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Gating

Intensifier Gating for Ultra-Short Exposure Times

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Intensifier Gating for Ultra-Short Exposure Times

The photocathode of an image intensifier can be used as an ultra-fast shutter. By varying the voltage on the photocathode, the image intensifier gate can be switched between open and closed. When the gate is open, incoming photons can enter the image intensifier and the light intensity is boosted. When the gate is closed, incoming photons can't enter the image intensifier.

Switching the gate between open and closed states can be done very quickly, thus allowing the gate to be opened for a very brief moment. This enables effective exposure times in the order of nanoseconds.

By opening the image intensifier gate only once during each exposure of the camera sensor, you can eliminate motion blur even when imaging fast-moving objects. The video below illustrates the effects of image intensifier gating.

The first part of the video shows a bullet recorded at 15000 fps. Despite the high frame rate, the bullet moves so fast that it is blurry. The second part of the video shows the benefits of image intensifier gating. By opening the image intensifier for just 2 microseconds during each frame of the camera, the bullet is no longer blurry. It is perfectly sharp in each frame of the video.

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Enhanced Intensifier Techniques

Apart from the obvious advantages of gain and intensification, the intensifier offers additional possibilities. It can serve as a fast shutter, by use of gating. At a negative cathode voltage, the intensifier is open. It closes at a positive voltage. Switching can be done very quickly and at high repetition rates, resulting in very short exposures (down to nanoseconds), synchronized with a camera that can operate at very high frame rates. Ultrashort exposure will reduce any motion smear to a minimum. The figure below shows a recording sequence of a combustion cycle of a fuel injection engine at 22000 fps, made with a gated intensified high-speed camera.

An image intensifier can also serve as a radiation converter. Images in the part of the spectrum that are invisible to the human eye (for example UV or NIR) can be converted to a different part of the spectrum that can be detected by an image sensor. The spectral sensitivity of the image intensifier is determined by the type of photocathode that is chosen.