With the advent of low-cost CMOS image sensors, high-speed cameras are becoming more and more popular. These cameras are increasingly appearing in applications including science, motion analysis, and automotive crash testing.
However, how do you decide which type of high-speed camera is best suited for a particular application? Before that, we have to carefully consider things like frame rate, resolution, storage capacity, and the type of interface between the camera and the computer. It's also important to consider what type of lens the camera can support and what type of illumination can be used to produce images with the highest contrast ratio.
shutter speed
Because high-speed imaging applications are often highly specific, these technical parameters need to be balanced to select the most cost-effective camera. The two most important considerations are the camera's minimum shutter speed and maximum frame rate.
Although the shutter speed is the exposure time of a single image, the maximum exposure time can never exceed 1/frame rate. The shutter speed of 1/1000 sec is not directly related to the frame rate of 1000fps, as the camera's frame rate can be set to a slower rate (e.g. 30fps) at an exposure time of 1/1000 sec per frame.
Therefore, using an exposure time of 1/1000th of a second and a frame rate of 30fps, compared with an exposure time of 1/500th of a second and the same frame rate (30fps), the motion blur will be minimized due to the shorter exposure time per frame, resulting in a relatively sharper image.
If you choose to use fast exposure times, you will most likely need to increase the illumination of the subject being photographed.
"For any desired contrast in the image, if the exposure time is halved, the illumination intensity needs to be doubled regardless of the sensitivity of the sensor. In other words, in order to achieve the same imaging effect, if the exposure time is reduced by half, the illumination intensity must be doubled. You need to hit the sensor with the same number of photons in half the time to get an image of the same illumination. Rick Robinson, vice president of marketing at Vision Research (www.highspeedcameras.com), said.
Although more light can be captured using a fast lens, the sensitivity of the detector in a high-speed camera is an equally important consideration, as the exposure time can be reduced as the sensitivity increases.
ISO standard
Some manufacturers of high-speed cameras use International Organization for Standardization (ISO) standards to prescribe this sensitivity. While ISO12232 defines three methods for measuring ISO sensitivity, the most common practice is to correlate sensitivity with the exposure required for camera system saturation. This ISO value can therefore be used to determine the nominal exposure time and/or illumination requirements of the camera. Details on how to get this value and what its mathematical description is, can be found at the URL www.bit.ly/1MMKUnN for further information. Companies such as Imatest (www.imatest.com) use this saturation-based ISO sensitivity (Ssat) standard to measure sensitivity related to the level of brightness that saturates a sensor or camera system. This value is used by Www.photron.com in its FASTCAMSA-Z camera products (see Figure 1). FASTCAM SA-Z is a 12-bit 1024×1024 resolution high-speed camera that runs at frame rates up to 20,000fps and ISO sensitivities of ISO50000 (monochrome) and ISO 20000 (color), respectively. Vision Research offers the Phantomv1612 high-speed camera with ISO 32,000 monochrome daylight sensitivity.
Figure 1: Photron's FASTCAM SA-Z is a 12-bit 1024×1024 resolution high-speed camera that runs at frame rates up to 20,000fps with ISO sensitivities of ISO 50,000 (monochrome) and ISO 20,000 (color).
Andrew Bridges, Photron's director of marketing, said, "Some manufacturers use 'T' and 'D' to describe ISO sensitivity; 'T' stands for tungsten filament lamp, removing the IR filter yields a higher value, and a lower 'D' stands for daylight value, obtained by adopting the published ISO 12232 Ssat standard. Bridges adds, "By removing the IR filter, more photons can be converted into signals, raising the ISO 'T' value to a level that the user cannot achieve." ”
In such cameras, the use of a Color Filter Array (CFA) on the image sensor will reduce the amount of light captured at each photographic position, thereby reducing the ISO rating. However, the ISO standard can be used to determine the sensitivity of a camera, but it has nothing to do with image quality, as increasing camera gain can be used to increase ISO speed, but at the expense of increasing image noise.
