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Clever use of digital cameras in combination with intensifiers and boosters allow us to create images of high-speed events, even when light is failing. In addition, fast gating offers possibilities to use extremely short exposures and to record multiple images in one frame. For creating images of events that are invisible to the human eye, like near-infrared (NIR) and ultraviolet (UV), radiation conversion techniques can be used. This technology note will review the techniques that make this possible.
 

Issues concerning high-speed imaging at low light levels

Ever since the invention of the digital camera, new imaging applications have been explored. The increasing possibilities of fast digital cameras have resulted in applications that were unthinkable only twenty years ago. High-speed cameras nowadays are widely used for recording of dynamic events at high frame rates (e.g. 10000 fps). The results can then be inspected by playing individual frames at a lower speed. 

High-speed imaging up to 100000 fps is easily feasible with current technology. But what if you need to create high-speed images when light conditions are far from optimal? Your high-speed camera will be no good under these circumstances, as a certain brightness of the object is required for the high frame rates that are used. The lack of light in combination with short exposure times will result in underexposed and noisy images. The obvious solution would be to increase the illumination level of the object. However, in some cases it is just not possible to add more light, for example because:

  • The object to be recorded generates light by itself. This may be the case for phenomena like the combustion process (flames and turbines), or in living cells that emit fluorescent light. 
  • The radiation level corresponding to the required brightness would cause an unacceptable temperature rise of the object. 

And what if the image signal has become too low because of the high frame rates? Camera noise will be an additional problem then.

Fortunately, there is a high-tech solution for these problems: the image intensifier. It is used to intensify the image before it is projected onto the image sensor of the high-speed camera. The intensified image results in a sensor signal that is typically 10000 times higher than without using an image intensifier - in the process elevating the signal above camera noise level. 

 

How does an intensifier work?

Figure 1. Photons are converted to electrons, accelerated and then multiplied in the MCP.

The image intensifier is a vacuum tube with a photocathode at the input, a micro-channel plate (MCP) in the middle and a phosphorescent screen at the output, as shown in figure 1. Photons are processed as follows:

  1. The image is projected onto the photocathode. The photocathode converts the incoming light (photons) into electrons. The electrons are emitted in the vacuum tube and accelerated towards the MCP by an electric field.
  2. The MCP is a thin plate consisting of many parallel micro channels; each channel works as an electron multiplier by secondary emission from the channel wall. The gain of this multiplier depends on the voltage that is applied between the input and the output of the MCP. Typical electron gain is in the order of 10,000. At the end of the channel, the electrons are accelerated by an electric field towards the anode screen.
  3. The anode screen is a phosphor layer deposited at the vacuum interface of the output window; it is covered by a thin aluminum film to prevent light feedback. The anode screen has a potential of 6 kV with respect to the MCP. The electron energy is absorbed by the phosphor material and converted into light. The result is a visibly intensified image at the output of the intensifier. 

The output window of the intensifier is usually fiber-optically coupled to the next component. This can either be the image sensor or to a next stage of the intensifier.
 

What if an intensifier is not enough?

The intensifier in combination with a high-speed camera offers great possibilities, but sometimes the quality of the resulting images is still insufficient. Light output limits the maximum frame rate that can be obtained when using an image intensifier. The light output increases linearly with the input as long as the gain is constant. However, the gain of the MCP is only constant up to a certain output level, even in the case of special lower-resistance MCPs that are used in high-speed applications. Above a certain level, the MCP becomes saturated and the number of electrons at the end of the MCP will no longer increase. This will result in a maximum output brightness that is insufficient for many high-speed applications.

Increasing the gain of the intensifier by applying multiple MCPs will not help either: the maximum output is limited by the same maximum output current of a single MCP.

What happens if we add another intensifier, but without the MCP, as a second stage? This is what we call a booster. There will be no saturation in the second stage, but the extra gain factor results in more light at the output of the second stage. If there is more light, a higher frame rate can be used. 

Figure 2. A booster is a second stage intensifier, but without the MCP. The advantage is that no saturation will take place in the second stage.

Figure 3 shows a comparison of three images of a blue gas flame, recorded with different techniques. The first recording (figure 3a) shows the gas flame to be studied in  detail. The light intensity of the flame is not very high. To see any details, especially in close-ups, very short exposure times are required.

Figure 3. Combustion research - three recordings compared.

Figure 3. Combustion research - three recordings compared.

Figure 3b was made with a standard high-speed camera at 1000 fps and a 1 ms exposure time. On the one hand a longer exposure time is needed to increase the sensitivity of the camera; on the other hand, a shorter exposure time is needed to prevent motion blur.

The third recording (figure 3c) was made with an intensified high-speed camera system at 2000 fps and a 15 us exposure time. The intensified high-speed camera is sensitive enough to image the flames at frame rates up to 100000 fps. By using gating (fast electro-optical shutter function of the image intensifier), the exposure time can be limited to a value at which motion blur is no longer an issue.
 

Boosting image brightness

So the output brightness of an image can be increased by adding a booster as the second stage intensifier. A booster is a so-called first generation (Gen1) image intensifier. It offers a relatively low but constant gain (ca. 10x) up to very high light levels. 

Gen1 image intensifiers are available as proximity-focused diodes. This is the type of intensifier schematically displayed in figure 2 as second stage. Gen 1 image intensifiers can also be used in electrostatic inverter tubes. This type offers electron-optical demagnification; it can be used to match the intensifier better to the image sensor format of the camera. The demagnification also increases the output brightness with a factor \(1/M^2\) (\(M\) is the magnification).

A demagnifying intensifier may be the ideal solution if either a booster is needed as a second stage intensifier or the format of the image sensor is considerably smaller than the 18 or 25 mm intensifier.

