Viewing entries in
Image Intensifiers

Intensifier Gating for Ultra-Short Exposure Times

Comment

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.

Comment

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.

Fiber-Optic Coupling

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.

Modulated Intensifiers for Lifetime Imaging

As part of its portfolio, Lambert Instruments designs and manufactures ICCD cameras based on modulated image intensifiers for frequency domain imaging techniques such as FLIM. Standard products such as the new TRiCAM M ICCD camera, the TRiCATT M modulated intensifier attachment, and the LIFA system are all based on a proximity-focused modulated Gen II or Gen III image intensifier.

 Figure 1. Schematic representation of a proximity-focused image intensifier, showing its photocathode, micro-channel plate and anode, fiber-optically coupled to a sensor.

Figure 1. Schematic representation of a proximity-focused image intensifier, showing its photocathode, micro-channel plate and anode, fiber-optically coupled to a sensor.

 Figure 2. Modulated detector gain (green), pulsed excitation (blue) and fluorescence emission (red) in a homodyne FLIM system. The phase and duty cycle of the detector gain is controllable.

Figure 2. Modulated detector gain (green), pulsed excitation (blue) and fluorescence emission (red) in a homodyne FLIM system. The phase and duty cycle of the detector gain is controllable.

The modulated sensitivity of an image intensifier is produced by high-frequency switching of its photocathode voltage. In this modulation mode the temporal and also the spatial characteristics of the image intensifier are different from the nominal characteristics under continuous operation.

The following table lists the nominal characteristics for the Gen II and Gen III intensifiers provided.

 

Gen II S20 & S25

Gen III GaAs

Gen III GaAsP

Spatial resolution
Photocathode QE at 550 nm

28 lp/mm
14%

17 lp/mm
28%

7 lp/mm
48%

The increased sensitivity of the Gen III image intensifier results in a better lifetime accuracy, although the amount further depends on the model and the modulated light source used. For a LIFA Gen III GaAs compared to a LIFA Gen II S25 at 550nm, both using the same Multi-LED light source, the increased sensitivity results in an increased lifetime accuracy of 40% at the same acquisition speed and fluorescence intensity.

For phosphorescence lifetime imaging with the TRiCAM GM and the LIFA-P the image intensifier is operated at lower frequencies spatial resolution is higher, reaching values in excess of 35 lp/mm for a Gen III GaAs image intensifier.

Please note that the characteristics of an image intensifier are unique and are known to vary between units.

 Figure 3. Image intensifier spectral response.

Figure 3. Image intensifier spectral response.

Dual-Stage Image Intensifier

In very low-light situations or when a very short exposure time is required, a dual-stage image intensifier may be required. The first stage is the same as a single-stage image intensifier; it has a micro-channel plate that multiplies the electrons emitted by the photocathode. The second stage is often referred to as a booster. This stage does not have a micro-channel plate, it multiplies the incoming photons without the saturation characteristics of a micro-channel plate.

Dual-stage image intensifier with fiber-optic coupling

The figure above shows a schematic representation of a dual-stage image intensifier that is fiber-optically coupled to the image sensor. The first stage is similar to a single-stage image intensifier.

Spatial Resolution of Image Intensifiers

The limiting spatial resolution of an intensified imaging system depends on several factors, including (but not limited to)

  • Image intensifier type
  • Image intensifier gain
  • Pixel size

Before we can discuss each of these factors, we need to define what limiting spatial resolution means. When characterizing an imaging system, the limiting spatial resolution describes the smallest features that can be distinguished. There are several ways of characterizing the spatial resolotion, most of them use a test chart like the USAF resolution test chart. Such charts have a series of lines on them, the smaller the lines an imaging system can distinguish, the better the spatial resolution.

  SilverFast Resolution Target USAF 1951 ( Creative Commons 3.0 )

SilverFast Resolution Target USAF 1951 (Creative Commons 3.0)

Spatial resolution is quantified in the number of line pairs that can be distinguised per millimeter (lp/mm). A line pair consists of a dark line and a bright line. So if one line is 5 microns wide, then a line pair will be 10 microns wide and there would be 1 mm/10 microns = 100 line pairs per millimeter.

