29 Jan 2003
CCD cameras may be a popular tool for scientfic imaging, but lots of technical jargon and a wide range of models can make it very difficult to select the most suitable product. Oliver Graydon offers some useful advice for the novice.
From Opto & Laser Europe February 2003
How it works At the heart of all CCD cameras is a charge-coupled device (CCD) image chip. This is an array of light-sensitive pixels that are electrically biased so that they generate and store electrons - electric charge - when illuminated with light (see figure 1). The amount of charge trapped beneath each pixel directly relates to the number of photons illuminating the pixel.
Before buying a CCD camera, you should decide on the key criteria for your needs. You should be confident that you can give the sales engineer the following details:
• The wavelength region you wish to image.
• The lighting level. Are you trying to detect single photons or bright events?
• The frame-rate required. Are you intending to perform very fast imaging?
Here is some general advice on what to look out for. Note that the performance characteristics of a CCD camera are often interrelated, and trying to optimize one parameter will often compromise another. For an explanation of the technical terms in italics, see the "Jargon Buster" section below.
What wavelength response do you need? The wavelength sensitivity of a CCD camera is usually determined by the quantum efficiency (QE) of the CCD chip.
Most CCD chips with no special coatings have a QE in excess of 30% in the visible and near-infrared (400-850 nm). However, if you need to perform imaging at slightly shorter or longer wavelengths, it is possible to obtain CCD chips that are coated with phosphors to increase their sensitivity in the infrared, blue and ultraviolet.
Buying a CCD camera with a back-thinned or deep-depleted CCD chip can also enhance a camera's sensitivity in the ultraviolet, visible and near-infrared, and may be useful if you are performing low-light imaging in these regions. It is important to note that the wavelength sensitivity of a CCD camera is temperature-dependent and will change if the camera is cooled (cooling is a popular way to reduce the dark current of a camera). By controlling the temperature of the CCD chip by thermoelectric cooling (rather than cooling with liquid nitrogen) it is possible to optimize the CCD's QE in a given wavelength range.
How strong is the lighting level? If you want to image very low light-levels, for instance in experiments involving fluorescence or bio/chemoluminescence, it is essential to use a CCD camera with an optimized signal-to-noise ratio. In reality that means choosing between a cooled high-QE CCD camera, which has very low noise, and an intensified camera (ICCD), which makes use of an image intensifier and can image single photons. Vendors that sell both types of devices indicate that CCD sensor technology is now so good that in many cases they recommend using cooled CCD cameras unless you need a measurement with a nanosecond response.
When imaging in good light conditions or attempting to capture bright events, the signal-to-noise ratio is no longer a pressing issue and a CCD camera will not need to be cooled or require an image intensifier. Instead, make sure that the CCD camera has good dynamic range, linearity and saturation characteristics. To obtain good-quality images it is important that the CCD camera does not saturate. The latter occurs when a pixel is close to storing its maximum amount of charge (known as its full-well capacity). When saturation starts to occur, the CCD's image becomes distorted as its response to light is no longer linear. In extreme cases, blooming occurs when the charge from a pixel starts to overflow into adjacent pixels. This shows up as a white streak around a bright point on the image. Many CCD cameras now come with sophisticated antiblooming features.
What frame-rate and time resolution? If you need to image very fast events, such as laser-induced breakdown spectroscopy, laser ablation or time-resolved fluorescence, you will need to use an intensified CCD camera. This offers very fast gating (shutter) times of a few nanoseconds. It is also important to consider the required frame-rate of the CCD or ICCD camera. If you want to perform imaging at video frame-rates or above then fast read-out cameras are necessary, often in the form of frame-transfer of interline CCD formats.
What does "dynamic range" mean? The term "dynamic range" is often a cause of confusion for buyers of CCD cameras, but the essential facts are as follows.
The dynamic range (DR) of a CCD camera is a measure of its ability to accurately record bright and dim parts of an image. A large DR (ideally 12 bits or more) is required to obtain high-quality images. The cause of the confusion is that different parts of the camera system, namely the CCD image sensor and the camera's electronics, often have a different DR. The important point to remember is that it is the weaker value of the two that ultimately dictates the true DR of the camera.
