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CCD

The Difference Between Rolling Shutter and Global Shutter Sensors

Global-Shutter Sensor

Image sensors are available in many shapes and sizes, and with different capabilities. But in this post, we will focus on one very important thing: the electronic shutter methods that are available.

Rolling Shutter

Most consumer cameras use a rolling shutter method. With this method, the pixels on the sensor are read sequentially. When you press the shutter button, the camera scans through all the pixels and stores the information digitally. This means that the first pixel will be read out at a different time than the last pixel. And everything that happens after the first pixel is read out will still be captured by the last pixel, and the pixels in between.

Global Shutter

Global-shutter sensors read out all pixels of the sensor simultaneously, so the entire frame represents image data that was captured at the same moment in time. This method is not subject to the same motion artifacts as the rolling-shutter method.
 

Consequences

In everyday use, you won't notice if your camera uses the rolling shutter method. Only when you're capturing an image of a fast-moving object (like a fan), you may notice some motion artifacts like deformed fan blades.

In situations that require high-performance imaging, rolling shutter can severely affect your data. In such cases, it is better to use a global-shutter sensor, to ensure that your image represents the same instant in time and to prevent rolling shutter artifacts.


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Fiber-Optic Coupling

It is important that image quality is maintained as much as possible when using intensifiers. At the same time, light efficiency should be maximized. This can be achieved by using a fiber-optic window as the output of the first stage and as the input of the second stage.

A fiber-optic window is a solid piece of glass consisting of millions of parallel glass fibers sealed together. Each fiber acts as an independent light conductor. The shape of the window can either be flat (parallel input and output faces), or concave. Fibers with a concave surface are used for distortion correction in electrostatic image inverters.

Often the second stage will also have a fiber-optic output to allow coupling to a third stage, or to the image sensor of the camera. In the latter case the image sensor of the camera should be equipped with a fiber optic input window. In addition, take the following into consideration when you need to make a choice for either fiber-optic coupling or lens coupling:

  • Fiber-optic coupling is a permanent connection; the connection is made during the manufacture of the integrated intensified camera.
  • A fiber-optic window transfers an image from one face to the other. If the fiber optic has a tapered form, the image is reduced or enlarged. This characteristic can be used to match it to the format of a coupled imaging component.
  • While fiber-optic coupling between intensifiers is the standard technique, coupling to the camera can also be done by lens optics. Disadvantages of lens coupling are the greater loss in efficiency (compared to fiber optics) and the lenses are more bulky.
  • Lens coupling offers the flexibility of easy decoupling, allowing you a choice to make camera recordings with or without the use of an intensifier.

Intensified CCD Cameras

An Intensified CDD (ICCD) camera is an electronic camera, equipped with an intensified CCD as image sensor. The sensor uses an image intensifier that is fiber-optically coupled to the CCD chip to increase the sensitivity down to single photon level.

An intensified CCD camera allows image acquisition at very low light levels over a wide light spectrum and at relatively high speeds. Single photons can be detected and discriminated from CCD noise. Ultra high-speed phenomena can be captured by using the image intensifier as a fast shutter (gating).

CCD Camera Sensitivity

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.

Cooled CCD

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

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Intensified Cameras for Lifetime Imaging

Intensified cameras enable full-field frequency-domain and time-domain FLIM. The image intensifier becomes an ultra-fast electro-optical shutter by operating it at radio frequencies allowing time-resolved imaging. The high-resolution image intensifier is the key component of the TRiCAM (part of the LIFA) and the TRiCATT camera attachment. Its photon gain is typically in the range of 100 to 10000. Lambert Instruments provides different image intensifiers based on photocathodes with different spectral sensitivity to match a range of applications in the UV, visible and NIR.

For FLIM in the lifetime range of 0 ps to 1 ms we provide S20 (UV) and SuperS25 (visual) image intensifiers. For increased quantum efficiency of the photocathode in the visual part of the spectrum in this lifetime range, a GaAs intensifier is available. For near-infrared applications up to about 1100 nm an InGaAs photocathode is available.

The graph below shows the spectral sensitivity of these photocathodes.