Image intensifier type

There is a wide range of image intensifiers available. We advise our customers on the type of intensifier they need for their application based on the wavelengths that are important for our customers, and the frame rates they need. High-speed intensifiers usually have a lower spatial resolution than image intensifiers that are optimized for lower frame rates.

Image intensifier gain

We can increase the MCP voltage of an image intensifier to increase its gain. But MCP noise and the size of the electron cloud at the exit of the MCP also depend on the MCP voltage, so the spatial resolution will be slightly reduced as the MCP voltage is increased. You can learn more about how an image intensifier works on our image intensifier page.

Pixel size

Finally, the limiting spatial resolution of an imaging system is determined by the size of the pixels that collect the light from the image intensifier. You can use our intensifier-sensor matching calculator to find the theoretical maximum sensor resolution. It is calculated using the size of the pixels.

  25 and 50 lp/mm (not to scale)

25 and 50 lp/mm (not to scale)

For example: If the pixels are 20 microns wide, we would need two adjacent pixels to distinguish a bright line and a dark line of a test chart. Those two pixels would have a total width of 40 microns, so the theoretical spatial resolution would be 1 mm/40 microns = 25 lp/mm.

The element of the imaging system with the lowest spatial resolution determines the limiting spatial resolution of the whole system. In our example, we have a sensor that has a limiting resolution of 25 lp/mm. If we have an image intensifier with a 50 lp/mm resolution, the size of the pixels would limit the resolution of the imaging system to 25 lp/mm.

However, if the pixels are smaller, 2 microns for instance, then the theoretical resolution of the sensor would be 250 lp/mm. In that case, the resolution of the image intensifier would determine the resolution of the total system.

Other factors

Many factors influence the spatial resolution of an intensified imaging system, like the size of the image intensifier, the number of image intensifiers and the optics. If you would like more information about the right image intensifier for your application, please contact us.


Related Posts

Third Generation Image Intensifier

The next step in technology is the third generation (GenIII) image intensifier in which the multi-alkali photocathode is replaced by a Gallium-Arsenide (GaAs) or a Gallium-Arsenide-Phosphide (GaAsP) photo-cathode. The quantum efficiency (QE) of these types of photo-cathodes are much higher as compared to the multi-alkali photocathode of the second-generation image intensifiers.

Recently, new filmless Gen III intensifiers have been developed that are using the high QE to its full extend. The higher QE results in a better SNR or in shorter exposure times at equal SNR. In the graph, spectral sensitivity curves of multialkali photocathodes, such as S25, S20 and broadband, are shown in comparison with GaAs and GaAsP photocathodes.

Second Generation Image Intensifier

In the second-generation image intensifier a so-called Micro-Channel Plate or MCP is added, improving the gain of the image intensifier enormously. The MCP is placed between the cathode and the anode and acts as an electron multiplier.

The MCP is a 0.5 mm thick plate with millions of 6 micron wide holes. An accelerated electron coming from photocathode will be accelerated towards the MCP. When the electron hits the wall of one of the MCP channels it will spawn secondary electrons. Due to the voltage over the MCP, these electrons will also be accelerated, and hit the surface of the MCP in their turn. Again spawning new (tertiary) electrons. This process is repeated several times, resulting in an electron gain far higher (several thousand times higher) than in first generation intensifiers.

When an electron leaves the MCP, it is propelled to the phosphor screen where it will generate multiple photons. The overall gain of an image intensifier is up to ten thousand. With two or three MCPs amplifications up to 10 million times gain is possible. The gain of the image intensifier can even be controlled by changing the voltage over the MCP.
 

Gating the Image Intensifier

An important feature of the MCP, and therefore the second-generation image intensifier, is that it can be gated. Gating the image intensifier offers a whole new possibility of using the image intensifier as an ultra fast (electro-optical) shutter. Gating is achieved by controlling the photocathode voltage of the image intensifier, creating a shutter with effective exposure times down to a few nanoseconds.