Increase sensitivity
To improve the sensitivity of high-speed cameras, many manufacturers use imagers with high quantum efficiency and high fill factors. With this in mind, many manufacturers of high-speed cameras choose sensors with large pixel sizes. For example, in the design of the i-Speed 726 camera (see Figure 2), the British company iXCamera (www.ix-cameras.com) uses a custom CMOS 2048×1536 pixel sensor with a pixel size of 13.5 μm, shot at full resolution, capable of running at a frame rate of 8,250fps, and the camera's maximum ISO sensitivity is 40,000 (monochrome) and 14,400 (color).
Figure 2: In the i-Speed 726 camera, iXCameras uses a custom CMOS2048× 1536 pixel sensor with a pixel size of 13.5 μm.
Although increasing the pixel size can improve the sensitivity of the camera, they have a certain negative impact on the resolution of the captured image, because when measured in line-pairs/mm, the resolution of larger pixels relative to smaller pixels will be slightly lower. However, in many high-speed imaging applications, this may not be a major consideration.
Frame rate
When choosing a high-speed camera, in addition to considering the sensitivity of the camera, which is an important parameter, the frame rate of the camera is also an important parameter to consider. To increase frame rates, camera manufacturers with CCD or CMOS sensors use different techniques. For CCD cameras, a technique called local scanning is used to read the variable band from the central part of the imager, reducing the need to output charge from each pixel of the CCD, thereby increasing the frame rate.
In cameras using CMOS sensors, the width and height of this region of interest (ROI) can be varied, increasing the flexibility of the frame rate that can be obtained. For example, in the design of the Q-MIZE HD v2 camera, the Swiss company AOS Technologies (www.aostechnologies.com) used a 1920× 1080 CMOS sensor that allowed the camera to operate in a variety of modes – 1,000fps at 1920×1080 resolutions, 1024×× 1024, 1280×720 and 853×480 resolutions, The camera can obtain frame rates of 2000fps, 2500fps and 5000fps, respectively.
Capture image data
When the camera is operating at high frame rates, the image data is either captured on the camera or transmitted to the host through a high-speed interface. In applications that require portable and non-connected cameras, the camera must capture sequences of images over a relatively long period of time. In high-speed applications, however, this "longer time" can be only a few seconds, as the camera can be triggered at a specific point in time, capturing an image of an event.
For other applications, image data over a longer period of time can be captured on the camera. For example, the TS5-D portable high-speed camera from Facec Imaging (www.fastecimaging.com) uses a 2560×2048 CMOS sensor that captures full-resolution HD images at a frame rate of 634fps and stores them in 8GB of memory (see Figure 3). In order to store these images on the camera board card at a speed of about 20GB/min, the camera's internal hard disk capacity must be up to 1TB; alternatively, the camera can store these images directly on the SSD card (720p@520fps), which takes about 35 minutes. The camera's Gigabit Ethernet port (GigE) allows image sequences to be transmitted to the host at rates of up to 90MB/s for post-analysis.
Figure 3: Fastec Imaging's TS5-D high-speed camera, using the 2560×2048CMOS sensor, can capture full-resolution HD images at a frame rate of 634fps and store the images in the camera's 8GB of memory.
Like Fastec Imaging, the German company www.mikrotron.de offers a range of portable and non-portable high-speed cameras. For example, Mikrotron's EoSens4CXP camera, using a CoaXPress (CXP) interface, can shoot images with a resolution of 2336×1728 (640×480 ROI mode) at frame rates in excess of 2000fps and transmit image data at rates of up to 25 Gbit/s to hosts located 100 meters away.
Since 2006, Vision Research has offered its CineMag interface, which enables data storage rates of 1G pixels per second on cameras. In this way, the camera can capture a one-megapixel image at a frame rate of 1000fps, and then transmit the image data to the computer via a 10Gbit/s Ethernet interface for subsequent downloads.
Today, high-speed cameras are widely used in many scientific, industrial, aerospace and automotive applications. To meet these application needs, while many high-speed camera manufacturers use custom multi-tap CMOS sensors in their designs, low-cost high-speed cameras have begun to make their way into various applications as sensors with faster data rates continue to emerge.
The original article is in the May 2016 issue of the English edition