Refocus criterion for both phase and amplitude objects in digital holographic microscopy

Abstract

For digital holographic microscopy applications, we modify the focus criterion based on the integration of the amplitude modulus to make possible its use regardless of the phase or amplitude nature of the objects under test. When applied on holographic data, the original criterion gives, at the focus plane, a minimum or a maximum, for amplitude or phase objects. The criterion we propose here operates on high-pass filtered complex amplitudes. It is shown that the proposed criterion gives a minimum for both types of objects when the focus plane is reached. Experimental results on real samples and simulations are provided, illustrating the efficiency and the potential of the method.

Wavelet-based tracking of bacteria in unreconstructed off-axis holograms

“…A phase particle is considered in focus when the amplitude of the particle in a holographic reconstruction is maximal [14]. When the particle's amplitude is at a maximum or a minimum, the gradient between the particle and the image background will also be at a maximum and therefore the mean modulus of a closed WTMM chain will be maximal.…”

“…A low-order approximation of this scattering, such as an Airy-like pattern, is less accurate but computationally more efficient. A phase particle, such as a transparent bacterium, is considered in focus when the amplitude of the particle in a holographic reconstruction is maximal [14]. As such, investigating gradients across reconstructed depths can reveal a particle's focal plane [11].…”

Extending the Depth of Field beyond Geometrical Imaging Limitations Using Phase Noise as a Focus Measure in Multiwavelength Digital Holography

“…As an alternative to the additional expensive lasers and further appropriate longer synthetic wavelengths, the axial measurement range in digital holography can be extended using shape-from-focus algorithms for focus detection, applied on amplitude or phase images. Promising approaches have been highly investigated in the past (e.g., amplitude stack [17], complex ratio [18], correlation coefficient [19], energy conservation [19,20], entropy [19], Fourier spectral [19][20][21][22], Gini index [23], gradient [22], Kirchhoff [24], L1 norm [23,25], Laplacian [21,22,26], multispectral images [19,27], self-entropy [28], speckle phase decorrelation [29], Tamura coefficient [30,31] variance [19,21,26,32,33], quadratic deformation of spatial coordinates [34], cubic phase plate [35], amplitude modulus [36], or numerical axicon transformation [37]). We present a shape-from-focus algorithm based on phase noise and use its output as a rough estimate of the object's surface shape, which allows us to extend the measurement range far beyond the limits of synthetic wavelengths and geometrical depth-of-focus.…”

Subaperture stitching interferometry based on digital holography

“…It means that the topography measurements on tested surface with optical interferometric precision and without vibration isolation. Moreover, due to the inherent advantages of DH such as numerical focusing [41][42][43][44], numerical aberration correcting [38,40,45,46], synthetic aperture imaging [47][48][49][50][51] and super-resolution imaging [52][53][54][55], the morphology measurement can be achieved with a large field of view and high resolution. Therefore, it has the potential for addressing rapid measurement of surface quality in realistic workshop conditions (e.g., under mechanical vibration, air turbulence, and so on) with high resolution and precision.…”