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Visual Performance
Published in Agnieszka Wolska, Dariusz Sawicki, Małgorzata Tafil-Klawe, Visual and Non-Visual Effects of Light, 2020
Agnieszka Wolska, Dariusz Sawicki, Małgorzata Tafil-Klawe
Increasing luminance increases flicker perception. The contrast detection threshold increases with luminance to approx. 300–450 cd/m2, where flicker sensitivity is highest at flicker of 10–20 Hz. Above this maximum level, an increase of luminance does not raise the maximum contrast detection threshold. In this case, the frequency range of flicker detection increases, but this change is small. The highest frequency perceived as flicker in optimal conditions and at maximum contrast is about 80 Hz. Above this frequency, flicker is not perceived. There is an effect of persistence of vision phenomenon (retinal persistence) [Kolb et al. 1995], in which the impression of an object image persists in the brain through neurons from the retina for about 1/20 s after the light stimulus. This phenomenon makes it possible to produce films, and also explains why the ability to detect contrast decreases with increasing flicker frequency.
Seeing with Sound: Diagnostic Ultrasound Imaging
Published in Suzanne Amador Kane, Boris A. Gelman, Introduction to Physics in Modern Medicine, 2020
Suzanne Amador Kane, Boris A. Gelman
The time taken to send and receive ultrasound pulses used in imaging is less than a millisecond in practice. The short times required for ultrasound scans mean that motions of the body can be imaged in real-time, and that many different views can be examined during a single session. Images can be recorded with an ultrasound B-scan as quickly as with a video camera, then played back as a video. Ultrasound images can be stored, printed or viewed as still images or videos by the radiologist or other physician. The brain has the ability to retain an image for an instant after it has disappeared, an effect called persistence of vision. In practice, this means that the images reproduced in texts seem lower in quality than what the physician sees on the monitor because a sequence of images tends to smooth out the speckled texture of the image.
Pitfalls in the Computation of DFT
Published in K. M. M. Prabhu, Window Functions and Their Applications in Signal Processing, 2018
The concepts of sampling, reconstruction, and aliasing can be demonstrated by means of a familiar example. While capturing motion pictures, the camera converts the dynamic scene into a sequence of frames. These frames are usually taken at regular time intervals of 24 frames/second. The frame rate has been chosen by taking into account the persistence of vision of the human eye. Sampling essentially selects a set of finite data points as a representation of the continuous-time signal at the corresponding time duration. Movie frames thus take samples of the scene information during each second. When it is played, our eyes and brain fill the missing data between the frames and thus provide the illusion of a continuously varying video. This operation of filling the breaks between the sampled data points is called reconstruction.
Unique Video Encryption Technique Intended for Smart City Application
Published in IETE Journal of Research, 2023
Hari Narayan Khan, Abhishek Das, Atal Chaudhuri
The strength of any encryption model depends on the difficulty of cracking the encrypted content [5,6]. Various existing algorithms have their own merits and demerits, and most of those are designed for either text or image, but this chapter presents a unique approach for video content fundamentally based on the symmetric key concept [7–9]. Any shoot video is conceptually a collection of a series of still images (frames) captured continuously (25 or 30 frames per second). For reproduction purposes, the captured frames are displayed in the same sequence at the same capturing speed. Because of the persistence of vision property of our eyes, it appears continuous. Thus, if the sequence or arrangement of captured frames is made shuffled among themselves, the corresponding reproduced video will become illegible and meaningless.