3
light-field is focused by biconvex Lens 1 not onto a CCD or CMOS sampling array but rather
onto a DMD consisting of an array of N tiny mirrors (see Sidebar: Spatial Light Modulators).
Each mirror corresponds to a particular pixel in x and φ
m
and can be independently oriented
either towards Lens 2 (corresponding to a 1 at that pixel in φ
m
) or away from Lens 2 (corre-
sponding to a 0 at that pixel in φ
m
). The reflected light is then collected by biconvex Lens 2 and
focused onto a single photon detector (the single pixel) that integrates the product x[n]φ
m
[n] to
compute the measurement y[m] = hx, φ
m
i as its output voltage. This voltage is then digitized
by an A/D converter. Values of φ
m
between 0 and 1 can be obtained by dithering the mirrors
back and forth during the photodiode integration time. To obtain φ
m
with both positive and
negative values (±1, for example), we estimate and subtract the mean light intensity from each
measurement, which is easily measured by setting all mirrors to the full-on 1 position.
To compute CS randomized measurements y = Φx as in (1), we set the mirror orientations
φ
m
randomly using a pseudo-random number generator, measure y[m], and then repeat the process
M times to obtain the measurement vector y. Recall from Sidebar: Compressive Sampling in a
Nutshell that we can set M = O(K log(N/K)) which is ≪ N when the scene being imaged
is compressible by a compression algorithm like JPEG or JPEG2000. Since the DMD array is
programmable, we can also employ test functions φ
m
drawn randomly from a fast transform such
as a Walsh, Hadamard, or Noiselet transform [6, 7].
The single-pixel design reduces the required size, complexity, and cost of the photon detector
array down to a single unit, which enables the use of exotic detectors that would be impossible in
a conventional digital camera. Example detectors include a photomultiplier tube or an avalanche
photodiode for low-light (photon-limited) imaging (more on this below), a sandwich of several
photodiodes sensitive to different light wavelengths for multimodal sensing, a spectrometer for
hyperspectral imaging, and so on.
In addition to sensing flexibility, the practical advantages of the single-pixel design include
the facts that the quantum efficiency of a photodiode is higher than that of the pixel sensors in a
typical CCD or CMOS array and that the fill factor of a DMD can reach 90% whereas that of a
CCD/CMOS array is only about 50%. An important advantage to highlight is the fact that each
CS measurement receives about N/2 times more photons than an average pixel sensor, which
significantly reduces image distortion from dark noise and read-out noise. Theoretical advantages