• Image sensor DR This is defined as the image sensor's full-well capacity divided by its read noise. It is quoted in bits: commercial CCD image sensors often have values ranging from 10 to 16 bits.
• Electronic dynamic range This refers to the resolution of the analogue-to-digital (A/D) converter in the camera's electronics, which is used to convert a pixel's electron count into an intensity value in an image. A 12-bit A/D gives 4096 intensity levels and a 16-bit A/D gives 16 384 levels.
On-chip multiplication/electron-multiplying CCD A relatively new technology that has another stage of electronics in the read-out circuit of the CCD sensor. This amplifies the signal so that it dwarfs the read-out noise of the sensor. Vendors of this technology say that the result is that read-out noise is essentially eliminated, while critics say that the effects are not as impressive.
Back-filled The housing of the CCD is filled with argon to prevent condensation from forming on the image sensor.
Back-thinned/back-illuminated A highly sensitive CCD chip image sensor that has been etched by acid so that it is just 10 µm thick. A back-thinned chip is illuminated from the rear so that incident light does not have to pass through the polysilicon gate electrode that is deposited on the top surface of a pixel (see figure 2). The process can enhance the QE through the visible, ultraviolet and near-infrared.
Binning The process of combining the charge from several adjacent pixels to create "superpixels" prior to read-out. Binning enhances the frame-rate and signal-to-noise ratio of the camera at the expense of reduced spatial resolution.
Blooming/saturation Saturation relates to the maximum light intensity that a pixel can cope with. If a pixel is thought of as a bucket of photoelectrons, saturation occurs when a pixel's bucket is full.
Blooming occurs when a bucket starts to overflow and charge spreads into neighbouring pixels, causing them to report false light levels. Blooming shows up as a white streak or blob around a bright point on the image.
CCD noise/dark current This refers to spurious electrons that are generated in the CCD chip by thermal and other effects in the absence of any illumination. The effects of CCD noise can be dramatically reduced by cooling the image sensor.
Deep-depletion CCD A CCD chip that is designed to offer superior sensitivity in the near-infrared and high-energy X-ray region. It contains a biased area of high-resistivity silicon for capturing photons that would not normally be absorbed.
Etaloning In a back-illuminated CCD, reflections between the front and back surfaces can lead to interference effects that degrade the performance of the camera in the near-infrared. Anti-etaloning technology in deep-depletion CCDs can overcome this effect.
Image intensifier A vacuum tube, usually 18-25 mm in diameter, that gives a significant boost in light sensitivity. It is placed in front of the CCD chip and converts incoming photons into electrons, which are multiplied before being converted back into photons. The intensifier can also be gated for time-resolved experiments.
Megapixel A term used to describe a CCD sensor that contains at least one million pixels.
Multipinned phase (MPP) A way of biasing the CCD chip so that the CCD's dark current is dramatically reduced. As a result, less cooling is required to reduce the noise of the camera, and compact thermoelectric coolers can be used instead of liquid nitrogen.
Photon/shot noise The laws of physics dictate that the number of photons striking a detector is inherently uncertain. This uncertainty is known as photon, or shot, noise and varies with the square root of the signal level. High QE cameras improve the signal to shot noise ratio.
Quantum efficiency (QE) The probability of a CCD chip converting an incoming photon of a given wavelength into an electron. It is measured as a percentage, and the higher the value, the more sensitive the camera.
Read-out noise This is noise that is generated by the electronics that convert the charge from each pixel into a digitized light-intensity value that is displayed in the image. Read-out noise increases with the speed of the read-out and consequently the frame-rate. It is reduced by technologies such as on-chip multiplication.
Read-out rate This is often quoted in MHz and refers to the number of pixels that can be digitized per second. It should not be confused with frame-rate.
Smearing If light is still falling on the CCD chip during read-out, image distortion called smearing can result. The use of physical shutters and frame-transfer or interline CCDs minimizes the effect.
Opto & Laser Europe would like to thank the following companies for their help in compiling this article.
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