HIGH-SPEED CAMERAS FOR AEROSPACE TESTING
By: Tim Callenbach, General Manager High-Speed Cameras – Photron
For years high-speed imaging has provided engineers with detailed analysis of projective tracking, missile launches, combustion testing, engine testing, fuselage testing, component testing, materials testing, flow visualization and more. There are several companies that manufacture high-speed cameras, so how do you decide which company to buy from and which model of camera to buy? There are many factors that are important to consider when purchasing a high-speed camera, these factors are discussed below.
The first and most obvious consideration in the purchase of a high-speed camera is frame rate. How many frames per second (fps) are required to capture sufficient video detail to allow you to analyze your high-speed event? Most high-speed cameras provide the ability to run at increasingly higher frame rates as the pixel resolution is reduced. You should be aware, however, that the increased frame rate is never completely proportional to the reduction in resolution and typically comes with restrictions regarding the specific numbers of horizontal and/or vertical pixels that can be selected with the reduced resolution.
High-speed cameras are available in a variety of resolutions. Most often, the resolutions are between 1-megapixel and 4-megapixel. Higher resolutions are important when the field of view of the camera is large, or when a very small spatial resolution is required because there are a lot of fine details in the high-speed event that need to be studied. It is important to understand that the higher the resolution of the camera, the lower the maximum frame rate will be. With a 4-megapixel camera you have 4x more pixels to process than with a 1-megapixel camera. Therefore, you will not be able to achieve the same frame rates with a higher resolution camera as with a lower resolution camera. In addition, higher resolution cameras are less sensitive than lower resolution cameras because their sensors have smaller pixels.
Bit depth is an important consideration as it impacts image quality and the ability to apply image processing to the images to enhance their usability. Most high-speed cameras capture image data that is either 8-bit, 10-bit or 12-bit. The higher the bit depth, the greater the amount of information that is captured by the camera.
In certain applications light sensitivity is the single most critical consideration when it comes to purchasing a high-speed camera because it affects so many things. Light sensitivity impacts the quality of your video because without sufficient light sensitivity your images will be dark and hard to analyze.
It is critical to understand that there is a huge variation in light sensitivity among the high-speed cameras that are out in the market. Light sensitivity is typically presented as an ISO value. The higher the ISO value, the more sensitive a camera should be. However, when you look at a datasheet or brochure for a given camera, you need to take the information regarding light sensitivity with a degree of skepticism. There are standards – such as ISO 12232 Ssat – that define how to measure light sensitivity, but many high-speed camera vendors do not adhere to the standards when they measure the ISO values of their cameras. Other vendors publish ISO values for their cameras but do not specify exactly which ISO standard (ISO 12232 Ssat, for example) the values conform to. Without such information the ISO values are meaningless.
Minimum Exposure Time
A camera’s minimum exposure time is often a critical factor in choosing a high-speed camera. Some very fast high-speed events require extremely short exposure times – sometimes even less than 1 microsecond – to stop the motion of those high-speed events. A camera’s ability to achieve a sub-microsecond exposure is dependent on two things. First, the camera’s sensor must be capable of performing such a short exposure. Second, the camera’s sensor must be sensitive enough that when it does utilize a sub-microsecond exposure it can capture enough photons of light during the exposure to be able to generate video that is of sufficient quality for analysis.
The amount of internal memory is an important issue for consideration when purchasing a high-speed camera, as a high-end, high-speed camera can generate a huge amount of data in a very short time. For instance, a high-end camera can generate 128GB of 12-bit image data in just under 4.5 seconds when run at 20,000fps at 1-megapixel resolution. Because no streaming mechanism exists to allow this amount of data to be transferred to a PC or external storage device in real time, high-speed cameras have internal memory to which the image data is initially captured. After the recording is completed, the image data can then be offloaded to a more permanent location.
Data Offload Speed
After you have captured video you will need to transfer it from the internal memory on the camera to a more permanent location for storage and analysis. It is important that there is a fast and convenient mechanism to facilitate this.
