Do ICCD cameras need cooling?

The advantages and necessity of actively cooled CCD sensors in ICCD cameras is frequently pictured with calculation of different imaging technologies. This article show considerations regarding the necessity of actively cooled CCD sensors exclusively for intensified imaging systems like the intensified CCD cameras from Stanford Computer Optics, Inc.

How much noise is generated by dark current of the CCD sensor compared to the signal?

The main difference to any other imaging system is that the intensified CCD cameras amplify incoming light BEVOR the signal reaches the CCD sensor. Therefore, any incoming signal – even single photons – are highly amplified when they reach the CCD sensor. This means, in contrast to EM-CCDs, CMOS or CCD cameras, the incoming signal is pre-amplified and, hence, much higher than the dark current of the CCD sensor. Due to the early signal amplification by the image intensifier within ICCD systems the downstream installed CCD sensor has less influence on the S/R ratio of the image. Therefore, the dark current of the CCD sensor has a minor influence in the image quality of intensified CCD cameras.

Pre-amplified signal reaches the CCD sensor and redundantize active cooling

The term “dark current” comes from the fact that the current has nothing to do with the incident light and is generated equally well in complete darkness. At the CCD sensor output, dark current generated electrons appear identical to signal generated electrons. Furthermore, dark current is depending on the sensors temperature and decreases dramatically with the sensors temperature. However, the pre-amplification of the incoming light within the image intensifiers of the ICCD cameras prevent further commitment in the reduction of the dark current.

Limiting factors of image quality by the intensified imaging systems

Far more important for the image quality of intensified CCD cameras are ion feedback and equivalent background illumination (EBI) [2] of the installed image intensifier. Ion feedback is a artificial second electron avalanche within the MCP of the image intensifier caused by free moving ions. The EBI is the amount of light you see in an image tube that is turned on but there is no light at all on the photocathode. It is affected by temperature where the warmer night-vision device, the brighter the background illumination. EBI is measured in lumens per square centimetre (lm/cm2) wherein the lower the value the better. The EBI is, therefore, the limitation of the intensified imaging systems and the equivalent to the dark current of EM-CCD, CMOS and CCD sensors.

Intensified CCD cameras do not need actively cooled CCD sensors

What clearly emerges from the above discussion and the simulations performed by D. Dussault [3] is that it is unnecessary to invest any special efforts into the CCD sensor cooling and readout in an intensified CCD camera. As a matter of fact, dark current and readout noise do only have an insignificant effect on the SNR of ICCD cameras.

 

 


References:

1) David W. Gardner, Does Your CCD Camera Need Cooling?, Jounal: Photonics Spectra (2002)

2) A.K. Musla and A.K. Jaiswal, Effect of Image Intensifier Tube Equivalent Background Illumination on Range Performance of Passive Night Sight, Defence Science Journal, Vol. 57, No. 6 (2007)

3) David Dussault ; Paul Hoess, Noise performance comparison of ICCD with CCD and EMCCD cameras, Proc. SPIE 5563, Infrared Systems and Photoelectronic Technology, 195 (2004)