Phantom Cameras and EMVA1288

Vision Research Inc has recently adopted EMVA1288 to measure and specify the performance of their Phantom high-speed cameras. For too long the high-speed camera industry has published specifications using standards that do not apply well to digital CMOS based sensors.

For many years the high-speed camera industry has adopted the old photographic film ISO standard and variations of it. ISO makes it difficult as a distributor or end user to compare digital CMOS high speed cameras from different manufacturers without testing them. It has always required a shootout which is often inconvenient and slow.

ISO is a standard that suited film-based imaging, but it is poor at characterising high-speed digital cameras because it misses some of the critical specifications required to truly evaluate high speed cameras.

ISO when used with digital CMOS high speed cameras is largely qualitative and inconsistent in its measurement of sensitivity, a critical factor of the performance of a high-speed camera. Often users interested in comparing cameras before purchasing, put them side by side to see which produces a brighter image in order to determine sensitivity. What they can fail to identify is the noise and loss of dynamic range in some cameras from manufacturers that choose to hide noise at the expense of true performance i.e. they create an artificially nice-looking image but compromise other performance parameters in doing so. This practice is not scientifically accurate and leads to false conclusions in analysis and measurement of images.

Developed by the EMVA the EMVA1288 standard is a camera standard that characterises digital CMOS camera performance in a scientifically robust way. Standardised equipment is supplied for testing cameras and to generate a consistently comparable report. The standard also defines the test method and so all facets of generating performance specifications is consistent and reliable regardless of the camera and its manufacturer.

The EMVA1288 standard treats a camera as a black-box model with light measured as an input and digital number measured as an output with both average and variance values of each being measured. The black box design is simple and sophisticated and models the camera with;
• A Quantum Efficiency
• A single source of Noise called Temporal Dark Noise or Read Noise (which is in fact a combination of several noise sources)
• A Gain
• A Quantisation (including error) to convert to Digital Numbers DN.

By measuring the inputs and outputs of cameras in the EMVA1288 test setup and using the Poisson relationship inherent in the photon shot noise behaviour of light, where the average value is equal to the variance, it is possible to determine the overall Gain factor in the camera. From there it is a matter of mathematics to accurately determine the other parameters that clearly and scientifically characterise a camera.

Understanding the key EMVA 1288 parameters:
The important output specifications that EMVA1288 produces are:

Quantum Efficiency
This defines the ability of the sensor to convert incident photons to electrons that collect in the pixel during exposure time. It is described as a % from 0% to 100%. The higher the number, the better the sensor is at converting photons to electrons and so the more sensitive it will be. Note that the measurements are done at a single wavelength and so it is useful to look at the camera’s normalised quantum efficiency curve which is supplied by the camera manufacturer in case you are using a different wavelength or using white light.

Temporal Dark Noise
This is also called Read Noise and is generated within the sensor. It occurs regardless of whether there is light incident on the sensor or not. It is measured when there is no light incident on the sensor. With low Read Noise the image is cleaner, sharper and image analysis measurements more accurate. Read Noise is also used to calculate Dynamic Range and the higher the noise the lower the Dynamic Range. Dynamic Range can be described as the ability to see contrasts details in both bright areas and dark areas of the scene. Read noise tends to be more important in low level light scenarios which is often the case with high-speed imaging, short exposures and the often-encountered difficulty in lighting scenes with enough light.

SNR Signal to Noise Ratio
The SNR value divides the signal power by the noise power. The noise that is considered when measuring SNR includes both Read Noise and Photon Shot Noise. The SNR is measured when the pixel is full of electrons and the signal is at its highest i.e. the SNR value quoted is the maximum SNR. It is important to note that as the number of electrons in a pixel increases the greater the influence of Photon Shot Noise, since it is directly related to the square root of the number of photons incident on the sensor. The Photon Shot Noise is typically higher than Read Noise and the dominant noise source as the pixels fills with electrons i.e. bright pixels are dominated by Photon Shot Noise while dark pixels are dominated by Read Noise.

Absolute Sensitivity Threshold
This is defined as the signal level at which the signal equals the noise level i.e. SNR=1. In itself, it is an arbitrary number. A signal that is lower than the read noise level may still be visible in the image and that will depend greatly on the scene being imaged, but the threshold allows users to directly and scientifically compare the sensitivity of two cameras, a critical factor in the choice of any high-speed camera.

Saturation Capacity
Saturation Capacity is the measure of a sensor’s ability to capture and store a maximum value of charge (electrons). It is the number of electrons that fill the pixel. It can also be called Full Well Capacity (FWC). The higher the FWC then the higher the SNR of a sensor.

Dynamic Range
Dynamic Range is defined as FWC divided by Read Noise. A sensor with a higher dynamic range has a greater ability to measure detail in both bright and dark areas of the image i.e. it has a greater ability to discern small brightness changes (contrast) in the image.

Vision Research is the first of the high-speed camera manufacturers to measure and publish the EMVA1288 specifications for their cameras, which demonstrates their willingness to be transparent and to give users the true ability to compare their Phantom cameras to other brands. It is now up to distributors and end users who need to choose between high-speed cameras, to pressure their prospective suppliers to measure and provide EMVA1288 specifications for their products. In this way cameras can be compared without costly and slow shootouts, where the truth can be obfuscated with misinformation and tricks. We at Adept Turnkey encourage you to contact your prospective high speed camera supplier and insist on EMVA1288 specifications that truly measure the performance of their cameras.

EMVA1288 is heavily reliant on scientific and mathematical methods and is not necessarily easy to understand or to visualise their effect on your images and analysis results. Engineers at Adept Turnkey have studied and understand the standard. Please contact us if you have any questions about EMVA1288. Our engineers will be happy to help you to more clearly understand what they can do for you.