Frequently Asked Questions
What is coded strobing camera/photography?
Coded strobing photography is a technique to capture and visualize high speed repetitive (periodic) phenomena using a much slower camera. For example, a phenomena such as a electric toothbrush which would normally need a 2000 frames-per-second camera can now be captured using a 25 frames-per-second camera. Such a significant drop in camera frame rate is achieved by two things a) exploiting the fact that the event is repetitive and b) by modifying the camera such that despite a much slower frame rate no detail is lost.
Why is it called coded strobing?
We term our technique coded strobing motivated by strobing often used for many applications. As opposed to strobing where light is flashed on a scene regularly, we flash light according to a predetermined random pattern. We refer to this random pattern as 'code' and hence 'coded strobing'. In practice, instead of flashing the light we perform a mathematically equivalent task of opening and closing the shutter according to the 'code'.
Why is it useful?
Fast cameras are considerably expensive due to sophisticated engineering needed to handle the great amounts of data generated by them. To capture and visualize fast repetitive phenomena we don't need as fast a camera. Since coded strobing camera uses a regular slower camera with an inexpensive shutter, it is much less expensive.
What is its applicability?
Coded strobing is applicable for capturing periodic phenomena. Our technique can also be used to capture phenomena which are not strictly periodic but whose period fluctuates over time. Periodic phenomena are a special class but can be found all around us. Industrial automation is necessarily periodic in its nature. So are many bilogical processes such as vibrating vocal folds and natural processes such as seasons.
What are its advantages?
Coded strobing is a generalization of other existing ways of capturing and visualizing periodic phenomenon as discussed in a subsequent question. The primary advantage in addition to using an inexpensive slow camera is the light efficiency of captured frames. As opposed to regular strobing which also captures periodic phenomena, coded strobing collects significantly more light in each frame, thus obviating the need to light the scene strongly. Coded strobing is agnostic to the period of the phenomena as opposed to regular strobing which needs prior knowledge of the period. This allows coded strobing to capture phenomena with multiple periods in the scene.
What are its limitations?
Coded strobing is currently limited by the fact that the computational algorithm is not real time. Since the algorithm relies on collecting frames from the camera before computationally reconstructing the underlying high speed frames, the system necessarily has a delay. Another drawback of the current system is that, the phenomena being captured cannot diverge significantly from periodicity.
How is it related to other techniques?
Coded strobing is a generalization of regular strobing used for capturing and visualizing periodic phenomenon. It also belongs to the broad class of emerging sensing/sampling techniques termed 'compressive sensing' and hence related to other sampling techniques such as Nyquist sampling and non-uniform sampling. Comparisons between coded strobing and related techniques can be found in table below.
Will it replace the current high speed cameras?
High speed cameras are useful for a wide range of phenomena and follow Nyquist sampling strategy. Coded strobing on the other hand is designed to capture periodic phenomena. If the phenomena is periodic then coded strobing offers significant advantage over high speed camera and strobing as shown above. Nevertheless, further research is needed to make a deployable coded strobing camera.
Can it really beat Nyquist rate?
The short answer is yes! Nyquist sampling says that a signal sampled at rate higher than inverse of its bandwidth can be reconstructed accurately. When the bandwidth is defined as the range of non-zero frequencies in a Fourier transform, then coded strobing beats Nyquist sampling. Compressive sensing research has shown that sub-Nyquist sampling is possible. We recommend the following papers in this regard.
Why is code strobing camera an example of compressive sensing?
Coded strobing camera uses a slower camera to sense a faster process. In a fixed duration, few frames of coded strobing camera capture as much information as many frames from a fast camera. This is achieved by coding the shutter in a pseudo-random way. For recovery we exploit the sparsity of periodic signals in Fourier basis.