Image sensors see new light

Developments in CCD and CMOS imagers are opening new applications

Manufacturers of CCD and CMOS imagers have long argued the merits of each technology. Generally speaking, for machine-vision applications, CCD imagers remain the detector of choice, while CMOS imagers are used in more specialized applications requiring high speed, low cost, random access, or high dynamic range. The reasons for this difference lie primarily in the specific design of the pixels and readout electronics associated with each device.

As a small-volume market, machine vision has depended on technology developed for consumer markets. Yet many machine-vision applications require higher performance than consumer-based devices can deliver. Some CCD and CMOS manufacturers have realized this opportunity and optimized their production for added-value niche markets.

Many scientific, medical, and security imaging applications demand low-light, solid-state sensors with photon-counting sensitivity that can capture hundreds of frames per second. Although CCDs are capable of high quantum efficiency, low dark current, high linearity, and uniformity, their serial-readout design limits readout speed. In addition, the on-chip output amplifiers of these devices need to be operated at high speed, increasing CCD read noise and compromising sensitivity in low-light-level applications.

To overcome these limitations, engineers at Fairchild Imaging (Milpitas, CA) have developed a low-light-level digital camera with a Camera Link interface that combines the best features of CCD imagers and CMOS technology. The camera, shown at Photonics West ( San Jose, CA), uses two CMOS readout ICs that are bump bonded to a split-frame transfer CCD. With very low dark current, the camera has been demonstrated to capture images at 30 frames/s where the brightest region within the image has less than 200 photoelectrons.

Low-light level

Scientific CCD image sensors with electron multiplication are also beginning to emerge for low-light-level applications. Before charge-to-voltage conversion, charge passes through a series of multiplication states, in which photogenerated electrons create more electrons with impressively low excess noise. While these devices still suffer flaws due to aging and at high temperature, they are already commercially available and permit low-light-level cameras to be built more easily.

The back-thinned CCD97 from e2v technologies (Chelmsford, England), for example, is available in a 512 times; 512 frame transfer format and is part of the company’s L3Vision range. For scientific imaging at low light, it uses an electron-multiplication output amplifier circuit that can operate with read noise of less than one photoelectron at pixel rates in excess of 11 MHz .

By coupling the quantum-efficiency performance of back-thinning with low read noise, the CCD97 complements the back-thinned CCD60. The back-illuminated CCD60 was the first CCD to offer sensitivity close to the theoretical maximum for a silicon imaging sensor, with quantum efficiency greater than 90% and read noise less than one electron, extending the scope of CCD sensor applications to include confocal microscopy, single-molecule imaging, and adaptive optics.

The CMOS counterpart to this CCD-based electron multiplication technology is called Geiger-mode avalanche pixels. At a pixel size of about 30 µm, the devices are capable of detecting a single photon. However, further developments are needed to increase their fill factor from their current 5%.

In operation, each single-photon avalanche diode (SPAD) is a p-n junction biased above breakdown voltage by an excess voltage of a few volts. A primary carrier resulting from the absorption of a photon can generate an infinite number of secondary electron-hole pairs by impact ionization. Upon arrival of a photon, the breakdown current discharges the depletion region capacitance, reducing the voltage across the SPAD to its breakdown voltage. The near-infinite internal gain requires no further amplification and the pixel output can be routed directly outside the chip.

At the Swiss Federal Institute of Technology (Lausanne, Switzerland), a single 32 × 32 SPAD imager has been developed specifically for use in a 3-D camera. Based on the pulsed time-of-flight (TOF) method, the sensor used in the camera is implemented in standard CMOS technology and consists of an array of single-photon avalanche diodes, capable of performing 1024 independent distance measurements.

Click here to enlarge image

CONARD HOLTON is editor in chief of Vision Systems Design; e-mail: cholton@pennwell.com.

Most Popular Articles

Webcasts

Femtosecond Lasers – Getting the Photons to the Work Area

Ultrashort-pulse lasers, both picosecond and femtosecond, are now available from a large number of manufacturers, with new players entering the field at a ra...

Laser Measurements Critical to Successful Additive Manufacturing Processes

Maximizing the stability of the variables going into any manufacturing process is what ensures ts consistency and high quality. Specifically, when a laser is...

Ray Optics Simulations with COMSOL Multiphysics

The Ray Optics Module can be used to simulate electromagnetic wave propagation when the wavelength is much smaller than the smallest geometric entity in the ...

Multichannel Spectroscopy: Technology and Applications

This webcast, sponsored by Hamamatsu, highlights some of the photonic technology used in spectroscopy, and the resulting applications.

Handheld Spectrometers

Spectroscopy is a powerful and versatile tool that traditionally often required a large and bulky instrument. The combination of compact optics and modern pa...

Fracking, climate change, and lasers:  new tools to reduce fugitive methane emissions

This webcast, sponsored by Hamamatsu Corporation, covers recent developments and field deployments of compact quantum-cascade-laser (QCL)-based methane senso...
White Papers

NIST Traceable Spectral Responsivity Calibration of Photodiode Detectors

All Newport optical detectors are recommended for a 12 month recalibration interval. Newport main...

Accurate LED Source Modeling Using TracePro

Modern optical modeling programs allow product design engineers to create, analyze, and optimize ...

Tailored bar concepts for 10 mm-mrad fiber coupled modules scalable to kW-class direct diode lasers

In this paper, laser modules based on newly developed tailored bars are presented. The modules al...
Technical Digests

OPTICAL COATINGS: Evolving technology produces new benefits

The antireflection, high-reflection, and/or spectral characteristics provided by optical coatings...

REMOTE FIBER-OPTIC SENSING: Data in abundance from difficult environments

The use of optical fibers to measure strain, temperature, and other parameters at desired points ...

SCANNERS FOR MATERIALS PROCESSING: Serving demanding applications

Galvanometer-based scanners are an essential component in laser-based materials-processing system...

Click here to have your products listed in the Laser Focus World Buyers Guide.

RELATED PRODUCTS

PRESS RELEASES

AFL Recipient of Three Technology Patents

10/03/2013 Five AFL associates were recognized for receiving patent awards for their work developing new pro...

AFL Introduces Fujikura Fixed V-groove Single Fiber

10/03/2013 AFL introduces the Fujikura 19S fusion splicer, a new fixed v-groove single fiber splicer, the la...

AFL’s Five-Year Warranty Sets New Standard

10/03/2013 AFL increased the warranty period on NOYES® Optical Power Meters (OPM), Optical Light Sources (OL...

AFL Introduces New Family of NYFORS™ Recoating Products

10/03/2013 AFL now offers Nyfors Teknologi AB’s new family of recoating products including the ReCoater 2™, ...
Social Activity
  •  
  •  
  •  
  •  
Copyright © 2007-2015. PennWell Corporation, Tulsa, OK. All Rights Reserved.PRIVACY POLICY | TERMS AND CONDITIONS