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Sensor Calibration & Testing
Uniform light source systems can provide uniform radiance or irradiance, with applications that include the test and calibration of focal-plane arrays and cameras for areas such as pixel gain normalization, photographic sensitometry and remote observation system calibration. Sources are used in the ultraviolet, visible, and near infrared regions of the spectrum.
Most uniform light sources use integrating spheres, while others are created from diffuse reflectance targets, diffuse lensed lamps, or projection sources. Integrating sphere systems are highly Lambertian and show very uniform radiance distribution. Labsphere's Integrating Sphere Uniform Source Systems feature a flexible design and can be modified for application specific requirements.
Labsphere has created thousands of uniform source systems for the testing and calibration of imaging and non-imaging devices with customers ranging from international aerospace agencies and government organizations to camera manufacturers. Labsphere Uniform Light Source Systems are designed to minimize color shifts with radiance level changes and provide the highest level of uniformity. Our calibration laboratory team has extensive experience in performing application specific calibrations, all of which are traceable to NIST.
CMOS TEST AND CALIBRATION
For complementary metal-oxide semiconductors (CMOS) typically the semiconductor is positioned at the exit port of the integrating sphere and is shielded from direct illumination by lamps within the sphere system. Illumination is provided indirectly, by multiple diffuse reflections, so that the device is exposed to a uniform irradiance across its active area.
The illuminance at the exit port of the sphere is factory-calibrated, and a monitor detector attached to the sphere provides a reading of the illuminance presented to the radiometer during the test procedure. The response of the device to this known uniform illuminance is evaluated to verify compliance to specifications, or to identify and correct process problems.
Different types of light sources may be used in this approach. The sphere may be illuminated with regulated incandescent lamps or the source may be designed to simulate a specified spectral distribution, such as that of average daylight. The integrating sphere may also be coupled to a monochromator for the purpose of spectral characterization.
Labsphere's Uniform Source Spheres and Systems test the responsivity and/or quantum efficiency of semiconductor wafers and devices used for photovoltaic or photoconductive applications.
FILM SENSITOMETRY TESTING
Manufacturers of silver-halide photographic materials must perform quality control tests on their products. If diffuse illumination is required, an integrating sphere uniform source is the best way to accomplish this. This is an illuminance application.
A few things necessary for this application are:
- The light source illuminating the target must be uniform over the film plane area.
- A sample holder is required for gray scale step wedges, color step wedges, as well as for the sample film under test
- Color temperature and illuminance at the film plane must be monitored and controllable.
- The illumination system must have a controllable shutter system for multiple exposure levels.
- Complete systems may incorporate the manufacturer's process control data collection requirements, such as logging data and providing statistical information.
To do this, typically the film sample is positioned at the exit port of the integrating sphere. Illumination is provided indirectly by multiple diffuse reflections within the sphere. These reflections create uniform irradiance across the sphere's exit port. The film sample is exposed to this uniform illuminance.
The illuminance responsivity at the exit port of the sphere is factory-calibrated. A lux meter provides a reading of the illuminance at the film plane. The response of the film sample to this known uniform illuminance and exposure time can be used to verify compliance to material specifications or to identify and correct process problems.
Labsphere's line of Uniform Source Spheres and Systems are optimally designed to characterize the response uniformity of photographic materials.
FOCAL PLANE ARRAY TEST AND CALIBRATION
Focal plane array applications can range from inexpensive video and consumer cameras to advanced scientific imagers for space-borne remote sensing. There are various types of focal plane arrays such as charge-coupled devices (CCD), charge-injection devices (CID), complimentary metal-oxide semiconductors (CMOS), and photo diode arrays (PDA). They can be linear or two-dimensional and not usually more than a few centimeters in size.
Once fabricated and packaged, the devices can suffer from some degree of non-uniform gain and offset coefficients. When exposed to an equal irradiance of light, each pixel in the array does not produce an identical electrical signal. Photo-response Non-uniformity (PRNU) is due to differences in responsivity (gain) among the pixels in the array. Fixed-Pattern-Noise (FPN) is due to variation in dark current (offset). In the presence of gain and offset variations the device produces images that have features that do not exist in the original object but are imparted on the image by the array. In other words, a picture of a uniform field is not uniform.
Offset normalization is simply performed by providing zero irradiance on the array and setting the output signal of each pixel to zero. During gain normalization, a uniform source produces irradiance on the array that is equal at each pixel. The gain, or responsivity, of each pixel is set so that each pixel produces an equal electrical signal.
Linearity can be measured by irradiating the array with varying light levels and measuring the signal produced. To do this, the FPA is placed at the exit port at a specified distance from the uniform source sphere exit port. At the exit port, the array will be irradiated from all directions. If a limited field is desired, the array can be located some distance from the exit port. This distance and the size of the exit port must be chosen to provide the required field angle and to ensure adequate uniformity.
The responsivity of an array may also be desired. A uniform source can provide a known amount of illumination. When the illumination level is varied and the array's response measured, the responsivity, linearity, and dynamic range can be characterized. By introducing narrowband light of various wavelengths, the spectral response can also be measured.