By applying a negative voltage to the photocathode, typically -200 V with respect to the MCP input, photoelectrons are generated in the photocathode. They are emitted and accelerated to the MCP to be multiplicated. In this situation the image intensifier is "gated on". When applying a small positive voltage to the photocathode, typically 50 V with respect to the MCP, the photoelectrons can not be emitted and the intensifier is "gated off". With this gating option the input light range is extended significantly and it offers unique options for time resolved experiments.

In the Lambert Instruments Fluorescence Attachment, a second-generation image intensifier is used as a detector. The image intensifier, combined with a CCD camera, is attached to a widefield fluorescence microscope. The photocathode of the intensifier is located in the image (focal) plane of the microscope. In the frequency domain LIFA, the photocathode is switched from positive to negative at the same frequency as the light source is modulated.

Furthermore gating can be used to reduce or prevent the effect of motion blur when capturing fast moving objects. In our Intensified cameras gating is standard synchronised with the exposure period of the CCD or CMOS sensor.
 

Advantages

  • Fibre-optic/glass/quartz/MgF2 input windows
  • Many photocathode types from UV to NIR
  • High gain
  • Fast shuttering is possible (gating)
  • Good over-illumination protection
  • Maximum output brightness control
  • Wide gain control range
  • Many types of output phosphors
  • Distortion free


Disadvantages

  • Limited intra-scene dynamic ranges
  • Low maximum output brightness for fast phosphors
  • No de-magnifying models
  • MCP introduces extra noise

First Generation Image Intensifier

A first generation image intensifier does not use a micro-channel plate. The electrons are guided from the input to the output by means of electrostatic focussing. Two types can be distinguished: proximity focussed diodes and electrostatic inverters. In the latter a structure of electrodes form an electrostatic lens that focusses the electrons coming from cathode onto the anode. The advantage of electrostatic focussing is that it allows de-magnification of the image. This is especially interesting when these devices are coupled to small CCDs.

Advantages of first generation tubes

  • Available in de-magnifying formats

  • Therefore no fiber optic taper required

  • No MCP noise

  • High intra-scene dynamic range

  • Low cost (standard models)

Disadvantages

  • Electrostatic inverters show a few percent of image distortion

  • Relatively low gain

  • Gating not possible

  • No UV sensitivity

  • Limited external gain control

  • Poor over-illumination protection

The Image Intensifier

An image intensifier is a device that intensifies low light-level images to light levels that can be seen with the human eye or can be detected by a camera. An image intensifier consists of a vacuum tube with several conversion and multiplication screens.

An incident photon will hit a light sensitive photo-cathode screen. Photons are absorbed in the photocathode and give rise to emission of electrons into the vacuum. These electrons are accelerated by an electric field to increase their energy and focus them on the multi channel plate (MCP).

 

Inside the MCP the electron image is multiplied, after which the electrons are accelerated towards an anode screen. The anode screen contains a layer of phosphorescent material that is covered by a thin aluminium film.

The anode contains a phosphor such that when striking the anode the energy of the electrons is converted into photons again. Because of the multiplication and increased energy of the electrons the output brightness is higher as compared to the original input light intensity.


Related Posts

Intensified Cameras for Lifetime Imaging

Intensified cameras enable full-field frequency-domain and time-domain FLIM. The image intensifier becomes an ultra-fast electro-optical shutter by operating it at radio frequencies allowing time-resolved imaging. The high-resolution image intensifier is the key component of the TRiCAM (part of the LIFA) and the TRiCATT camera attachment. Its photon gain is typically in the range of 100 to 10000. Lambert Instruments provides different image intensifiers based on photocathodes with different spectral sensitivity to match a range of applications in the UV, visible and NIR.

For FLIM in the lifetime range of 0 ps to 1 ms we provide S20 (UV) and SuperS25 (visual) image intensifiers. For increased quantum efficiency of the photocathode in the visual part of the spectrum in this lifetime range, a GaAs intensifier is available. For near-infrared applications up to about 1100 nm an InGaAs photocathode is available.

The graph below shows the spectral sensitivity of these photocathodes.