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Things to consider when buying a high-speed camera

Overview
Here we outline the top considerations for buying a high-speed camera. This represents the key information required to make an initial informed decision. Check the technical specifications first, but nothing replaces using the camera itself. Before purchasing, schedule a demo to ensure that the camera works well for you: make sure the camera is easy to use, the specifications align with your specific application, and the results you are getting are exactly what you want to see. Every major high-speed camera distributor will allow you to "test drive" a camera before purchasing.

Throughput
The single most important aspect to consider when purchasing a high-speed camera is the potential throughput which the camera can achieve. You need to be sure the camera is capable of capturing the number of pixels you want, at the frame rate you need.

Throughput is the direct relationship between the resolution and speed of the camera and can easily be calculated by multiplying the maximum resolution by the maximum frame rate at that resolution.

Throughput = Width (pixels) x Height (pixels) x Frames Per Second

Throughput is the most important factor when purchasing a high-speed camera, as applications require a combination of both resolution and frame rate. In fact, by examining the growth in throughput of high-speed cameras, we are able to see the exponential technological growth of Moore’s law more than in any other single camera specification.

Throughput is often compared to plumbing, with an increased pipe diameter representing a higher throughput. Just as a wider pipeline can transport a greater volume of water from a reservoir, a higher throughput is capable of capturing more pixels from the sensor.
Frame Rate

Harold E. Edgerton, Bullet through Apple, 1964
The technology has existed for several decades to capture a single still photograph in very high resolution. It is lacing these high-resolution frames together sequentially which is important in high-speed video capture.
The frame rate of a high-speed camera truly differentiates it from other camera types, and is one of the essential elements used to calculate throughput. The frame rate used on a shot is rarely considered in isolation, but usually drives the need for a high-speed camera. Typically, a required frame rate is chosen and then the resolution is adjusted based around the available options and the region of interest.

High frame rate cameras have become more common in consumer-grade electronics like mobile phones and consumer cameras, which provide frame rates of 120-240 fps at relatively low resolutions and with a rolling shutter. High-speed cameras are necessary for events that start around 500 frames per second and go up to 1,000,000 frames per second; but even at slower frame rates the consumer-grade products may often produce images which are unacceptable for the application.
Resolution
A camera’s resolution represents the number of pixels available in the camera’s sensor and determines the detail of the image and the precision with which motion can be measured.

Resolution can be conveyed in a few different fashions, including a width x height measurement in megapixels (millions of pixels) or by use of a common resolution standard. In this fashion, 1080p, 1920 x 1080 and 2.1 MP each represent of the same number of pixels and are equivalent to what is available in most monitors and TVs. Starting with a higher resolution image allows you to digitally zoom in on an area of interest and presents a much wider choice of potential aspect ratio.

More Pixels = More Versatility
Comparing two cameras of equal throughput but different resolutions can often be difficult; however, it can universally be stated that the higher resolution camera ALWAYS offers greater versatility.
As stated above, the resolution is directly linked to the frame rate of the camera, but the only fixed values are the number of pixels transferred and the number of pixels on the image sensor. This means that as long as you do not exceed the horizontal and vertical resolution of the image sensor, your image can be a rectangle that is long and narrow, short and wide, square shaped, or anything in between.
Shutter Speed
Also known as "exposure time," shutter speed measures the length of time a camera shutter is open to expose the camera's sensor to light. Shutter speed is measured in fractions or multiples of a second and can be designated as an absolute duration, a fractional value, or a multiplier (1x, 2x, 3x of the current frame period). Longer shutter speeds are required for low-light images, while shorter shutter speeds are required for fast-moving subjects to avoid motion blur.

Shutter speed is especially important in applications which require very high frame rates and very little motion blur, such as ballistics testing. Often, recording at thousands of frames per second isn’t enough to capture what you are looking for, as you also need the image to be crisp and without motion blur. Sometimes you will get a better image capturing a scene at 500,000 fps with a 4x shutter than you would capturing a scene at 1,000,000 fps with a 1x shutter, as the shutter duration of the 500,000 fps shot is half that of the 1,000,000 fps shot and the camera will achieve a higher resolution at 500,000 fps.
Imager


A rolling shutter is the typical method of capture for consumer-grade cameras offering slow motion options. A rolling shutter doesn’t capture the entire scene at once, but instead starts capturing at the top of the screen and scans down. This means that significant distortion occurs when trying to capture very high-speed events. To avoid this effect, ensure that your camera makes use of a global shutter, capturing the entire image at the exact same time.
Many people will want a color high-speed camera for their video application over a black-and white monochrome imager. A color camera will generally produce prettier, more interesting pictures in a demo; however, purchasing a color camera is often a mistake for many applications.