Most camera suppliers have chosen to implement Gigabit Ethernet to enable the transfer of image data from the high-speed camera. You should be aware that not all Gigabit Ethernet implementations are the same, for instance, using TCP/IP protocol tends to be inefficient for downloading large video sequences because there is a lot of overhead involved with that protocol. Gigabit Ethernet using UDP protocol, on the other hand, is quite efficient and can result in image data transfer speeds of up to 5GB per minute. Not all camera manufacturers implement UDP protocol. Further, some camera manufacturers provide two Gigabit Ethernet connectors (instead of one) so that data transfer speeds can be doubled.
As an alternative some cameras have the ability to download images to removable nonvolatile memory such as SD cards and CFast cards, or to integrated SSD drives. Such download methods can be very useful, but when you calculate the overall transfer time required to get your image data from the internal camera memory to a PC, you will need to consider both the time required to transfer images from the camera to the nonvolatile memory and the time required to transfer images from the nonvolatile memory to the PC.
Color vs Monochrome
When you purchase a high-speed camera, you will have to decide between a color and a monochrome camera. Oftentimes when a camera purchase is being considered, the marketing department wants a color camera because color video is more impressive to the viewer. However, it is important to understand that because of the way high-speed sensors are made, a monochrome camera will be two to three times more sensitive than the equivalent color camera. Unless color changes within a high-speed event are critical to the analysis of the event, the monochrome camera is typically a better choice.
The physical size of a high-speed camera is an important consideration if your application provides only a limited space in which to fit the camera. It is important to understand that there are some important trade-offs involved in purchasing a smaller size camera. Small cameras are often less sensitive than larger cameras due to the implementation of small sensors with small pixels. In general, they also have lower frame rate performance and less memory because they contain less internal space for the electronics that impact these performance parameters.
Aerospace Success Story
Laboratory Director Gerardo Olivares has four Photron SA-Z cameras, two of which are color plus two monochrome models in The Virtual Engineering Laboratory Wichita State University’s National Institute for Aviation Research (NIAR). Olivares says the flexibility of the frame rate and excellent quality of image resolution make the FASTCAM SA-Z an ideal high-speed camera for their testing processes.
Photron cameras were used recently during a drop test of an airplane fuselage. The test was done as research for the Federal Aviation Administration (FAA), to examine the behavior of composite materials when used for main aircraft structures. In this case, the materials were a honeycomb structure covered by a carbon fiber laminate.
The ten-foot section of composite fuselage was brought into the lab and filled with appropriate ballast to simulate the 1,500 pound weight of an operational fuselage. Extensive accelerometers and strain gages were attached with two SA-Z high-speed cameras placed on each side of the test area. Two monochrome cameras were set up to record the entire width of the fuselage and two-color cameras were placed on the other side, focusing primarily on the emergency exit door area. The test represented a pure vertical crash at 30 feet per second.
A laser beam was used to trigger the cameras and instrumentation so that image collection began when the fuselage was about a foot away from impact. Set to record at 20,000fps and using the Photron camera’s Pre-and Post-Triggering feature, 25% of the images were captured prior to impact and the remaining 75% of the images were recorded during and after impact. Using Digital Image Correlation (DIC), researchers were able to see levels of deformation and performance of the composite materials on the fuselage.
The FAA is studying results of the test to support future regulations on the use of composite materials versus metallic in 14 CFR PART 25 commercial aircraft. Composite materials are lighter and manufacturers design and build their products based on certification requirements set forth by the FAA showing safety levels equivalent or better to metallic materials.
Frame rate, resolution and light sensitivity are typically the most important factors to think about when purchasing high-speed cameras, but as discussed above there are a number of other things that you need to think about as well. Aerospace testing environments can be very challenging for high-speed camera suppliers. To guarantee the successful implementation of high-speed cameras within such environments it is important to select an experienced supplier who can provide a broad range of highly-reliable cameras along with exceptional customer support.
In additional to testing being performed at NIAR Photron high-speed cameras are used to support many important aerospace and defense applications, including testing conducted at NASA, Sandia National Labs, the DOD, Blue Origin and more.