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 industry standard. Therefore, this technology can be found on small compact entry-level cameras through to the latest back-illuminated sCMOS cameras and other high-end scientific cameras. A permanent vacuum sealed enclosure is a technically more difficult technology, and therefore costly to implement so it is found less frequently, and only available from a small number of camera manufacturers, including Andor. 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?
Camera designs will vary, but a back-filled enclosure refers to filling the sensor chamber with dry gas. O-ring seals are used to keep the dry gas in, and the air, moisture and other contaminants from the external environment out. This means you can generally avoid the moisture that would otherwise be present in the air from degrading sensor performance.
4.What Andor cameras use back-filling technology?
Of the current Andor camera portfolio examples that use this technology include the Andor Zyla range (Zyla 4.2 PLUS and Zyla 5.5) of compact, cost-effective sCMOS solutions. The warranty for the 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 chamber seal is formed from a compressed O-ring, so it is not a permanent metal-on-metal seal as used with vacuum chamber technology. For ALL non-vacuum sensor enclosures, one expects a low-level moisture ingress into the sensor chamber over time, pervading through the O-ring seal. Furthermore, in cameras that thermoelectrically (TE) cool the sensor to reduce darkcurrent, this moisture would then condense onto the cooled sensor, affecting image quality and also presenting some risk of sensor malfunction. To mitigate this, all such cameras utilise desiccant material in the sensor enclosure to absorb any moisture that has entered.
However, eventually the desiccant can become saturated, indicating the need for the camera’s sensor chamber to be serviced, whereupon the chamber will be re-backfilled and the desiccant replaced. There is some anticipated variability as to when moisture ingress can be expected to present itself as an issue to camera performance following saturation of the desiccant, based on variability in environmental humidity and air flow around the camera. For example, while a camera may be operated in a dry air-conditioned laboratory environment throughout the working day, the laboratory air con may be turned off overnight, during which the camera chamber can become exposed to high heat and humidity. Under such circumstances, back-filled cameras can be more exposed to elevated rates of moisture ingress into the sensor chamber.
A vacuum enclosure provides the ultimate protection of the sensor over an extended service life. UltraVac™ is Andor’s proprietary permanent vacuum enclosure technology. UltraVacTM uses a hermetic metal-on-metal seal, avoiding need for O-rings. 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 many thousands of Andor cameras over more than 25 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 enclosure will have a warranty period which will cover any failure within this time, typically 3 years. The warranty period of the sensor enclosure is specified separately to the rest of the camera. If the condensation occurs 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 back-filled camera throughout the industry. To avoid the risk of requiring back-filling service throughout the operational lifetime of the camera, a camera utilizing UltraVacTM vacuum technology should be considered.
Customer support information including warranty information for both back-filled and vacuum sensor enclosure technology cameras is available at https://andor.oxinst.com/support/