What are Camera Sensor Enclosures and why do they Matter
In this article Microscopy Camera Specialist Dr Alan Mullan answers questions on the different types of camera sensor enclosures that are used for scientific cameras and how they affect camera performance and longevity.
Figure 1: The Andor Sona camera features an UltraVacTM permanent vacuum seal for long term reliability and deep cooling.
1. Why are sensors enclosed?
The sensors used for scientific cameras are very delicate and easily damaged from physical handling and mechanical damage. In addition to, moisture, hydrocarbons or other gas borne contaminants all of which may degrade performance over time. For this reason, camera manufacturers need to protect the sensor from the external environment. For most cameras there is a camera window so that light may pass through to the sensor while dust and other contaminants may not. However, it is not easy to determine how the sensor is sealed within this enclosure or chamber and what the significance of this is to the performance of the camera and its longevity.
2.How are sensors sealed in scientific cameras?
There are two main technologies which are generally used, “back-filled" and “vacuum”. Back-filled cameras are the most common type. This method can be found on small compact entry-level cameras through to the latest back-illuminated sCMOS cameras and other high-end scientific cameras. A vacuum sealed enclosure is a technically more difficult technology and therefore costly to implement so it is found less frequently. It is normally only found on higher performance cameras where long-term reliability and the highest sensitivity or deep cooling is required.
3.What is a back-filled camera?
A back-filled enclosure refers to back-filling a sensor chamber with gas. Often the term backfilled may not be used and they will just be referred to as a “sealed” enclosure. The chamber of a back-filled camera is filled with an inert, dry gas such as Nitrogen or Argon. Once the air has been pumped out and replaced with a positive pressure of the gas used for “back-filling” the sensor chamber will be sealed up. O-ring seals are used to keep the gas in and the air, moisture and other contaminants out. This means you can avoid the moisture and other organic contaminants that would otherwise be present in the air from degrading sensor performance.
Figure 2: For back-filled cameras the sensor enclosure is evacuated and filled with a dry, inert gas and sealed using O-rings as summarized in this representative image.
4.What Andor cameras use back-filling technology?
Of the current product portfolio examples would be the Andor Zyla range (Zyla 4.2 PLUS and Zyla 5.5) and the Andor Apogee cameras (Alta and Aspen) use back-filled technology. The warranty for the Zyla and Alta and Aspen models sealed sensor enclosure is three years.
5.Are there any disadvantages to cameras that are back-filled?
The main downside of this approach is that the seal is formed from a compressed O-ring, so it isn’t a permanent hermetic seal like the vacuum technology is. Over time, the gas will leak past the O-ring seals and be replaced by moisture laden air. When this happens, condensation can form on the inside of the camera window. This condensation may be observed in the image and on visual inspection of the camera window. You may also notice a higher noise floor in the images. If condensation forms on the sensor, moisture droplets can cause short circuits and ultimately the sensor would be damaged. Therefore, if condensation is spotted it is important to stop using the camera and contact your support representative for further guidance.
A vacuum enclosure provides the ultimate protection of the sensor over an extended service life. UltraVac™ is Andor’s proprietary permanent vacuum enclosure technology. UltraVac uses a permanent hermetic seal so there are no O-rings to leak. It has a proven reliability having been used on the Andor iXon EMCCD, Newton, iDus, iKon and the latest Sona and Marana back-illuminated sCMOS. UltraVac™ technology has been implemented in 1000s of Andor cameras over the last 20 years.
To find out more about UltraVac technology click here.
7.Other than long-term reliability are there any other advantages or disadvantages of vacuum technology?
Figure 4: Deeper cooling of CCD models can help provide a reduction in dark current and drop the noise floor even between -70°C and -95°C. UltraVacTM ensures the deepest possible cooling and lowest noise floor of the sensor can be attained.
Aside from providing long-term protection of the sensor, a vacuum enclosure has a few other benefits. UltraVac™ enables deeper cooling which means that dark current can be reduced to the minimum for when longer exposures are required.
Hot pixels and other pixel blemishes are also reduced when the sensor is cooled meaning that the overall image quality can be improved. This can be particularly relevant to sCMOS cameras due to the higher pixel to pixel variation that these cameras have compared to CCD.
Figure 5: Cooling of sCMOS sensors can reduce dark current as well a¬s reduce the impact of blemishes
Vacuum technology requires production facilities, equipment and complex processes to implement it in a camera which means the associated costs are higher than for back-filled enclosures. However, the UltraVac™ process is a key part of many of our camera platforms at Andor so the cost of this is minimized through the volume of cameras we produce with this process.
8.What Andor cameras use vacuum sealed technology?
Cameras that use UltraVac™ technology include iXon Ultra and Life, iKon series (iKon-M, iKon-L and iKon-XL), Newton, Idus, iVac and Neo. The new back-illuminated sCMOS Sona and Marana models also feature UltraVac™ technology. Some variants of these cameras are used for high energy applications and may be open fronted so, in these cases, cameras may be mounted to a vacuum chamber directly.
9.What other considerations are there when enclosing a sensor in a sealed chamber?
Sensors generate heat during operation which we would see as dark current and a high noise floor if it is not removed. So, a cooling system is needed to remove this heat, which becomes more important for low light applications and longer exposure times. For most scientific cameras the sensor will be coupled to a thermoelectric cooler or a passive cooler assembly for compact designs. Therefore, heat from the sensor can be removed from the camera via air cooling using a fan or water cooling using a water chiller. The sensor also needs wires passing through to the electronics boards on the other side of the sensor enclosure so that the signals from the sensor itself can go for amplification or processing so that an image can be displayed. The camera window is also important as this will have its own transmission efficiency profile. The camera window will be selected for optimal transmission efficiency to suit both the sensor and the application that the camera will be used for. To find out more about camera windows click here.
10.What should you do if your backfilled camera needs to be refilled?
The camera backfill or vacuum will have a warranty period which will cover any failure within this time. The warranty period of the camera seals may be specified separately to the rest of the camera. If the seals fail outside of the warranty period the camera will need to be serviced, at cost to the customer. It is therefore important to be aware of this and that this is a normal part of the life of any camera.
Customer support information including warranty information for both back-filled and vacuum sensor enclosure technology cameras is available at https://andor.oxinst.com/support/