Smartphone microscope quickly reconstructs 3D holograms

Researchers have developed a new smartphone-based digital holographic microscope that enables precision 3D measurements. This highly portable and inexpensive microscope could help extend 3D measurement capabilities to a wider range of applications, including educational uses and point-of-care diagnostics in resource-limited environments.

Holographic microscopes digitally reconstruct holograms to extract detailed 3D information about a sample, enabling precise measurements of the sample’s surface and internal structures. However, existing digital holographic microscopes typically require complex optical systems and a personal computer for calculations, making them difficult to transport or use outdoors.

“Our digital holographic microscope uses a simple optical system created with a 3D printer and a smartphone-based computing system,” said Yuki Nagahama, head of the research team from Tokyo University of Agriculture and Technology. “This makes it inexpensive, portable, and useful for a variety of applications and settings.”

In the newspaper Applied optics, Researchers demonstrate the ability of the smartphone-based digital holographic microscope to capture, reconstruct, and display holograms in near real-time. The user can even use a pinch gesture on the smartphone screen to zoom in on the reconstructed holographic image.

“Because our holographic microscope system is inexpensive to build, it could be useful for medical applications, such as diagnosing sickle cell disease in developing countries,” Nagahama said. “It could also be used for field research or in education by allowing students to observe living organisms at school and at home.”

Quick rebuild from smartphone

Digital holographic microscopes work by capturing the interference pattern between a reference beam and light scattered by the sample. The hologram is then digitally reconstructed, generating 3D information that can be used to measure features of the sample, even those below the surface.

Although smartphone-based digital holographic microscopes have already been developed, available technologies do not allow for the reconstruction of holograms on a separate device or do not allow for their reconstruction in real time. This limitation results from the limited computing and memory capacity of most smartphones.

To achieve fast reconstruction on a smartphone, the researchers used an approach called band-limited double-step Fresnel diffraction to calculate the diffraction patterns. This method reduces the number of data points, allowing for faster computer reconstruction of images from holograms.

“When I was a student, I worked on portable digital holographic microscopes, which initially used laptops as the computing system,” Nagahama said. “With the rise of smartphones, I began to explore their potential as computing systems for broader applications and considered exploiting them for tasks such as removing artifacts from observed images, which ultimately shaped the development of this microscope.”

To facilitate portability, the researchers created a lightweight housing for the optical system using a 3D printer. They also developed an Android app to reconstruct the holograms acquired by the optical system.

The microscope generates a reconstructed image of the hologram on the image sensor of a USB camera integrated into the optical system. This hologram can be observed by the Android smartphone, which provides a computer reconstruction of the image in real time. The reconstructed hologram is then displayed on the smartphone, where users can interact with it via the touch screen.

Near real-time reconstruction

The researchers evaluated their new microscopy system by using a prepared object with a known pattern and then checking whether the pattern on the object could be accurately observed under the microscope. They were able to successfully observe the pattern on the test target and also used the microscope to image other samples such as a cross-section of a pine needle.

The researchers showed that band-limited, double-step Fresnel diffraction could reconstruct holograms at a frame rate of up to 1.92 frames per second. This made it possible to display images in near real time when observing stationary objects.

Next, they plan to use deep learning to improve the quality of images generated with the smartphone-based microscope. Digital holographic microscopes often generate unwanted secondary images when reconstructing the hologram, and the researchers are exploring how deep learning could be used to remove these unwanted images.