Introduction

Video contains more information about an event or action than any other type of recorded media.  A video clip of an event will require >1,000x more disk space to store and bandwidth to transmit than any textual description of the same event – thereby lending credence to the old adage that “a picture is worth a thousand words.” High-speed video in particular generates a lot of information in a short amount of time. The proliferation of on-line video repositories such as YouTube, GoogleVideos, and Vimeo among others validates video as a communication, educational, and entertainment media. This application note explores the acquisition, synchronization, and storage of high speed video for use in post-processing video analysis software. It is intended for end-users and integrators with some exposure to video analysis but who may be new to high-speed cameras and the types of video they produce.

Specific topics discussed include:

Whether and what type of high-speed camera is needed
Synchronizing multiple cameras for one event
Accessories and supporting equipment
Acquisition and storage of high-speed video
Software features and vendors

What is a High Speed Camera?

High-speed cameras are similar to conventional video cameras but have the ability to capture images at higher rates and increased shutter speeds (reduced exposure) beyond that of conventional video recorders. Consumer video cameras typically have image resolutions of 640x480 (VGA) and record images 10-60 frames per second. High speed cameras have VGA or higher resolution and typically record images at 200 frames-per second or higher. Frequently, consumer video cameras that operate at high shutter speeds are erroneously referred to as high-speed cameras even though the frame-rate is standard 30-Hz. A true high-speed camera will operate at high frame-rate (≥200-Hz) and high shutter speed (≤1/1000th of a second). Lambda Photometrics offers several high-speed camera lines with frame-rates from 200-90,000-Hz and shutter speeds as low as 1/500,000th of a second.

Check out our range of High Speed Cameras

Although capable of high frame-rates, any high speed camera should also be capable of operating at 30-Hz if desired. The function of high-speed cameras is to generate video files and the good news is that files generated by high-speed cameras are of the same format as those generated by conventional video recorders. The most common audio/visual format is the Audio Visual Interleave (or AVI) format defined by Microsoft. An AVI file created by a high-speed camera should playback just as one created by a conventional video recorder. Most high-speed cameras do not record audio because of the audio sampling problems arising from the variable frame-rates used during recording.

Who Needs High Speed?

Most users who venture into the world of high-speed cameras have already tried capturing video with a home camcorder and observed that they cannot see the necessary details of the event under study because of motion blur or because the event was missed entirely. In many cases, the shutter speed and frame-rate can be determined fairly quickly by trial and error. But that assumes that the user already has a high-speed camera and can play around with it. What if you’re interested in acquiring a high-speed camera but don’t know which one to purchase based on frame-rate and shutter speed? As an example, let’s assume the user wishes to record a batter in a cricket game in order to improve their stroke. It helps to capture the bat and ball with minimal motion blur in order to determine their relative positions. A short exposure may be necessary to determine the batter’s motion throughout the swing. But how short is short? Assume that the ball is moving at 98-mph or approximately 144 feet-per-second and that the camera’s field-of-view (FOV) is approximately 10-feet x 10-feet centred on the batter. The ball takes about 70-milliseconds to traverse the 10-foot window and the duration of the batter’s swing is about quarter of a second. At 30-Hz video rates, approximately 5-10 frames will be captured during this interval and will probably be sufficient to see the various stages of the swing. However, motion blur should be kept to <1-pixel for clear stop-action images. For a 640x480 image with 10-foot FOV, the ball traverses all 640 vertical columns in 70-milliseconds or 1-pixel in 70/640=0.11-milliseconds. This is equivalent to a shutter speed of 1/10,000.

Now assume that, not only do you want to record the swing, you want to actually record the instant the ball contacts the bat. At 30-Hz, the ball moves almost 5-feet between consecutive frames so that chances are very small that you will get the instant of impact. So how fast of a frame-rate do you need? To capture the impact, the ball must move less than the diameter of the bat between frames. At 144 feet-per-second, the ball will move 3-inches in 1.7-milliseconds so that a frame-rate greater than 1/0.0017=600-fps is required to have a reasonable chance of recording impact. On the other hand, if a batter’s complete swing lasts 0.5-seconds, a 200-Hz frame-rate will give 100 images from start to finish. This is more than enough detail to record every nuance of the swing/stroke.

Golf swing taken with a standard camcorder @30fps

Golf swing taken with a high speed camera @ 200fps

Synchronizing Multiple Cameras

Some video analysis software packages have the ability to process multiple AVIs of one event taken from different views. In doing so, three dimensional information about an object or motion in the event can be extracted and stored for applications such as tracking, accurate timing, and precision measurements. For these applications, all cameras must be synchronized to a high degree – typically to a time interval much less than the frame period (33-milliseconds for conventional video recorders). For more information regarding ways to this please speak to our product specialist.