Color sensors have multiple filters over the imager, including a Bayer filter to split incoming light into red, green and blue, an optical low pass filter to remove any false coloring, and an IR cut filter to block non-visible light from entering the sensor and tinting the image. All of these filters block light in some fashion and are akin to wearing a pair of sunglasses indoors: you can still see, but everything is darker and a little more blurred. In an industry where light is a precious resource, every bit counts.

A monochrome camera produces a brighter and crisper image, and most times what you are viewing is evident regardless of the presence or lack of color. In addition, motion analysis software compares pixel intensity and relative location while totally ignoring pixel color, so it has no effect on measurements made. If your application is laser based, near-IR based, or low-light, a monochrome camera is a better choice.
Light Sensitivity


Here are two identical images, the upper one, a RAW image file, and the lower one with applied digital gain. While the lower one is significantly lighter in appearance, there are spots where significant discoloration is present. Often ISO camera values are given for images which contain artificial gain.
Several attributes affect the light sensitivity of the camera, but the general rule of thumb is that a larger resolution sensor is less sensitive to light than a smaller resolution sensor, as each individual pixel is smaller in size and has fewer photons hitting it at any given time.

The industry standard for camera light sensitivity is the ISO value. ISO values were initially created for still film cameras and have not yet been updated for modern digital sensors. As such, ISO camera values vary wildly between manufacturers and are based on subjective things like perceived image quality, noise present and gain added. Gain is an artificial method of boosting the light sensitivity of the image while allowing for more signal noise in the image. It can be added as either analog signal gain or digital signal gain. In any photographic setting, the most crucial element is light. The faster the shutter, the shorter the exposure time, the more light is needed. Running a camera at high speed reduces the exposure time, so high-speed video is notorious for requiring high illumination levels. Lights for high-speed cameras provide a great quantity of flicker-free and high-intensity illumination – recent advances in LED technology make some specific lights suitable. Available illumination and light sensitivity are directly related, with the general rule for high-speed photography being: you can never have too much light!

Because of all of the factors involved, the only true way to compare light sensitivity of different cameras is to place them side-by-side, viewing the same event, with the same lens and the same lens iris setting.
Event Duration
The many types of high-speed cameras available can be broken down into two general categories: cameras that can offload the video at the same speed or faster than it is captured, and cameras that capture an entire event and must offload that event later. The category of camera that you want depends upon the length of recording time you need. Long-record events and transient events necessitate different capture methods.

A transient event happens within a few seconds and is typically captured at several thousand frames per second at a high frame rate, resulting in several GB worth of RAW data. In this instance, to increase the event duration you need a camera with as much temporary storage capacity (RAM) as possible. Transient events require cameras with extremely high throughput capabilities.

Circular Buffer
Recording memory in a high-speed camera is constantly cycled, recording over itself until it is triggered. This means that while a camera may only save a few seconds total duration, it can be recording for hours waiting for the event to occur.
Long-record events typically take several minutes or hours at 100-300 fps and a moderate resolution. During these events the camera is unable to store all of the video necessary and must offload it in real time to a computer. This offloading speed is determined by the throughput capability of the transport medium instead of the capture potential of the camera. In this instance, the storage space on the recording computer is what affects the potential event duration and not the camera’s memory.
Video Storage
Shooting high-speed video at high frame rates creates a great deal of data. You need an adequate amount of internal memory and additional storage for offloading the images from the camera to permanent and/or portable non-volatile storage media. In addition to storage space, another factor to consider is the affordability and speed of the transfer medium. Since some high-speed cameras are capable of recording bursts of up to 288 GB of video in just a few seconds, transfer speeds from the camera's temporary RAM into permanent storage are almost as important as the medium itself. Typically, all high-speed cameras can be connected to a computer via Ethernet and can offload the video that way, but the transfer rate may be limited. Because of this, faster portable offload options are usually preferred. Several portable transfer options exist on the market, but currently none are ideal. The faster options are quite expensive and sacrifice image bit depth to offload the video quickly, while the slower options are more cost-effective but…well…slower.
Synchronization
High-speed cameras capture thousands of individual photographs in every second. These images are captured at perfectly even time intervals, but those intervals are determined by the camera. If two cameras are in use simultaneously, for example in a 3-D experiment, they will inevitably capture their images at slightly different times. For this reason, most cameras can lock the timing of image capture to an external reference, called a sync source. Some cameras are more flexible than others, providing different modes of use, the ability to follow varying speeds of external syncs, and even the ability to be a sync source for other cameras.
Support and Service
High-speed cameras are complex and expensive scientific devices requiring periodic updates, troubleshooting and support. Before purchasing, take a look at the resources and support which the company provides. Download and run their software, ask a question on social media, or reach out to an engineer at the company with an industry-specific question. Visit the company's website and see what educational materials are available; content could be video tutorials, tips and/or techniques, technical papers or specific information on a camera feature. The goal should be to interact with the company outside of your direct sales contact. This will give you a good sense of how easy the company will be to work with after the sale is made in case you ever need replacement parts, additional service for your camera, or need to perform an update.



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