Picosecond high speed ICCD camera family:
4 Picos

What is the difference between field rate and frame rate?

In contrast to progressive scan in interlaced video technique the pictures on the screen are built up by a sequence of fields which are only the half of a frame. Each frame is divided into two fields where one of both contains the even rows only and the other one contains the odd rows only. Instead for showing 30 frames per second for example, 60 fields are shown per second. Because of the doubled repetition frequency this concept results in a much more flicker-free video reproduction.

What does GenII or GenIII mean?

When night vision devices were originally developed their image intensifiers were bulky and less sensitive units. Improvements were developed later on and from the second generation of image intensifiers, the GenII intensifiers, the innovative multi channel plate was utilized. GenII intensifiers are equipped with different types of photocathode materials, that allow to customize the image intensifier to the spectral requirements of the specific application. However, photocathodes of GenII intensifiers provide quantum efficiencies of the order of only 25%.

GenIII intensifiers, the third generation, also use the highly effective multi channel plate. But, in addition, they offer gallium arsenide photocathodes that feature quantum efficiencies of more than 50%. A selection of available photocathode materials is shown here.

What is the main functional difference between ICCD cameras and EMCCD cameras?

An ICCD camera contains a CCD sensor and an image intensifier mounted in front of it. The image intensifier multiplies the incoming photons and supplies the CCD sensor with a large number of photons even under extreme low-light conditions. Thus, the number of photons collected by the CCD sensor is always much larger than the number of photons originating from the darc current noise. For this reason, there is indeed no need for cooling an ICCD camera.

An EMCCD camera does not contain an image intensifier but an electron multiplying CCD sensor. The incoming photons are directly collected by the CCD sensor. The photo electrons generated by the sensor are then read out and afterwards multiplied electronically in a multi-stage gain register. For this reason, EMCCD cameras need extremely strong cooling, because the electrons originating from the darc current noise are of the same order of magnitude than the signal electrons under low-light conditions and are amplified together with them to the same extend.

Because EMCCD cameras do not include an image intensifier they do not provide fast gating capability as ICCD cameras do.

What is the free running mode?

In the free running mode the camera is periodically gated by the built-in trigger source. The trigger frequency and the field and frame rates resulting therefrom depend on the camera model. E.g. a camera containing an EIA standard video unit runs with 30/60 Hz frame/field rate, whereas a CCIR standard video unit gives 25/50 Hz frame/field rates. Thus, in the free running mode your ICCD camera behaves like a normal video camera.

My camera always shows perpetual fluctuations in image brightness when operated in the free running mode?

This is a typical effect when the camera is operated in free running mode in rooms where artificial lighting is present. The lighting runs with 60 Hz alternated current, hence the illumination of the scene will fluctuate by 120 Hz. On the other hand the cameras shutter is not synchronized to the lightings mains and the exposure time will be much shorter than 1/120 second. So, successive images are taken at different illumination levels each resulting in the observable image brightness fluctuations.

Is cooling always necessary for ICCD cameras for low light conditions?

Yes and no. Cooling the CCD sensor strongly reduces the darc current. Because the noise of the darc current is derived as the darc currents square root the noise is also reduced by cooling the sensor. And, the lower the darc current noise compared to the signals noise the better.

But, ICCD cameras amplify the incoming light itself by means of the image intensifier, so that the CCD sensor detects the already amplified light signal. Hence, the sensor delivers a large signal, also under low light conditions, and thereby also a larger signal noise which again is given by the square root of the signal itself. For this reason its normally not neccessary to decrease the sensors darc current noise by cooling because the noise of the already amplified signal is even higher anyway, at least as long as the sensors temperature does not significantely exceed 30° Celsius.

Please note: This does not impact the signal to noise ratio, because the signal itself is still amplified and scales with the square of the noise. For more information, please see also here.

Are less bits in the A/D conversion would work as well as superior 16 bit A/D converters.

This can obviously not be true. You should not start thinking about this matter by calculating the mathematical resolution of technical A/D converters. Just start with considering the physically given shot noise of the measured light signal. The shot noise is given by the square root of the signals average value. Hence, the shot noise always confiscates the lower significant half of the number of bits that are needed to code the signals average value. So, the usage of more bits in the A/D conversion can indeed not increase resolution or dynamic range because both are in fact limited by the signals shot noise level that lies far above the technical resolution limit anyway. For moe information, please see also here.

I cannot believe its possible to increase resolution and dynamic range without increasing the number of bits in the A/D conversion?

Each A/D conversion gives a certain minimum quantization step. The more bits are used, the smaller the resulting quantization step. However, the resulting values are still digital values. On the other hand a simple mathematical average process following the A/D conversion is of course able to yield real analog values that are not restricted to any quantization levels. This is what frame adding does. Instead of increasing the exposure time of the CCD sensor it adds an equivalent number of short time exposures together in the computer after the A/D conversion. The result is a real analog value. For mor information, please see also here.

I was told that bright light conditions will damage the image intensifier of my ICCD camera?

You can put your mind at rest, they do not. What gives you "watermarks" on the photocathode is not the light intensity onto the camera but a much too high photocurrent over a longer period of time. The photocurrent is proportional to the product of light intensity and gating time which means, that if the light intensity is high, the exposure time has to be short to avoid watermarks. This indeed is a intuitive control process, because if the light intensity is high and the exposure time too long the image will be strongly overexposed. In this case you will reduce exposure time anyway.

If you would like to feel more confident about this we would be pleased to upgrade your camera to automatic exposure control.