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The human eye can see ‘ghost images’

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Scientists have discovered that the human eye has an uncanny ability. It can detect “ghost images.”

These are the images that are encoded in a random pattern, previously thought only detectable by the computer. But in a new document posted online to the preprint server arXiv, scientists in Scotland at the Heriot-Watt University in Edinburgh and the University of Glasgow have found that the human eye can determine the necessary calculations.

“Although the brain can not separately see, the eye should be one way or the other is the detection of all the patterns, and then keep the information is there, and summing everything together,” said study co-author Daniele Faccio, a physics professor at Heriot-Watt University. [The Most Amazing Optical Illusions (and How They Work)]

How to create floating images

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In a regular camera, multiple pixels in the light from a source such as the sun, to create an image. Ghost images are actually the opposite: They start with multiple light sources in a predictable array, Faccio told the Science, the light that is captured by a single-point detector, usually called a “bucket.”

A simple way to imagine how this works is to think about lidar, which uses a single-point laser scan of a scene. The detector records how the laser light bounces back from each spot in the scene, which can then be reconstructed into an image.

But there is a faster way to do floating images, Faccio said. Instead of scanning the scene with a single light source, researchers have discovered that they can project patterns on a scene. The light that bounces off the object, plus the pattern can then be measured. The difference between that pattern of light and the original pattern is projected, includes the “ghost image” that the computer can then mathematically wring out of the data. These images seem to be a vague grayscale representation of the original image.

Ghostly visions

Computationally speaking, this method of creating floating images consists of two mathematical steps, Faccio said. The first is a combination of the original patterns and the patterns as they appear after being projected on the object. This is mathematically done by multiplying the original pattern against the light signal made by the object, and the pattern on each place. The second is to the sum of all those numbers over the whole scene. [The 11 Most Beautiful Mathematical Equations]

“The question we asked ourselves is, ‘Can the human brain do this?'” Faccio said.

The researchers decided to focus on the second half of the computation, the summation of all the patterns together. To do this, they started by projecting the checkerboard-type patterns called Hadamard patterns against the famous photo of Albert Einstein with his tongue sticking out. They then used a single-pixel detector for collecting the resulting light patterns, which they fed into a LED projector.

That LED projector shone the Einstein-plus-Hadamard patterns on a screen of the original Hadamard patterns, in essence, is the multiplication of the two together. Step one: complete.

The next step was to see what people can see when you come to this summation. The researchers found that when the Einstein-plus-Hadamard patterns were projected slowly, in pulses of 1 second or longer, people just saw black-and-white checkerboard — no ghost images. But if the researchers accelerated the projections, Einstein’s goofy face appeared. The researchers also did experiments with digits and letters and found they were readable in the “spirit” versions.

“Blacks and whites will begin to disappear,” Faccio said. “They will be grey and you actually start to see the image appear in front of you.”

Click on the refresh

The reason that this works, Faccio said, is that the human eye has a slow refresh rate. It is no different than the reason that movies work: When the images flickered on the screen faster than the refresh rate, it creates the illusion of smooth movement.

The eye “is very quick in the acquisition of information,” Faccio said. “It’s just very slow in getting rid of it.”

The researchers figured that the flickering patterns remained in the eyes of the “memory” for approximately 20 milliseconds, slowly over that time. As the 20-millisecond patterns overlap, the eye sums they like a film, making the ghost image to arise.

The exciting part of this discovery, Faccio said, is that the ghost-imaging system can be used for the study of the human visual system. The researchers of the paper is now under review in a peer-reviewed journal. The team’s next step is to find out if the human eye can also the behavior of the first step of the view of the spirit images, perhaps by multiplying together different inputs to the right and left eyes.

Original article on Live Science.

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