on the left side of the graphic (Figure 6).

In conclusion, the “flares” or parasite flashes are not unidentified flying objects, but rather light effects produced by the camera that captures the images.