A uniform source system is an excellent tool to measure an array's photon transfer curve. By varying the level of input illumination, one can measure the noise in the array and determine the sources of that noise: noise "floor" under low photon flux conditions; shot noise as illuminance increases: and FPN at higher illuminance. This technique also gives the dynamic range of the array, including its associated readout electronics.
IMAGING DEVICE TESTING AND CALIBRATION
Digital cameras, remote-sensing systems, and other array-based electronic imagers must be normalized in much the same way as bare focal plane arrays (FPA). However, one more element is introduced that must be accounted for — the optical imaging system.
Imaging systems, whether they are refractive, reflective, or both (catadioptric) suffer from irradiance that varies with field angle. The most common variation is the cosine-fourth law. A procedure similar to that described for simple FPAs will correct for cosine-fourth irradiance falloff and other sources of irradiance variation in the image.
To calibrate a digital camera with an integrating sphere system, typically the camera is positioned at the integrating sphere's radiance port and is focused at infinity or at the plane of the sphere's exit port. The sphere's exit port should be large enough to fill the camera's field-of-view, ensuring that this area is completely featureless and devoid of shadows.
The integrating sphere is equipped with one or more lamps positioned outside the camera's field-of-view. Multiple diffuse reflections within the sphere integrate the light creating uniform sphere wall radiance which is collected through the camera's optics to achieve uniform irradiance on the array. The uniform distribution of light on the array is then used to identify and correct for non-uniformities in the signal conversion of the array (see the Focal Plane Array section for additional information).
For camera applications requiring absolute photometric calibration as well as spatial uniformity, the luminance of the uniform source system may be calibrated so that a monitor detector attached to the sphere provides a reading of the sphere luminance output during the camera test procedure. The system then becomes a source of known uniform luminance, which can be used to calibrate the camera's photometric scale.
An integrating sphere provides the best means of characterizing and calibrating the response of an imaging system, significantly out-performing alternatives such as reflective diffuser targets.
Labsphere's line of Uniform Source Spheres and Systems are specifically designed for camera calibration applications and are readily adaptable to provide variable levels of radiance and spectrum without affecting the uniformity of the scene presented to the camera being tested.
MACHINE VISION TEST AND CALIBRATION
Machine vision is automated visual inspection. The components of a machine vision system include one or more cameras, optics, lighting, a computer and software to process images of a part in production to discern features of interest in the image. These features could be the width, surface quality, reflectance, transmittance and/or location of an object. Machine vision software isolates these features and interprets them to quantify a parameter of interest. In general, machine vision can reduce labor costs, reduce worker boredom, ensure consistent product marking, allow increased production volumes due to high speed product inspection and ensure all products are inspected in the same way.
The success of a machine vision project demands integration of the vision system within the existing production process. The right selection of the components can make all the difference in the performance and efficiency of the machine vision system. One key component is the lighting. Front end lighting systems feature high contrast and backlit systems which require uniform radiation distribution. Labsphere's integrating sphere systems can be configured with current regulated incandescent lamps to produce a uniform field of radiance or irradiance appropriate for an application to create a total solution.
Labsphere's Uniform Source Systems are used to uniformly back light systems for machine vision applications.
SEMICONDUCTOR TESTING
Typically, the semiconductor is positioned at the integrating sphere uniform plane of irradiance. Illumination is provided indirectly by multiple diffuse reflections so the device is exposed to uniform irradiance across its active area.
The semiconductor is located at the exit port of the uniform source or at a specified distance from the exit port. An illuminance or irradiance monitor, which is mounted on the sphere wall, is calibrated to monitor the power per unit area at the integrating sphere's exit port. This provides the amount of light on the surface of the semiconductor, which is then used to determine the electro-optical response of the device.
Different types of light sources may be used in this approach. The sphere may be illuminated with current regulated incandescent lamps or with more sophisticated sources to simulate a specified spectral distribution, such as that of average daylight. The integrating sphere may also be coupled to a monochromator for the purpose of spectral characterization.
Labsphere's Uniform Source Systems are used to uniformly irradiate semiconductor wafers and devices to test their responsivity and/or quantum efficiency.
SPECTRORADIOMETER CALIBRATION
For imaging applications, where the radiometers are fitted with an optical device or an optical train, systems can suffer from irradiance that varies with field angle, whether they are refractive, reflective, or both (catadioptric)
To calibrate the spectroradiometer, the spectral radiance of the integrating sphere is imaged onto the spectroradiometer's entrance slit. With the known spectral radiance of the uniform source, the spectrometer can be calibrated for spectral radiance responsivity for the UV-VIS-NIR spectral region. Linearity and dynamic range can be characterized as well.
The sphere's exit port should be large enough to fill the instrument's field of view and is equipped with one or more lamps positioned outside this field of view. The region of the sphere wall viewed by the instrument is illuminated by multiple diffuse reflections so that a uniform field of radiance is presented to the instrument.
The luminance of the sphere system may be used to calibrate a luminance monitor as well.
Labsphere's line of Uniform Source Systems solve common problems in the calibration of both imaging and non-imaging radiometers. The integrating sphere approach eliminates sensitivity to alignment and polarization, which can affect other systems. Uniform Source Systems are readily adaptable to provide variable levels of spectral radiance, radiance, irradiance luminance, or illuminance, at constant CCT or variable spectrums.
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