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Image Sensor
ISO MeasurementISO Measurement

There are many ways to describe the sensitivity of animaging system to light. It is traditional for the sensitivity ofa 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 (mJ/cm2).
In some cases the exposure is defined in photometric units(lux × seconds).
Traditionally, film sensitivity has been specified in termsof speed or “ISO”. ISO gives the exposure index at which
specified image quality parameters are achieved. Exposureindex is given by:
EI + 10H

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TECHNICAL NOTE
where ‘ISO’ represents the base or saturation−based ISO,‘f#’ is the effective f number of the camera lens used in themeasurement, ‘L’ is the luminance of an 18% reflector in thescene (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 laboratory sothis is a convenient choice. The constant in the front isdetermined by making some assumptions about a typicalimaging system − 90% lens transmission, a vignetting factorof 0.98, and an image point that is 10° off axis, which folds
into the cos4 q part of the calculation of the actual exposureon the image plane.
To measure the scene luminance, the lighting is varied
until the camera outputs an average value equal to 18/106 offull scale when imaging the 18% reflecting surface. During
this adjustment the exposure time is held constant. Full scaleis either where the imager saturates or where the camera’s
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% forimage highlights. Very demanding applications might needmore headroom for highlights, and so might use a higher
number when calculating base ISO.
The scene luminance is measured with a light meter in
cd/m2 and entered into the equation with the f number andintegration time. For a color imaging system, the ISO
measurement is performed on the highest speed
channel − green for an RGB imager.
This measurement depends on just about everything −lenses, IR cut filters, camera gain and offset settings,
mapping of the output voltage to the ADC, just to name afew. So take care that the camera system is set up exactly as
it will be used in the end application.
The KAF-16801 Image Sensor, a 16 megapixelfull−frame CCD image sensor with 9 micron pixels, aRed−Green−Blue color filter array, and no microlenses has
a base ISO just over 100 assuming a demanding 170%
reflectance tolerance for highlights or base ISO around 62assuming 106%. This ISO is more than adequate for thestudio lighting conditions under which this sensor istypically used.

(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 themeasurement of ISO for digital imaging systems. Using aparticular ISO speed value as the exposure index on anelectronic still camera should result in the same focal plane
exposure as would be obtained using that exposure index ona film camera.
There are two basic types of ISO: saturation-based andnoise-based. The saturation-based ISO is also referred to as
the ‘base ISO’. The type of ISO of interest depends on theapplication at hand. In applications where the lighting
conditions are controlled, like studio photography, exposureindex settings are selected to give the best possible image,with the image highlights falling just below the saturationlevel. This exposure situation is described by thesaturation-based ISO.
When lower than ideal lighting conditions are expected,
a noise−based ISO is more useful. In this calculation thesignal to noise ratio (SNR) that gives a reasonable image forthe application of interest is used to calculate the ISO. ThisISO corresponds to the maximum EI setting to achieve thatSNR. It is common in photography to designate SNR = 40
as an ‘excellent image’ and SNR = 10 as an ‘acceptableimage’ and calculate a corresponding noise-based ISO foreach.
The ISO of ON Semiconductor image sensors isdetermined through direct measurement and application ofthe following formula:
ISO + 15.4
Lf# 2t© Semiconductor Components Industries, LLC, 2014July, 2014 − Rev. 2(eq. 2)

1

Publication Order Number:
TND6115/D

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Reference

International Organization for Standardization standard
12232, Photography – Electronic Still Picture Cameras –Determination of ISO Speed. (See International
Organization for Standardization, www.iso.ch)

ON Semiconductor and theare registered trademarks of Semiconductor Components Industries, LLC (SCILLC) or its subsidiaries in the United States and/or other countries.
SCILLC owns the rights to a number of patents, trademarks, copyrights, trade secrets, and other intellectual property. A listing of SCILLC’s product/patent coverage may be accessedat www.onsemi.com/site/pdf/Patent−Marking.pdf. SCILLC reserves the right to make changes without further notice to any products herein. SCILLC makes no warranty, representation
or guarantee regarding the suitability of its products for any particular purpose, nor does SCILLC assume any liability arising out of the application or use of any product or circuit, andspecifically disclaims any and all liability, including without limitation special, consequential or incidental damages. “Typical” parameters which may be provided in SCILLC data sheets
and/or specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including “Typicals” must be validated for eachcustomer application by customer’s technical experts. SCILLC does not convey any license under its patent rights nor the rights of others. SCILLC products are not designed, intended,or authorized for use as components in systems intended for surgical implant into the body, or other applications intended to support or sustain life, or for any other application in which
the failure of the SCILLC product could create a situation where personal injury or death may occur. Should Buyer purchase or use SCILLC products for any such unintended or
unauthorized application, Buyer shall indemnify and hold SCILLC and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, andexpenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claimalleges that SCILLC was negligent regarding the design or manufacture of the part. SCILLC is an Equal Opportunity/Affirmative Action Employer. This literature is subject to all applicable
copyright laws and is not for resale in any manner.

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TND6115/D