Image intensifiers: How to make the right choice?
The image intensifier is a key component of each ICCD camera. This section deals with the fundamental characteristics of image intensifiers and their options.
The structure of image intensifiers in a schematic drawing. The main components of image intensifiers are photocathode, multi-channel-plate and phosphor screen.
Characteristics of image intensifiers are adjusted to experimental conditions
Different applications of ICCD cameras and intensified imaging solutions have special demands and requirements to the camera and thus on the image intensifier. This guidance deals with some fundamental characteristics of image intensifiers. The way the image intensifiers will perform does strongly depend on the scenery observed. Therefore, the following questions need to be addressed:
- What are the spectral characteristics of illumination?
Does determine the suitable photocathode.
- What image field of view is necessary?
Does determine the size of image intensifiers.
- How fast need to be the shutter/shortest gating time?
Highest shutter speed does have some constrains e.g. size of image intensifiers.
- How much light is there?
Dual stage multi-channel-plates have better performance at low light environments.
- High speed or low light imaging?
Does determine the suitable phosphor screen.
Diameter of image intensifiers
The diameter of the image intensifier is one key parameter. Image intensifiers are available with diameters up to 40mm, though 18mm and 25mm diameter are the most common.
The 18mm image intensifier provides highest shutter speeds due to its smaller size of the photocathode. It also has a substantially higher specific spatial resolution than the bigger 25mm image intensifier. This makes the 18mm image intensifier to the standard and best choice of many applications of ICCD cameras. The total spatial resolution in the total field of view of the 18mm image intensifier si typically almost the same at 18mm and 25mm intensifiers.
The 25mm image intensifiers are mosly suitable for specific applications, e.g. spectroscopy. At a given combination of focal length and grating of a spectrograph the 25mm has a 20mm wide field of view, which is 39% more than the 14.4mm which are available at a 18mm image intensifier.
Summary in short: The 25mm image intensifier does not lead to hte higher total reolution that could be expected at first sight but does have a significant lower gate time. It is the best choice only for special applications like spectroscopy.
The shutter speed is limited by the electrical supply of and the electrical properties and connection to the photocathode. Stanford Computer Optics is the only manufacturer that uses coax cables for the photo cathode voltage supply. This enables gate times less than 3ns which are not possible with standard single wirecabling.
However, the shutter speed is mostly limited by the size of the photocathode as the propagation of any electromagnetic signal is limited by the speed of light. Therefore, the electromagnetic signal just need more time to spread over a larger photocathode. Due to this physical constraint, only the 18mm image intensifiers can provide highest shutter speeds with gating times down to 200 picoseconds.
The standard input window is made of quartz. This limits the UV spectral range at about 165nm. The optional Magnesium Fluoride (MgF2) window enables measurements down to 110nm.
Photocathodes define the sensitivity and the spectral response of image intensifiers. More about photocathodes ...
Image intensifiers can be equipped with single or double stage MCP's. The single MCP is standard and fits most of the applications of ultra high speed ICCD cameras. The V-stacked double MCP's are especially used for extreme low light environments. The increased electron multiplication enables single photon detection and reduces the ion feedback noise. Therefore, the double MCP is mainly used for long exposure measurements, extreme low light applications and single photon counting. More about multi-channel plates ...
There are three important considerations in choosing a luminous (phosphor) output screen. First the efficiency, second the phosphor decay time and last the spatial resolution. The two mostly used phosphor screens are P43 and P46. The P43 phosphor screen has a higher efficiency and higher spatial resolution due to smaller grain size. However, it has a long decay time. For fast applications e.g. double frame mode with a interframing time of 500ns the faster decaying P46 phosphor screen is necessary to avoid gost images from the previous exposure. The trade-off of the P46 phosphor screen are lower efficiency and lower spatial resolution. more about phosphor screens ...