TND6115-ImageSensorISOMeasurement-TND6115D.PDFVIP

? Semiconductor Components Industries, LLC, 2014 July, 2014 ? Rev. 2 1 Publication Order Number: TND6115/D TND6115/D Image Sensor ISO Measurement ISO Measurement There are many ways to describe the sensitivity of an imaging system to light. It is traditional for the sensitivity of a digital image sensor to be specified in terms of its quantum efficiency ? the number of electrons generated for each incident photon. It is also common to measure responsivity, or the output in volts generated during an exposure (J/cm2). In some cases the exposure is defined in photometric units (lux × seconds). Traditionally, film sensitivity has been specified in terms of speed or “ISO”. ISO gives the exposure index at which specified image quality parameters are achieved. Exposure index is given by: EI  10 H (eq. 1) where ‘H’ is the exposure in lux × s. Because it is often useful to compare digital imaging systems to conventional silver-halide based photographic systems, standards have been developed that allow the measurement of ISO for digital imaging systems. Using a particular ISO speed value as the exposure index on an electronic still camera should result in the same focal plane exposure as would be obtained using that exposure index on a film camera. There are two basic types of ISO: saturation-based and noise-based. The saturation-based ISO is also referred to as the ‘base ISO’. The type of ISO of interest depends on the application at hand. In applications where the lighting conditions are controlled, like studio photography, exposure index settings are selected to give the best possible image, with the image highlights falling just below the saturation level. This exposure situation is described by the saturation-based ISO. When lower than ideal lighting conditions are expected, a noise?based ISO is more useful. In this calculation the signal to noise ratio (SNR) that gives a reasonable image for the application of interest is used to calculate the ISO. This ISO correspon