Characterization of the performance of image sensors and image sensor systems is critical to success in many defense and security applications. From image sensors used in the recording of fast processes, to the use of sensors in munitions development and performance analysis, to the "eyes" on un - manned vehicles and targeting image sensors, new and better methods for testing and characterization of sensors are now being employed.

Important performance parameters that are now being used to determine the limits of field application include testing for quantum efficiency, noise floor, gain, and wavelength of available optical energy. No one would want to deploy a sensor or system in the field that would not provide adequate imaging in all scenarios of a mission.

Figure 2. Close-up of the digital light source instrument utilized in Figure 1.

The advent of digitally controlled, solid-state lamp (SSL)-based light source instruments has greatly simplified the testing process and substantially lowered costs associated with performance characterization in image sensor components production and integrated camera testing. New techniques for testing the offset bias, gain, noise floor, spectral gain, and quantum efficiency characteristics of focal plane arrays (FPAs), CCDs, and CMOS image sensors are now available with digital control devices that offer stability, programmability, linearity, and uniformity.

Many image sensor performance evaluation and testing applications are well suited to utilize the advantages of single-spectral and multi-spectral channel digital-control standard light sources. A multi-channel light source instrument can emulate and switch illuminants at will, with repeatability and consistency, outperforming conventional sources.

Testing and evaluation applications include testing and calibrating monochrome sensors and cameras for all their response characteristics (for low end to high imagers); testing and calibrating color sensors and cameras for all their response characteristics - with the single exception of color reproduction (for low-end to high-end imagers); and the multi-channel version can be used as a brightness, colorimetric, and/or spectral reference to determine, test, control, and confirm color gamut differences and ranges for any and all color metric applications. Processes that relate to night vision goggle compatibility, paints, pigments, fabrics, plastics, and displays all benefit from this advanced digital light control testing technology.

Benefits and Advantages

Figure 1. A SolidWorks electronic assembly drawing of the custom configuration of a digital light source instrument coupled to the Agilent 93000 test head.

The features of these digitally controlled light sources that provide an advantage over tungsten, xenon, or other sources, are the long-term stability of the output and the long, usable life. Stability is achieved through the use of onboard monitoring photodiodes that are integrated into both the optical delivery system and the illumination power control electronic circuitry. This combination allows a 65,000 to 1 intensity setting resolution (16-bit) with an optical output power linearity of better than 99.9 percent over this range; in other words, a deviation from perfect linearity of less than 0.1 percent over a 65,000 to 1 dynamic range. This performance allows characterization of high dynamic range and resolution scientific- grade CCD image sensor chips so they can be used to replace process monitoring functions previously only possible with film.

The long-term stability of these light sources has been proven in production line testing applications of CMOS image sensors. Usually using white, high-correlated, color-temperature spectral-power distributions, the digital light sources are coupled to the multi-signal test heads manufactured by Agilent, KYEC, Teradyne, and others. These test systems can be configured to evaluate image sensors in wafer format and also individual sensors after the wafers are diced into single components. Long-term use and statistical process control monitoring of the digital light sources in image sensor fabrication test houses has proved to provide output power stability of less than 2 percent drift in the output power over a two-year time period. Verification of the stability is accomplished using NIST-calibrated temperature-controlled standard detectors with annual drift rates of 0.2% as the primary reference standards, which can also provide on-site low uncertainty calibration.

The newer, multi-channel version of this successful digital illumination source technology provides up to 12 independent channels of essentially the same linearity and stability performance of the single-channel source into a single, integrated microcomputer- controlled package. The advantage of having up to 12 individual, digitally addressable channels is the ability to create, using nonlinear and iterative weighting, to fit a desired target spectral power distribution (TSPD) with adjustable intensity. This iterative weighting algorithm is able to maintain a close fit to the TSPD over a wide range of intensity settings, even though the individual channels can vary in spectral output with intensity. In addition to these capabilities, the intensity and spectral content switching speeds (on the order of a few milliseconds, including the output settling time) is not possible with conventional mechanical means of adjusting intensity and spectral content.

Figure 3. The image sensor test system features a custom configuration with the digital light source instrument attached as the illumination source.

Implementation of the channel intensity hardware features of the 12- channel source is accomplished through a comprehensive software control/ graphical user interface package, Synthicolor™. Spectrums can be created by the user through simple Windows slider controls, or by importing ASCII format spectral waveform data to be matched. Spectrums can be saved, recalled, displayed, and quantified with regard to their spectral and colorimetric characteristics. Individual spectral profiles can be imported, matched, displayed, and saved using adjustable matching and illuminant criteria, and can even conserve relative 'whiteness' and 'lightness' between different spectral profiles.

Image Sensor Testing

In image sensor testing, costs associated with test time, throughput, and profitability are an important factor. Reducing the down time of test equipment components and the time required to change out components of the test system is critical in maintaining efficiency. Replacing more traditional tungsten lamp-based light sources with digitally controlled solid-state light sources has several time and efficiency advantages. The first is the amount of time required to optically align the source mounted to a multi-signal test head and obtain a uniform illumination field in the image sensor test plane. Tungsten-based sources can take hours to align to obtain the "sweet spot" of desired uniformity. Once aligned, maintaining the position to tight tolerance is critical. In contrast, the digital solid-state light sources require only minutes to attach and provide superior uniformity in the test illumination plane.

The second advantage over tungsten-based sources is the long lifetime of the digital solid-state light sources. This translates to time saved, by eliminating the need to remove the source from the test head and replace the lamp every 500 to 2,000 hours. The solid-state sources deployed to date have given over two years of operation without replacement.

Figure 4. Oscilloscope trace of the intensity settling characteristics of a 2-millisecond pulse of light from the multi-spectral digital source.

The third advantage is also related to the limited lifetime of the tungsten lamps and the drift in the output over its limited life. There is improvement by a factor of ten in the stability of the digital solid-state sources compared to tungsten sources. Over the 500- to 2,000-hour life of a tungsten-based light source, the drift in the light output is the limiting factor in the ability to determine differences in the test image sensor performance. This limits the chip manufacturers' ability to improve fabrication processes and improve yield of the image sensor production. These factors, in addition to the lower initial cost, produce a payback time of a few months for replacement of the tungsten-based test head illumination systems with the digital solid-state sources.

Digitally controlled, single-spectral-channel, solid-state-lamp-based precision test sources have proven to be economical and useful in the testing to a broad range of image sensor products, from low-cost, high-volume CMOS sensors to high-performance CCD sensors integrated into high-performance camera systems. The availability of a 12- spectral-channel source allows even greater efficiency and performance advantages.

This article was written by Richard L. Austin, President of Gamma Scientific, San Diego, CA. For more information, click here .