Reference Light Source for EMVA 1288 Testing of Imaging Sensors and Cameras
The selection of a suitable sensor is an important step in the design of image processing systems. For the user, however, the choice is not always easy, as the information provided by different sensor manufacturers does not support a direct comparison of the available products. This problem has motivated the European Machine Vision Association to generate the EMVA 1288 standard [1]. The parameters listed therein and the description of the measuring methods offer the user the necessary comparability in the selection of imaging sensors and cameras.
Measurement Setup Solution for EMVA 1288 Testing
To measure sensitivity, linearity and non-uniformity, the EMVA 1288 standard recommends that integrating sphere based light sources should be used as the reference [2]. As a special feature, the standard states that spheres must be configured with LED sources.
To measure the sensitivity of imaging sensors, light sources with quasi-monochromatic emission spectra can be used. For this purpose, their intensity and emission spectrum must be known by calibration in an absolute unit of measure. The sensitivity measurement described in EMVA 1288 is not the same as the much more complex measurement of full spectral sensitivity of imaging sensors which is only listed as an option within the standard. For determining the sensitivity of monochrome image sensors, a wavelength close to the maximum sensitivity of the image sensor is recommended. Typically, NIR LEDs with a peak wavelength at 850 nm are used. For color image sensors, three light sources are needed to measure the sensitivity. Their wavelengths correspond with the maximum sensitivity of each of the color sensor’s RGB channels. Typical sources are monochromatic red, green and blue LEDs with peak wavelengths of 455 nm, 532 nm and 630 nm.
To measure the linearity of imaging sensors, the intensity of the reference light source is varied and compared with the corresponding response of the sensor or camera. Intuitively, the use of LEDs as a light source seems ideal for this because their luminous flux or radiant flux is almost directly proportional to the operating current. In practice, however, the limited dynamic range of the permissible operating current and the careful implementation of temperature management must be taken into account.
Flexible Uniform Light Source Solutions
The non-uniformity of imaging sensors is measured by comparing the signal from each pixel when the sensor is uniformly illuminated. According to the specifications in EMVA 1288, the light field of the integrating sphere light source used must be larger than the illuminated sensor surface. The modular sphere concept from Gigahertz-Optik GmbH can be configured to provide a suitable reference source for a wide range of sensor sizes and types. With clever arrangement of the external sources on the integrating sphere that ensure diffuse illumination of the hollow sphere, luminous fields with a surface uniformity distribution of better than the required 97% are achieved.