Pepperl+Fuchs is a worldwide operating manufacturer of sensors and components for process and automation technology. Their portfolio encloses, among others, camera-based systems such as vision sensors and optical identification systems, which are being developed at daughter company Omnitron located in Griesheim, Germany. At Omnitron, within the framework of a diploma thesis, a procedure was introduced for the qualification of the camera sensitivity taking into account the influence of noise. Embedded in this procedure is the camera standard EMVA 1288.
EMVA 1288, hosted by the European Machine Vision Association, was originally developed to enable the users of industrial and scientific cameras to compare the technical data of the camera data sheets provided by the camera vendors. The standard provides a defined test frame as well as the defined presentation of the resulting data. Within this standard, one method is the "photon transfer method" (PTM). The work for the diploma thesis employed the EMVA 1288 standard to ensure a later comparability of the results.
The Formation of Noise
As an introduction to the standard, this section will briefly explain the functionality of a camera. Light, in the form of photons, shines on the image sensor. The photon transfer creates an electrical charge in the image sensor. With the help of a capacitor, this charge is converted into a voltage, which is amplified before an ADC digitizes it. The digitized steps can be shown as grey values in digital images.
In this process a noise will develop that adds to the signal. This noise is comprised of in three main types: a temporal noise, a spatial noise and quantization noise.
As to the temporal noise, the digital signal of an individual pixel varies from frame to frame. Reasons for the spatial noise are due to the production process of the image sensor. In an image one can recognize it as a fixed pattern. It also exists when every pixel is exposed to precisely the same amount of light and the temporal noise is removed. Particularly CMOS image sensors suffer strongly from spatial noise. Quantization noise stems from converting the amplified voltage into a digital signal with discreet steps: The coarser these steps are the bigger is this specific noise.
Standardized Measurement Set-ups
The standard EMVA 1288 describes a complete set-up for the experiment and the measuring requirements.
First, it is important to shield the camera against any ambient light and to work with a defined monochrome illumination. The wavelength can be chosen arbitrarily. Moreover, the image sensor should be illuminated very homogeneously. This is accomplished best by a set-up without an optical lens and by making use of a Lambert's emitter. The standard´s description of the geometrical configuration is comprehensive and very easy to understand. The diameter of the emitter in combination with the distance to the camera should result in an f-number of 8. Figure 1 shows the set-up used in our experiment.
The light source is an approximate Lambert's emitter with a diameter of 25 mm. Consequently, the distance to the camera is 200 mm. It is important that the geometrical centers of both components are positioned along the same axis and that they stay parallel to each other.
In addition the following settings on the camera are necessary:
The grey level resolution (number of bits per pixel) must be as high as possible. Through a higher bit number the ADC receives more steps to discretize the analogue signal. The higher the resolution, the lower the quantization noise.
The amplification must be as low as possible but high enough to ensure that the noise is greater than or equal to one grey value.
The Offset (Black Level Calibration) must be as low as possible but high enough to ensure that the signal in darkness is greater than or equal to one grey value.
No automatic corrections and/or settings may be used.
The measurement has to be executed in the linear mode of the camera.
Signal and Noise
According to standard EMVA 1288, the signal of an image is represented by the average of all grey values of the image.
Two noise sources dominate the amplitude of the signal, the "Photon Noise" and "Amplifier Noise". Photon Noise originates from the nature of the light itself. The photons do not hit regularly onto the image sensor. The irregularity is given by a certain mathematical expectation. A Poisson distribution determines this expectation.
The Amplifier Noise is a white noise and is added when the analogue signal is amplified.
Both dimensions belong to the type of temporal noise. Therefore, in order to scrutinize the sensitivity, only the temporal noise needs to be observed.