Optical axis | EPFL Graph Search

An optical axis is a line along which there is some degree of rotational symmetry in an optical system such as a camera lens, microscope or telescopic sight. The optical axis is an imaginary line that defines the path along which light propagates through the system, up to first approximation. For a system composed of simple lenses and mirrors, the axis passes through the center of curvature of each surface, and coincides with the axis of rotational symmetry. The optical axis is often coincident with the system's mechanical axis, but not always, as in the case of off-axis optical systems. For an optical fiber, the optical axis is along the center of the fiber core, and is also known as the fiber axis. See also

Ray (optics)
Cardinal point (optics)
Antenna boresight

3D nanometrology of transparent objects by phase calibration of a basic bright-field microscope for multiple illumination apertures

Martinus Gijs, Daniel Migliozzi, Bingying Zhao

Optical retrieval of the structure of transparent objects at the nanoscale requires adapted techniques capable of probing their interaction with light. Here, we considered a method based on calibration of the defocusing with partially coherent illumination and explored its phase retrieval capability over a wide range of illumination angles. We imaged: (1) commercial dielectric nanospheres to assess the phase calibration when measured along the optical axis,(2) custom-made nano-steps micropatterned in a glass substrate to assess the phase calibration when measured along the transversal axis, and (3) human cancer cells deposited on a glass substrate to assess the results of the calibration on complex transparent 3-dimensional samples. We first verified the model prediction in the spatial frequency domain and subsequently obtained a consistent and linear phase-calibration for illumination numerical apertures ranging from 0.1 to0.5. Finally, we studied the dependence of the phase retrieval of a complex nanostructured object on the illumination aperture.

2020

Phase calibration of a basic bright-field microscope for 3D metrology of transparent samples at the nanoscale

Martinus Gijs, Daniel Migliozzi, Bingying Zhao

Optical retrieval of the structure of transparent objects at the nano-scale requires adapted methods capable of probing their interaction with suitable light. Scattering events, for instance, depend on the content and the arrangement of the medium encountered by the light along its propagation path. Physical models of scattering were used in the past to understand image formation and phase retrieval in transparent objects. Here, we considered one of them, which is based on the acquisition of defocused images obtained with partially coherent illumination, and explored its phase retrieval capability over a wide range of illumination angles. We used a basic transmission bright-field microscope with a bandpass filter and an adjustable illumination aperture. The bandpass filter was used to select a specific wavelength range to avoid light dispersion in the sample and maximize illumination capabilities. The adjustable illumination aperture was used to probe and assess the calibration over a wide range of illumination angles, which give access to different parts of the spatial frequency spectrum of the sample. We subsequently employed a computational algorithm to retrieve the local 3-dimensional phase-shift induced on the light field by the scattering through the sample. We imaged several types of samples to explore the calibration and the results in different experimental configurations. We employed: (1) commercial dielectric nanospheres to assess the phase calibration when measured along the optical axis, (2) custom-made nanosteps micropatterned in a glass substrate to assess the phase calibration when measured along the transversal axis. We first verified the model prediction in the spatial frequency domain for the scattering induced by our samples, and subsequently obtained a consistent and linear phase-calibration for illumination numerical apertures ranging from 0.1 to 0.5. This enabled us to calibrate a very simple optical system in a wide range of illumination angles to obtain quantitative 3D metrology of transparent samples at the nanoscale.

SPIE-INT SOC OPTICAL ENGINEERING2021

Extended Depth-of-Field for Color Images in Light Microscopy: Image Fusion and 3D Visualization

Niels Quack, Daniel Sage, Michaël Unser, Dimitri Nestor Alice Van De Ville

Bright-field microscopy suffers from a relatively small depth-of-field. Typically, the specimen’s profile covers a range larger than the depth-of-field, and parts of the specimen that lie outside the object plane appear blurred. The specimen can be ‘scanned’ by moving the object along the optical axis, and different images will contain different areas that are sharp. The purpose of image fusion is to combine those images into one single image with an extended depth-of-field. One promising method is based on the wavelet transform. We show how the wavelet-based image fusion technique can be successfully applied to obtain a sharp composite color image. The selection of the proper type of wavelets can improve the results. We also introduce a way to apply this technique for color images without introducing false colors. Finally, we show how height information on the specimen, which we obtain as a side product from our algorithm, can be used for 3D visualization.

2004

Phase calibration of a basic bright-field microscope for 3D metrology of transparent samples at the nanoscale

Martinus Gijs, Daniel Migliozzi, Bingying Zhao

SPIE-INT SOC OPTICAL ENGINEERING2021