Digital holography is a technology that uses light and computation to create three-dimensional (3D) images. Unlike a photograph that captures only the brightness of light, a hologram records the entire light field, including its intensity and phase. The “digital” aspect signifies that the recording is done with a digital sensor, and the image is reconstructed using computer algorithms, replacing older methods that required chemical processing.
This digital format allows for a versatility in processing that physical holograms cannot match. By using a digital sensor, the hologram is captured quickly and becomes a set of data ready for computational analysis. The result is a true 3D representation of an object that can be manipulated and viewed from different angles.
The Science of Capturing a Hologram
Capturing a hologram relies on the wave-like properties of light, specifically interference and diffraction. Interference happens when two light waves meet and combine, while diffraction occurs when a light wave bends as it passes an object. To control these phenomena and create a stable interference pattern, a coherent light source like a laser is used.
The process starts by splitting a laser beam into two. The first, the object beam, is directed at the subject being recorded. As this light reflects off the object, its wave pattern is altered, picking up the surface’s physical characteristics. The second beam, the reference beam, is left undisturbed and aimed directly at the digital sensor.
The digital sensor, such as a charge-coupled device (CCD) or complementary metal-oxide-semiconductor (CMOS) sensor, is placed where the two beams intersect. It records the interference pattern created by the meeting of the object and reference beams. This pattern of light and dark fringes holds all the information about the object’s 3D structure, including depth and texture.
Digital Reconstruction of the Image
Once the interference pattern is captured, the digital data is transferred to a computer for reconstruction. The computer uses algorithms based on diffraction theory, which mathematically describes how light propagates, to generate a 3D image. This computational approach replaces the need for a physical laser to illuminate a holographic plate.
The core of the process is a numerical simulation. The computer simulates shining a light source back through the recorded interference pattern. By calculating how this virtual light would diffract, the algorithm reconstructs the original light field that came from the object.
The final output is a 3D image that can be displayed on a screen and manipulated. For instance, it can be rotated to view different sides or refocused to examine different depths. A single hologram contains enough information to reconstruct the optical field at any distance.
Distinguishing Digital Holography from Other 3D Technologies
Digital holography differs from analog holography in its recording and reconstruction. Traditional holography records the interference pattern on a physical medium like a photographic plate, requiring chemical development and a laser for viewing. Digital holography uses a digital sensor and reconstructs the image computationally, eliminating physical processing.
It is also different from stereoscopic 3D, used in 3D movies and some VR headsets. Stereoscopy presents two slightly different 2D images to each eye to create an illusion of depth. Digital holography recreates the actual 3D light field of an object, providing true depth and parallax—the way an object’s position appears to change when viewed from different lines of sight.
The “holograms” seen in concerts are also not true holograms. These are two-dimensional projections onto a transparent screen, an illusion known as Pepper’s Ghost. True digital holography creates a three-dimensional image that appears to float in space and is viewable from multiple angles.
Applications Across Various Fields
A primary application of digital holography is in microscopy. Digital holographic microscopy (DHM) is used in biology and medicine to create 3D images of living cells over time. It can be done without stains or dyes that could harm the cells, allowing for the observation of natural cellular processes.
In manufacturing and engineering, digital holography is used for industrial inspection. It performs non-destructive testing by measuring microscopic defects, stresses, and vibrations on material surfaces with high precision. This allows for quality control without damaging the object being inspected.
Medical imaging also uses digital holography. It has applications in ophthalmology for imaging the retina and in endoscopy for providing 3D views from inside the human body.
The principles of digital holography are also applied to data storage and security. Holographic data storage can place vast amounts of information in a small space. The complexity of holographic patterns also makes them useful for creating anti-counterfeiting tags for currency and other items.