The testing and characterization of devices such as image sensors, industrial cameras, hyperspectral imaging sensors and optoelectronic detectors requires the use of light sources with highly uniform light distribution. Gigahertz-Optik GmbH produces a wide range of homogenous (uniform) light sources with integrating spheres and LED panels with which the uniformity and performance of image sensors and industrial cameras can be determined. Some application examples are presented on this page.
Integrating sphere light sources with continuous emission spectrum
To measure the sensitivity, linearity and non-uniformity of imaging sensors, a test setup with a reference light source is required which homogeneously irradiates the test specimen. Integrating sphere light sources with quasi monochromatic emission spectra can be used for qualifying grayscale and color sensors and camera systems. However, for the characterization of imaging sensors and systems required for more demanding photometric and spectroradiometric measurement tasks, light sources with a continuous emission spectrum are necessary. The standard illuminant for such applications is the quartz halogen lamp.
The only suitable light source with a broadband continuous emission spectrum from ultraviolet to infrared is the quartz-halogen lamp. Their spectral emission curve closely matches that of a black body over the wavelength range from 250 nm to 2500 nm. Therefore, a quartz-halogen lamp’s color temperature is approximately the operating temperature of the lamp’s filament.
Quartz halogen lamps are used as calibration standards for spectral irradiance and spectral radiant flux. For use as a calibration lamp for spectral radiance, quartz halogen lamps can be combined with an integrating sphere. Such integrating sphere light sources provide a highly uniform luminous field with near perfect Lambertian distribution .
For sizing the integrating sphere, the rule of thumb is that the total area of all openings in an integrating sphere light source (required for the illuminated field, sources and any monitor detectors) should not exceed 5% of the total sphere surface to ensure uniform distribution of light in the sphere. The uniformity of the light distribution within the integrating sphere also depends on suitable coating and, importantly, the design of internal baffles which are required to prevent any direct optical paths between the light source(s) and the light field at the sphere’s output port.
The spectral radiance level of the illuminated field depends on various parameters:
- Diameter of integrating sphere and size of any ports (The larger the sphere diameter and the port area, the higher the attenuation).
- Reflectance of the integrating sphere coating (the higher the reflectance, the higher the light throughput). The wavelength-specific reflectance must be taken into account.
- Size and design of internal baffle(s).
- The radiant flux within the integrating sphere:
- The radiant flux of quartz halogen lamps depends on the lamp power
- The radiant flux of quartz halogen lamps increases with their color temperature
- The full radiant flux of the quartz halogen lamp can only be coupled into the integrating sphere if the lamp is arranged inside the sphere
The linearity of imaging sensors is measured by illuminated them with different intensities. An important prerequisite for the linearity measurement is that the emission spectrum and the spatial uniformity of the reference light source must not change when the intensity is varied. For integrating sphere light sources with internal lamp arrangements, a change in intensity can only be achieved by switching the lamp on and off. This is because any change in the electrical power to the lamp causes a change in the color temperature and is therefore inadmissible. For variable intensity adjustment, the quartz halogen lamp can be placed outside the sphere. In this way the radiant flux entering the sphere can be controlled by a variable aperture. It is most important that any mechanical iris is uniformly illuminated by the quartz-halogen lamp to ensure a color temperature-neutral intensity adjustment. For this purpose, the use of reflectors with a diffuse surface has proven itself.
The possibility to use optical correction filters in front of an externally mounted quartz halogen lamp is limited because of the very high level of infrared radiation (heat) emitted by the lamp. Only filters with very low infrared absorption can be used.
To measure the sensitivity of the imaging sensors and cameras, the intensity of the luminous area can be calibrated in units of spectral radiance (W. sr-1.m-2.nm-1) and luminance (cd/m2 ). It can be provided at different intensity levels. In addition, there is the possibility of incorporating a monitor detector into the sphere’s design which is matched to the output radiance/luminance of the sphere.
Gigahertz-Optik GmbH manufactures integrating sphere light sources ranging in sphere diameter from 50 mm to 1000 mm. Standard models ISS-17-VA and ISS-30-VA, for example, are popular for calibrating the spectral radiance and uniformity of spectroradiometers, multispectral and hyperspectral sensors, etc. Additionally, the modular sphere concept from Gigahertz-Optik GmbH is geared to configuring application specific integrating sphere light sources.
Reference light source for EMVA 1288 testing of imaging sensors and cameras
The selection of a suitable sensor is an important step in the design of image processing systems. For the user, however, the choice is not always easy, as the information provided by different sensor manufacturers does not support a direct comparison of the available products. This problem has motivated the European Machine Vision Association to generate the EMVA 1288 standard . The parameters listed therein and the description of the measuring methods offer the user the necessary comparability in the selection of imaging sensors and cameras
To measure sensitivity, linearity and non-uniformity, the EMVA 1288 standard recommends that integrating sphere based light sources should be used as the reference . As a special feature, the standard states that spheres must be configured with LED sources.
