EDNE October 2012 - (Page 47)

Image sensors evolve to meet emergIng embedded vIsIon needs By M i c haE l Tusc h • Read this article stand-alone, or as part 2 of the cover story article from our September issue. That article (www.edn-europe. com/article.asp?articleid=5722) looks at examples of embedded vision and how the technology transition from elementary image capture to more robust image analysis, interpretation and response has led to the need for more capable image sensor subsystems. parT 2: hDr procEssiNg algoriThMs aND harDWarE a p ica l IN aNy revIew of Image seNsor techNology, It’s ofteN ImportaNt to dIscuss so-called hdr (hIgh dyNamIc raNge) or wdr (wIde dyNamIc raNge) seNsors. maNy embedded vIsIoN applIcatIoNs (as wIth photography aNd other Image capture applIcatIoNs IN a more geNeral seNse) requIre robust fuNctIoNalIty eveN wIth challeNgINg real-lIfe sceNes. hdr aNd wdr meaN the same thINg –It’s just a matter of how you use each axIs of your dyNamIc raNge graph. the commoN termINology “hdr” Is used throughout thIs dIscussIoN. C ameras, even high-end DSLR (digital single-lensreflex) types, aren’t often able to capture as much information in high contrast scenes as our eyes can discern. This fact explains why we have rules of photography such as “make sure the sun is behind you.” Although conventional image sensors struggle in such conditions, the industry has devoted significant work over many years to developing HDR sensors that extend the raw image capture capability. The reliability of the image capture component is, of course, one key element of the overall system performance. The dynamic range (DR) of the sensor is the ratio of the brightest pixel intensity to the darkest pixel intensity that the camera can capture within a single frame. This number is often expressed in decibels (dB), i.e., DR in dB = 20 * log10 (DR) The human eye does very well with respect to dynamic range and, depending on exactly how the quantity is measured, is typically quoted as being able to resolve around 120 to 130 dB (i.e. around 20 bits) in daytime conditions. Image sensors are analogue devices that convert pixel intensities to digital values via an analogue-digital converter (ADC). The bit depth of the output pixels sets an upper limit on the sensor’s dynamic range (Table 1). In reality, the maximum dynamic range is never quite achieved, since in practice the noise level reduces the useful pixel range by up to approximately 2 bits. Standard CMOS and CCD sensors achieve up to around 72 dB dynamic range. This result is sufficient for the great majority of scene conditions. However, some commonly encountered scenes exist which overwhelm such sensors. Well-known examples are backlit conditions (i.e., a subject standing in front of a window), outdoor scenes with deep shadows and sunsets, and nighttime scenes with bright artificial lights (Figure 1). Such scenes typically exhibit a dynamic range of around 100 dB and, in rare cases, up to 120 dB (Figure 2). If captured with a conventional sensor, the image either loses detail in shadows or has blown-out (i.e., clipped, also known as “blooming”) highlights. Numerous attempts have been made to extend standard CMOS and CCD image sensor technologies, overcoming Type of sensor Very low cost standard Average standard Higher quality standard HDR HDR Bits/pixel 8 10 12 16 20 Maximum intensity levels recorded 256 1024 4096 65536 1048576 Maximum sensor dynamic range (dB) 48 60 72 96 120 Table 1: The dynamic range potential of various image sensor types. www.edn-europe.com OCTOBER 2012 | EDN EuropE 47 http://www.edn-europe.com/article.asp?articleid=5722 http://www.edn-europe.com/article.asp?articleid=5722 http://www.edn-europe.com

Table of Contents for the Digital Edition of EDNE October 2012

Cover
Contents
International Rectifier
Microchip
RS Components
Masthead
Microchip
EDN Comment
Pulse
Analog Devices
Farnell
Baker's best
Omicron
Test & Measurement World
Agilent Technologies
Maxim Integrated Products
Maxim Integrated Products
Digi-Key
Mixed-signal SOC verification using analog behavioral models
Messe München
Bergquist
The future of connected-device security
Silicon Laboratories
Digi-Key
Coilcraft
Messe München
Image sensors evolve to meet emerging embedded vision needs
Digi-Key
Chip Expo
National Instruments
Supply chain
Introducing software-designed instrumentation to test-and-measurement
Digi-Key
Mechatronics in design
Design ideas
Product Roundup
Tales from the cube

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