The SPAD512S from Pi Imaging Technology is the first commercially available SPAD camera with fast gating capability. This lends itself to many applications from Fluorescence Lifetime Imaging to Quantum Optics. This gating capability combined with single-photon detection, zero read noise, and fast acquisition speed makes the SPAD512S a truly unique camera.
In gated imaging mode, it can collect light during a time window as short as 6 ns and can shift this window with a resolution of 17 ps. When synchronized with a pulsed laser, one can reconstruct light propagating from the laser by collecting thousands of frames, each with a slight shift of the gated window to observe the light at a slightly different time. By creating a time-lapse of all these images, we can reconstruct the pulse of light propagating at the speed of light through a scattering medium.
This experiment is both fascinating and mesmerizing. In the movie below, we image light in slow motion propagating through water. A glass vial containing water is located in the center, for this first experiment.
We observe the scattering of the light pulse propagating through the tank and the vial, then the pulse disappears when it exits the scattering media. From the timestamp, we can see that the recording of the pulse lasted about 6 ns, which corresponds to the short pulse duration from the laser (about 100 ps) combined with the gated window of the camera acquisition(6 ns).
Fluorescence is the property of certain molecules to absorb a photon and then emit another photon of lesser energy. When a fluorescent molecule absorbs a photon that energy pushes an electron into a higher energy orbital. The unique property of fluorescent molecules is the radiative decay of the excited orbital state. This results in emitting a photon of a lesser energy (due to the conservation of energy). For light, energy and wavelength are directly related, so from a practical perspective the molecule absorbs light at one wavelength and emits light at a longer wavelength. The amount of time these electrons stay in the excited orbital state is a characteristic of the molecule and is known as the fluorescent lifetime. This lifetime is on the order of nanoseconds, and so it cannot be measured directly using conventional camera technology. In the next experiment, instead of water, we placed regular canola oil in the vial, which is known to exhibit strong fluorescence when excited with visible blue light. Here is what we recorded:
What we see here is the pulse of light propagating through the water in the tank, through the oil sample creating a bright fluorescence signal in the vial, then continuing through the water on the other side. The pulse of light disappears from the water as it does in the first video, but the fluorescence signal persists for tens of nanoseconds in the oil, even though the laser pulse is long gone.
This video clearly illustrates the decay of the fluorescence signal. If we look at the intensity signal in the small container as a function of time for both videos, we get the following curves:
The decay of the signal in water is extremely fast. Indeed, there is no fluorescence in water so what we observe (essentially a square wave ) results from the 6ns gate of the SPAD512S camera. On the other hand, the decay of the intensity signal from the oil container takes much longer, with a fluorescence lifetime probably in the range of 10 to 20 ns.
Although it appears much longer in this video, keep in mind this is orders of magnitude faster than the fastest non-gated cameras can ever record. The unique properties of the SPAD512S, combining high-speed single-photon sensitivity and gated capability, make it an ideal option for fluorescence lifetime imaging applications.
To learn more about the SPAD512S, click HERE.