Technologies

Electron Multiplication from Texas Instruments

An EMCCD (Electron Multiplication CCD) is identical to its conventional cousin except for the fact that an additional (high voltage) serial register is placed between the read-out register and the output amplifier. During the read-out process, all the charge in each of the image-area pixels passes in turn through this 400-pixel long register and each time the electrons are clocked from one pixel to another at around 20V a small increase in signal is seen due to Impact Ionisation.

The probability of multiplication per stage is between 1.5% to 2%, but as there are 400 stages the overall increase in gain in the register is substantial and sufficient to render the effects of read-out noise negligible even at pixel frequencies in excess of 35MHz.

The next important source of noise, other than shot noise, is that on the dark current. Cooling the silicon using a thermo-electric cooler, however, reduces the dark current and consequently the dark current noise. The degree of cooling depends on the integration time but is typically between -5 deg C and -10 deg C for video use.

When used in this way, Texas Instruments' Solid state, Impactron Electron Multiplication (EMCCD) sensors are able to image at the photon-counting level.

In conclusion:-

EMCCDs are not damaged by over-exposure and are not ‘lifed' items.

They do not require intensifiers to work satisfactorily even at the lowest light levels and so the MTF is not degraded by them.

Furthermore, the excess noise factor (around 1.4) is better than that for currently available intensifiers. The image is consequently easier to view at low light levels as there are virtually no high-intensity scintillations.

Image Intensification from Photonis-DEP

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An image intensifier first converts photons into electrons, then increases the number of electrons whilst maintaining their spatial integrity and finally converts these electrons back into photons to be viewed by the user directly.

An image intensifier is a vacuum device consisting of:-

a photocathode layer deposited on the inside surface of a glass substrate
a Micro-Channel Plate (MCP)
a Phosphor Screen deposited on the inside surface of an output substrate.

Very small gaps exist between the Photocathode and the MCP, and between the MCP and the Phosphor Screen, allowing Proximity Focussing to take place, whereby the free electrons inside the tube are given little opportunity to repel one another. In this way any degradation in the spatial resolution of the device is minimised. The necessary (high voltages) voltages are generated by a miniature power supply which is generally encapsulated around the tube at the anode end. A small dc voltage of 2.7V energises the device.

Step 1: Photon to electron conversion

When an image is focussed onto the photocathode layer by the objective lens some of the incident photons are absorbed by it in accordance with the spectral absorption of the layer and their energy is transferred to electrons, which are released into the lattice. Some of these electrons find their way to the surface of the layer and have sufficient energy to overcome the surface potential barrier there to escape into the vacuum. The combination of the photon absorption and escape probability of the photo-electrons is a useful parameter called the Quantum Efficiency. The way in which the QE varies with the wavelength of the photon is given by the Spectral Response.

Step 2: Electron multiplication

The photo-electrons emerging from the photocathode experience an electric field and are accelerated towards the MCP, which is made of a specially treated lead glass plate less than 0.5mm thick, having an array of 6um diameter (8um pitch) circular channels cut right through at a slight bias angle (approximately 5 degrees with respect to the normal).

The bias angle of the channels ensures that an incoming electron does not pass straight through the plate but is forced to hit the inside wall of a particular channel where secondary emission takes place. The resulting electrons are further accelerated down the channel by virtue of a potential gradient across the MCP (around 800V) and strike the channel wall again and again, each time releasing additional charge from the channel wall. Up to 40 collisions may take place before the cloud of electrons, perhaps 500 to 1000 strong emerge from the output surface of the MCP, resulting from a single photo-electron.

Step 3: Electron to photon conversion

The electrons emerging from the MCP experience a very high electric field and are accelerated towards and bombard the high-resolution phosphor screen where their energy is converted into radiated light. A very thin aluminium layer covering the inside of the screen prevents the light getting back to the photocathode and indeed reflect the photons towards the user's eye.

In conclusion:-

Image intensifiers are very low power devices and have a very high resolution built-in screen. As such there is no substitute for them in direct-viewing applications.

They can be coupled to CCD and CMOS sensors very easily and can be gated on and off at high speed - typically around 1 ns . This makes them attractive for many time-dependent imaging applications such as time-resolved fluorescence imaging.

Note: synchronised gating can eliminate frame-shift smear from CCDs in low light images in which a bright point of light exists.