An extra brightness gain is achieved that increases the sensitivity of the intensified high-speed camera. No tapered fiber optics nor demagnifying relay lens, both cutting down the coupling efficiency, are needed. The output of the demagnifying image intensifier is 1:1 imaged onto the image sensor, either by a straight fiber optic face plate or a 1:1 relay lens.
 

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. Figure 4 shows a recording sequence of a combustion cycle of a fuel injection engine at 22000 fps, made with a gated intensified high-speed camera.

Figure 4. Combustion cycle of a fuel injection engine.

Figure 4. Combustion cycle of a fuel injection engine.

Figure 5. Sensitivity of photocathodes. GaAs and GaAsP cathodes are usually offered in Gen3 intensifiers, while S20, S25 and broadband are common in Gen2 intensifiers.

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. Figure 5 shows the spectral sensitivity curves of various photocathode types.

When not to use a booster

As explained earlier, adding a booster to your intensifier will allow for higher frame rates. However, as the booster is an extra component in the imaging system, it will also lower the resolution of the total system. For comparison: an intensifier has a typical resolution of 45 line pairs per mm while an intensifier combined with a booster will bring the resolution down to 25 line pairs per mm.

This is when camera pixel size becomes important: when the camera has large pixels (e.g. the 20 um typical for a high-speed camera), the change from a 45 to a 25 lp/mm resolution will be negligible. When a camera with small pixels is used, however, the change in quality of the image will be significant.
 

Fiber-optic coupling or lens coupling?

Obviously, it is important that image quality is maintained as much as possible when using intensifiers. At the same time, light efficiency should be maximized. This can be achieved by using a fiber-optic window as the output of the first stage and as the input of the second stage.

A fiber-optic window is a solid piece of glass consisting of millions of parallel glass fibers sealed together. Each fiber acts as an independent light conductor. The shape of the window can either be flat (parallel input and output faces), or concave. Fibers with a concave surface are used for distortion correction in electrostatic image inverters.

Often the second stage will also have a fiber-optic output to allow coupling to a third stage, or to the image sensor of the camera. In the latter case the image sensor of the camera should be equipped with a fiber optic input window. In addition, take the following into consideration when you need to make a choice for either fiber-optic coupling or lens coupling:

  • Fiber-optic coupling is a permanent connection; the connection is made during the manufacture of the integrated intensified camera. 
  • A fiber-optic window transfers an image from one face to the other. If the fiber optic has a tapered form, the image is reduced or enlarged. This characteristic can be used to match it to the format of a coupled imaging component. 
  • While fiber-optic coupling between intensifiers is the standard technique, coupling to the camera can also be done by lens optics. Disadvantages of lens coupling are the greater loss in efficiency (compared to fiber optics) and the lenses are more bulky.
  • Lens coupling offers the flexibility of easy decoupling, allowing you a choice to make camera recordings with or without the use of an intensifier.
  • Governed by the laws of optics no increase in brightness is achieved when demagnifying the image either by use of fiber optics or by lenses.
     

How to choose the correct image intensification solution

Before you start to look for a solution for a high-speed imaging problem, you will need to determine the following:

  • What exactly would you like to record?
  • How many frames per second (fps) do you need to record?
  • What is the required gating repetition rate?
  • Do you wish to use a camera that you already have? If yes, what type of camera is it and what type of lens mounting is required.
  • Which photocathode will be suitable? You may want to consult a specialist in intensified high-speed imaging for help.
  • Will you need a booster? Again, you may want to consult a specialist in intensified high-speed imaging for help.
 

Single-stage intensifier

Dual-stage intensifier

Max. gain
Effective diameter (mm)
Max. gating (ns)
Gate repetition rate (kHz)
Max. sensor framerate (fps)

10000
18 or 25
40 (fast gating: < 3)
100 (gast gating: 200)
10000

100000
18 or 25
40 (fast gating: < 3)
100 (fast gating: 200)
100000

Generally, a two-stage intensifier (with the booster as second stage) is recommended at frame rates of 1000 fps or more. If the object to be imaged consists of individual light emitting events against a dark background, a single stage might be adequate up to much higher frame rates, as the charge capacity of the MCP allows a temporal high-brightness output when such an event is imaged.

Depending on the sensitivity of the high-speed camera and the light-coupling efficiency between intensifier and camera, frame rates of up to 200000 fps are feasible. If higher frame rates are required, more than one booster stage can be used.
 

Looking for a specialist in intensified high-speed imaging?

When looking for an intensified high-speed imaging specialist, be sure to consider the following requirements:

  • At least 20 years market experience: make sure you contact a company that has experience with intensified high-speed imaging, has a variety of customers and extensive market knowledge.
  • Extensive support: you will want a company that provides consulting, implementation and support.
  • Variety of solutions: the ideal company offers a range of practical solutions for high-tech imaging issues.
  • Integration with current equipment: you will want a company that offers options for integration of high-speed cameras, intensifiers and/or boosters with existing equipment.
     

Lambert Instruments

Lambert Instruments is a company devoted to development, production and worldwide sales of products for high-speed imaging at low light levels. The aim of Lambert Instruments is to provide a well-defined product that fits budget and planning. 

Lambert Instruments has successfully provided solutions with intensified high-speed imaging for a wide range of applications:

  • Combustion research
  • Plasma physics
  • Time-Resolved Fluorescence
  • Dynamic phenomena in microscopy (imaging of rotation of single molecules of ATPase, detection of Brownian motion)
  • Laser-induced fluorescence (LIF)
  • Particle image velocimetry (PIV)
  • Microfluidics

We welcome feedback on this technology note and we encourage discussion about the principles of high-speed imaging at low light levels. Also, we consider it a challenge to solve any problem you may have concerning high-speed imaging at low light levels. Please feel free to contact us.

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