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.

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.

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

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.

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.

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

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

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.

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.

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.