At low light levels standard CCD/CMOS cameras are not sensitive enough to capture useful high-contrast images. There are ways to increase the sensitivity of such cameras. The first method is to allow the CCD to integrate for much longer times. In order to prevent high background noise, CCD cooling is applied when using long exposure times. A second method is to use an image intensifier to boost the input signal.
At longer integration times of a CCD, more light is captured to enhance images. However, not just more input signal is collected, but also more dark current from the CCD itself. The amount of dark current depends strongly on the temperature; for every 6 degrees C the CCD is cooled down, the noise (dark current) halves. When the CCD is cooled to -25 degrees C, integration times up to minutes can be applied. This enhances the sensitivity of the camera immensely.
To better improve the cameras SNR, the read-out noise is reduced by using a lower read-out speed. These techniques are used in high performance 14 and 16 bits digital cameras.
Intensified CCD with Fiber-Optic Coupling
An image intensifier helps to increase the sensitivity of a camera by amplifying the input light-signal before relaying it to the CCD/CMOS sensor of a camera. Roughly, there are two ways to relay the output image, from an image intensifier, to a CCD/CMOS sensor. The first is by means of a relay lens. A lens coupling is flexible, but the downside is that a lens coupling has a low transmission efficiency, caused by the limited aperture of a lens. A more efficient way is to use a fiber-optic window to transfer the image from the intensifier to the CCD. A fiber-optic window contains a large number of microscopic (6-10 micron) individual fibres and acts as an image guide. A tapered fiber-optic window will magnify or demagnify input images. Generally, demagnification is chosen to match the image intensifier to the CCD/CMOS sensor.
In summary, the advantages of a fiber-optic coupling are:
- Low light losses
- Intensifier/CCD combination is more compact
- Camera design is sturdier
- No optical adjustments are needed