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Electronic Imaging & Signal Processing

Understanding sensitivity

In order to meaningfully evaluate imaging system sensitivity, you must consider the requirements of the end application.

From oemagazine January 2002
31 January 2002, SPIE Newsroom. DOI: 10.1117/2.5200201.0008

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 (µJ/cm2). In some cases, the exposure is defined in photometric units (lux · s).

Traditionally, film sensitivity has been specified in terms of speed under standards set by the International Standards Organization (ISO).1 This sensitivity measurement, informally dubbed ISO, gives the exposure index at which specified image quality parameters are achieved. We define exposure index EI as

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, an ISO standard has been defined 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.

choosing your ISO

There are two basic types of ISO: saturation-based (also referred to as 'base ISO') and noise-based. In applications for which the lighting conditions are controlled, like studio photography, users select exposure index settings 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, we determine the ISO using a signal-to-noise ratio (SNR) that gives a reasonable image for the application of interest. This ISO corresponds to the maximum EI setting necessary to achieve that SNR. It is common in photography to designate SNR=40 as an 'excellent image' and SNR=10 as an 'acceptable image' and calculate a corresponding noise-based ISO for each.

At Kodak, we define the ISO for our image sensors by:

where ISO represents the base-ISO, f /# is the effective f-number of the camera lens used in the measurement, L is the luminance of an 18% reflector in the scene in cd/m2, and t is the length of the exposure in seconds. The choice of an 18% reflector is somewhat arbitrary, but generally an 18% gray patch is available in the lab, so this is a convenient choice. The constant in the front is determined by making some assumptions about a typical imaging system—90% lens transmission, a vignetting factor of 0.98, and an image point that is 10% off axis, which folds into the cos4 part of the calculation of the actual exposure on the image plane.

To measure the scene luminance, we vary the lighting until the camera outputs an average value equal to 18/106 of full scale when imaging the 18% reflecting surface. During this adjustment we hold the exposure time constant. Full scale is either the point at which the imager saturates or the point at which the camera's analog-top-digital converter (ADC) reaches its maximum value, whichever happens first.

The use of the number 106 is not arbitrary—it means that we want to accommodate a reflectance of up to 106% for image highlights. Very demanding applications might need more headroom for highlights, so we might use a higher number when calculating base ISO.

We measure the scene luminance in cd/m2 using a light meter and calculate the equation with the f-number and integration time. For a color imaging system, the ISO measurement is performed on the highest speed channel—green for an RGB imager or yellow for a CMY imager.

The accuracy of this measurement depends on just about everything—lenses, IR-cut filters, camera gain and offset settings, and mapping of the output voltage to the ADC, just to name a few. For best results, take care that the camera system is set up exactly as it will be used in the end application. oe

Reference

1. International Organization for Standardization standard 12232, Photography—electronic still picture cameras—determination of ISO speed (see www.iso.ch).


Gloria Putnam

Gloria Putnam is an applications engineer in the Eastman Kodak Company's Image Sensor Solutions division. Gloria Putnam serves on the oemagazine editorial advisory board and the new ESTEP task force on work-force development. She was formerly chair of both SPIE's Education Committee and the Women in Optics working group. She is currently WiO adviser.