Accessories and Supporting Equipment

In many cases, the same lenses, tripods, and lighting used for conventional photography can be used for high-speed cameras. Below is a list of accessories and some of the things to consider when purchasing.

Lenses: Unlike most conventional video recorders, high-speed cameras to not come with lenses and one must be specified on purchase. While there are a wide variety of fixed focal length and zoom lenses available in C-, F-, or M-mount formats, very few high-speed cameras are available with auto-focus and auto-exposure features. Lambda Photometrics has a range of cameras which has an auto-exposure feature that adjusts each frame’s exposure on a frame-by frame basis to compensate for fluctuating light conditions. This feature does not mechanically adjust an iris within the lens as do most video-cameras but the effect is the same.

C-mount and F-mount lens adapters are the most common adapters for high-speed cameras. If the imager size is >2/3” CCD, a C-mount to F- or M-mount adapter allows use of a less-expensive 35-mm lens. Because of the short exposures typically used with high-speed cameras, lenses with large apertures (f/# ≤3) are required. Navitar, Pentax, and Schneider are some lens manufacturers with large area, large aperture lenses. The Navitar DO-5095 lens, for example, has a 50-mm focal length, 0.95 aperture, and comes with manual focus and iris. The focal length of the lens will depend on the desired field-of-view of the video and the distance of the camera from the object being recorded.

Lighting: Ensuring sufficient illumination during high speed video recording is essential. The best, least-expensive, easiest-to-use light source is the sun. However, not all recording can take place outdoors and there are several alternate illumination options. One lighting feature that should be carefully considered is the fluctuation or flickering caused by ballast-driven lights such as fluorescent or HMI (hydrargyrum medium-arc iodide). These types of lights are driven by AC currents and show significant changes in intensity at recording frame-rates ≥120-Hz. High-frequency ballasts have been proposed for HMI lights but are not commonly available. This doesn’t mean do not use these types of lights – just be aware of the fluctuation tendencies.

Halogen – this type of lighting is the least expensive, brightest, most common illumination source on the market. When powered with an AC/DC converter, halogen lights exhibit minimal fluctuations or flickering. However, these lights put out a lot of heat and care must be taken not to overheat any object in the recorded scene. They also emit a significant amount of infrared radiation that can degrade resolution and colour clarity. Some cameras has an infrared blocking filter to prevent this wavelength from affecting the image but, even with a filter, the images may look redder under halogen lighting.

HMI – these lights are bright, have very good colour temperature, and do not emit significant infrared compared to halogen lights. Unfortunately, intensity fluctuations caused by ballast operation limit their use for high speed video. Several lighting providers now offer “flicker-free” ballast options but these are generally relegated to professional supply shops. Count on paying several hundred of pounds for flicker-free HMI lights.

Fluorescent – probably the most common light source next to incandescent lights, fluorescent lights yield acceptable colour clarity. However, they tend to be too dim for most frame-rates over 200-Hz and are susceptible to intensity fluctuations caused by ballast operation. These can be purchased at any lighting or home supply store.

Compact Fluorescent – rapidly becoming more common with new models on the market every day, these lights typically use a high-frequency flicker-free ballast. Although most compact fluorescent bulbs are between 7-30W, their efficiency makes a 30-W bulb almost as bright as a 100-W incandescent.

Quartz halogen spotlight – most of the light sources mentioned above have a fairly large emission angle (10o or larger) and are intended to deliver light over a fairly broad area. Many times, the object of interest is relatively small but without access so that the camera must be located several feet away. For those situations where a lot of light needs to be delivered to a small object a distance away, a battery-powered quartz halogen spotlight is inexpensive and relatively efficient at keeping a small divergence angle

Computer

A host computer is required for most high speed cameras to configure the camera, store recorded video, and playback in reasonable detail. The major features affecting the type of host computer are interface and hard disk capacity. The most common interfaces for high speed cameras are USB, Ethernet, and CameraLink interfaces. It would be hard to find a computer without one of the first two interfaces while the third type interface requires an additional PCI card to be installed on the host computer before it will recognize the camera. RAM and hard disk capacity should be sufficient to store any conceivable quantity of video during a recording session.

Software Techniques

Any high speed camera should come with software that allows the user to control the camera, capture video, and download to hard disk. Sometimes the same software package will allow video analysis after recording. All software video analysis packages start by importing a video – whether AVI, raw, or a format proprietary to the camera manufacturer. From there, common video editing features are contrast/brightness adjust, object tracking, colour enhancement, edge detection, test and graphics overlay, smoothing, image subtraction, rotation, cropping, and resizing to name just a few. The most common one used in the market today is StreamPix

For more information or for a free consultation please contact our machine vision team on 01582 764334. Or email [email protected].

Accessories

Lenses
Lighting
Cameras