The whole project proposed in this roadtest is based on a particular technology used in CMOS camera sensors: Rolling Shutter
Global shutters, which are commonly implemented on CCD sensors (although CMOS variants exist), expose all pixels on the sensor simultaneously and gather incoming light over all pixels for the exposure time (Te). After collection has stopped, the data is transferred.
Rolling shutters on the other hand, consecutively expose and read-out individual rows of pixels in a pipelined fashion. The exposure is performed in rapid succession, producing adequate images for scenes with minimal motion. Te for a rolling shutter is defined as the time each individual row is exposed before being sampled. Though rapid motion or changes in lighting can result in significant geometric distortion of the captured images, rolling shutter technology is prevalent in CMOS sensors used in smartphones, tablets, and consumer computer peripherals due to the advantages of low power-consumption, complexity and cost.
The rolling shutter mechanism can capture a timevarying light as a spatially-varying image. A LED pulsed at a period TLED (less than the the frame duration) will produce bright and dark bands coinciding with rows exposed during the on-time Ton and off-time Toff of the LED respectively.
The duty-cycle of the LED PWM signal determines the ratio of the height between the bright and dark bands. The number of bands per image is proportional to the LED’s frequency fLED = 1/TLED, as well as the vertical resolution of the image. The height of each band is determined by fLED and the row transfer time. The key phenomena is that the frequency of the image bands is proportional to the LED PWM frequency.
also shows that the smoothing between light and dark bands can be minimized by shortening Te.
However, care needs to be taken to not under-expose an image which would decrease the SNR of the received signal. In order to maximize the dynamic range between light and dark bands, Te must be smaller than min(Ton,Toff).
