Optical imaging is widely used for detection, inspection, and diagnostics in numerous industrial, biomedical, and scientific applications. In particular, imaging modalities, such as differential interference contrast (DIC) microscopy [1
] and phase-contrast (PC) microscopy [2
], which can capture images of transparent objects without the need for chemical staining, are of a significant importance. Metallurgy and semiconductor processing heavily rely on these modalities for imaging surface scratches, lines and edges, defects, and contaminations inside the material being tested [3
]. Also, such modalities play a crucial role in detection and diagnostics of diseased cells (e.g., leukemia) in advanced biomedical research and clinical settings [6
High-throughput imaging in such applications is highly desirable [10
], but extremely challenging. For instance, high-throughput screening of biological cells (that show nearly no contrast with respect to their aqueous surroundings) enables finding of rare diseased cells in a large population of healthy cells. However, conventional techniques (e.g., DIC microscopy) with the ability to perform this task have relied on CCD (charge-coupled device) and CMOS (complementary metal-oxide-semiconductor) image sensors [11
]. Hence, their image acquisition throughput is limited by that of CCD and CMOS cameras. More importantly, the shutter speed of even the fastest cameras is too slow, resulting in significant blurring of images during high-speed screening. Due to these technological limitations, conventional DIC and PC microscopy have not been commonly used for applications that require monitoring of dynamic samples in real time with high throughput.
The recently introduced imaging technology known as serial time-encoded amplified imaging or microscopy (STEAM) [12
] overcomes limitations in conventional imaging and provides ~1000 times higher frame rates and shutter speeds than conventional image sensors. STEAM employs an amplified space-to-time mapping technique to encode the spatial information of an object into a one-dimensional (1D) serial time-domain optical waveform and optically amplify the image, simultaneously. The high-speed capability of the STEAM imager enables its use for applications in which high-throughput screening of an object is of interest, such as blood cell screening [13
]. Unfortunately, the STEAM imager is inadequate for imaging of transparent objects but rather limited to opaque samples or samples with high refractive-index or absorption contrast. While the objects (e.g. biological cells) can be stained with dyes to increase their contrast, chemical staining requires time-consuming sample preparation and is often accompanied by cell death.
Here we present a method that overcomes the contrast limitation of STEAM and hence performs high-speed contrast-enhanced imaging of transparent media without the need for chemical staining. The presented technique, which we refer to as Nomarski serial time-encoded amplified microscopy (N-STEAM) combines STEAM’s high-speed capability and DIC/Nomarski microscopy’s ability to image unstained transparent objects. To demonstrate our method, we show contrast enhancement in imaging of a transparent refractive-index-modulated structure and white blood cells in flow with 33 ps dwell time (shutter speed) at 36.1 MHz frame rate using a single-pixel photodetector. This method is expected to be a valuable tool for high-throughput screening and sorting of unstained cells.