Classic Lock-in amplifiers measure at a single point. In Lock-in imaging, on the other hand, the same demodulation is performed pixel by pixel – amplitude and phase for each pixel. This enables experiments that are difficult or impossible to perform with point measurements: spatially resolved phase information, fast mapping, and robust evaluations even with strong noise.
In applications with challenging lighting conditions – be it a weak signal in strong noise or very strong background or interference light – conventional cameras reach their limits. Either saturation (clipping) occurs early, or the weak signal must be averaged for noise reduction, which slows down the process or even makes it impossible. In both cases, the useful signal is dominated by interference sources. However, if the useful signal can be modulated and thus separated from the interference signals, the heliCam™ C4 continues to deliver high-quality images.
Figure 1: A camera with pixel-level Lock-in technology can almost completely suppress specific sources of interference such as constant background or 1/f noise.
A Lock-in amplifier makes extremely weak signals visible, even when they are superimposed by noise and extraneous light. It compares the input signal s(t) synchronously with a reference signal of the same frequency and phase (phase-sensitive detection).
Brief operation:
1. Input: s(t) contains useful signal + noise + interference signals
2. Synchronous Multiplication: with references cos(ωt) and sin(ωt) → two channels I (In-Phase) and Q (Quadrature).
3. Integration / Low-pass: Averaging over the measurement time T reduces the noise bandwidth (ENBW ≈ 1/(2T)) and suppresses components outside the reference frequency.
Result:
• Amplitude: R = √(I2 + Q2)
• Phase: φ = atan2(Q, I)
• Broadband noise and non-synchronous extraneous light are largely averaged out; the useful signal remains as a DC component.
• In practice, the Lock-in acts like a very narrow-band, phase-sensitive filter, the selectivity of which is determined by the integration time T.
In Lock-in imaging, the same process is performed pixel by pixel: Each pixel provides I/Q (or amplitude/phase) – the basis for spatially resolved, phase-sensitive measurements under demanding lighting conditions.
Figure 2: Highly noisy useful signal in the time domain (left) and frequency domain (right). Despite larger interference amplitudes, the useful signal can be successfully extracted and measured thanks to the Lock-in principle.
The Lock-in pixel signal processing developed and patented by Heliotis is the core of all our products. In our proprietary CMOS image sensor, phase-sensitive demodulation is performed directly at each pixel, rather than in the downstream signal path. Each pixel performs a synchronous I/Q measurement and outputs a 10-bit In-Phase (I) and a 10-bit Quadrature (Q) value (dual-phase demodulation). Amplitude and phase of the useful signal can be determined directly from I and Q – area-based, stable, and reproducible.
This pixel-parallel demodulation suppresses constant background, 1/f noise, and non-synchronous interference light already during acquisition, so that even very small, modulated signals become reliably visible – without long averaging times.
The application areas in which Lock-in cameras are most frequently used can be roughly outlined as follows:
Imaging in scattering media
A large part of research aims to capture optical images in turbid structures or, more generally, to focus light that interacts with a scattering medium.
Interferometric 3D Imaging
In interferometric 3D imaging, both improvements to scanning white light interferometry and alternative measurement methods have been proposed.
Quantitative Phase Imaging Methods
Several research groups have proposed quantitative phase imaging methods to obtain label-free and high-contrast images of, for example, biological cells, which appear largely transparent in classical, absorption-based optical measurements.
Quantum Sensing
Applications in quantum sensing are gaining increasing importance, particularly with regard to a widefield quantum diamond microscope for dynamic imaging of the smallest magnetic fields.
Spectroscopy
Spectroscopic methods that detect the smallest and fastest periodic changes in the spectrum, caused by various mechanisms, have been repeatedly implemented with Lock-in cameras from Heliotis.
Further Applications
More information on the mentioned and other application examples of the Lock-in camera can be found in our collection of research publications.
Lock-in cameras are a comparatively new but extremely powerful tool in research. They open up exciting possibilities for phase-sensitive imaging and high-precision experiments.
Below you will find answers to some frequently asked questions.
The heliCam™ C4 with 542×512 Lock-in pixels demodulates signals in the range from 305 Hz to 134 kHz.
The heliCam™ C4M with 1024×1102 Lock-in pixels demodulates signals in the range from 305 Hz to 50 kHz.
Further information can be found in our documentation and on the following pages:
Yes, your questions are usually answered by our support team (for installation, technical questions, etc.) and our application experts (for application-related questions) in Switzerland. Before purchase, we would be happy to provide individual and non-binding advice.
Rentals are possible after applicability has been validated with our experts.
Our experts will help you develop the right setup for your specific question, choose optimal parameters, and quickly lead your experiment to success.
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