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. Figure 5 shows the spectral sensitivity curves of various photocathode types.
When not to use a booster
As explained earlier, adding a booster to your intensifier will allow for higher frame rates. However, as the booster is an extra component in the imaging system, it will also lower the resolution of the total system. For comparison: an intensifier has a typical resolution of 45 line pairs per mm while an intensifier combined with a booster will bring the resolution down to 25 line pairs per mm.
This is when camera pixel size becomes important: when the camera has large pixels (e.g. the 20 um typical for a high-speed camera), the change from a 45 to a 25 lp/mm resolution will be negligible. When a camera with small pixels is used, however, the change in quality of the image will be significant.
Fiber-optic coupling or lens coupling?
Obviously, 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.
- Governed by the laws of optics no increase in brightness is achieved when demagnifying the image either by use of fiber optics or by lenses.
How to choose the correct image intensification solution
Before you start to look for a solution for a high-speed imaging problem, you will need to determine the following:
- What exactly would you like to record?
- How many frames per second (fps) do you need to record?
- What is the required gating repetition rate?
- Do you wish to use a camera that you already have? If yes, what type of camera is it and what type of lens mounting is required.
- Which photocathode will be suitable? You may want to consult a specialist in intensified high-speed imaging for help.
- Will you need a booster? Again, you may want to consult a specialist in intensified high-speed imaging for help.