To measure the sensitivity of imaging sensors, light sources with quasi-monochromatic emission spectra can be used. For this purpose, their intensity and emission spectrum must be known by calibration in an absolute unit of measure. The sensitivity measurement described in EMVA 1288 is not the same as the much more complex measurement of full spectral sensitivity of imaging sensors which is only listed as an option within the standard. For determining the sensitivity of monochrome image sensors, a wavelength close to the maximum sensitivity of the image sensor is recommended. Typically, NIR LEDs with a peak wavelength at 850 nm are used. For color image sensors, three light sources are needed to measure the sensitivity. Their wavelengths correspond with the maximum sensitivity of each of the color sensor’s RGB channels. Typical sources are monochromatic red, green and blue LEDs with peak wavelengths of 455 nm, 532 nm and 630 nm.
To measure the linearity of imaging sensors, the intensity of the reference light source is varied and compared with the corresponding response of the sensor or camera. Intuitively, the use of LEDs as a light source seems ideal for this because their luminous flux or radiant flux is almost directly proportional to the operating current. In practice, however, the limited dynamic range of the permissible operating current and the careful implementation of temperature management must be taken into account.
The non-uniformity of imaging sensors is measured by comparing the signal from each pixel when the sensor is uniformly illuminated. According to the specifications in EMVA 1288, the light field of the integrating sphere light source used must be larger than the illuminated sensor surface. The modular sphere concept from Gigahertz-Optik GmbH can be configured to provide a suitable reference source for a wide range of sensor sizes and types. With clever arrangement of the external sources on the integrating sphere that ensure diffuse illumination of the hollow sphere, luminous fields with a surface uniformity distribution of better than the required 97% are achieved.
Compact integrating sphere calibration lamp for the spectral radiance
The need for a compact spectral radiance calibration standard arises when the available space in or around the instrument to be calibrated is limited or from the need of a transfer standard or for service purposes. Depending on the application, additional requirements may arise in addition to the need for a compact version of integrating sphere.
An example of limited space availability is the calibration of fluorescence spectrophotometers with respect to the absolute spectral radiance sensitivity of the luminescence measurements. The development and characterization of the necessary spectral radiance transfer standard was carried out by Gigahertz-Optik GmbH and Physikalisch-Technische Bundesanstalt (PTB).
In order to compare the results of luminescence measurements from fluorescence spectrophotometers of different types and from different manufacturers, the instrument-specific properties must be compensated for. With this in mind, a joint project moving towards quantitative fluorometry was started in 2013 . The participants were the Federal Office for Materials Testing BAM, Berlin; Physikalisch-Technische Bundesanstalt, Berlin; Fluka GmbH, Switzerland and Gigahertz-Optik GmbH, Germany. The task of Gigahertz-Optik GmbH was to design, build and qualify a compact integrating sphere source for use as a transfer standard for spectral radiance.
In order to meet the requirements of a suitable transfer standard, an integrating sphere of only five centimeters in diameter was developed and qualified. The model ISS-5P-SR-FS integrating sphere is manufactured from the neutral and diffuse reflective plastic ODM98 from Gigahertz-Optik GmbH which is more durable than Barium sulfate coating. The design of the in-line baffle between the 5 W quartz halogen lamp and the 20mm diameter light output port ensures uniformity and provides attenuation of the radiance level. The photomultiplier detectors used within fluorescence spectrophotometers typically have a limited linearity range and therefore the radiance of the transfer standard should be as low as possible. The transfer standard for the spectral radiance was calibrated in the wavelength range from 380 nm to 1700 nm.
Reference light source for sensitivity adjustment of combined rain and ambient light sensors
Integrating sphere light sources are widely used in the characterization and calibration of imaging sensors and cameras. In this application, the integrating sphere light source serves as a reference light source that can uniformly illuminate all of the many individual light-sensitive detectors (pixels). This enables the relative sensitivity of the pixels to be adjusted as required.
Integrating sphere light sources emit a constant radiance in all directions of a hemisphere (i.e. Lambertian distribution)  and are therefore most suitable for adjusting the optoelectronic sensors found in a wide variety of applications. One such application is the adjustment of combined rain and ambient light sensors used in the automotive industry.
For improved comfort and safety, optoelectronic sensors are used as combined rain and ambient light sensors in vehicles. These automatically switch on windshield wipers when rain starts as well as control the wiper speed based on the amount of rain. In addition, they switch on the driving lights when twilight sets in or when entering a tunnel.
The rain sensor is based on the principle of total internal reflection and constructed from an NIR source set at an angle to the windscreen and an NIR detector, forming a detection area within a section of the windshield. Raindrops falling on this sensor section reduce the level of NIR radiation reflected back to the detector enabling the control of the windscreen wipers.
The ambient light sensor measures the brightness in front of the vehicle and automatically switches the driving lights on and off when a limit values are reached. An additional sensor can support more accurate tunnel detection. Ambient light sensors are sensitive in the visible spectral range so that they closely match the sensitivity function of the human eye.
The tolerances of the detectors used within the sensors and their measuring geometry set-ups require individual adjustments. An integrating sphere light source with an external halogen lamp provides a suitable reference light source for these adjustments. Since the sensor unit is positioned directly at the sphere output for adjustment, the sphere surface provides uniform illumination of all the individual detectors with diffuse radiation characteristics over their particular field of view. This results from the homogeneous light distribution of the integrating sphere. A monitor detector with photometric sensitivity fitted to the integrating sphere provides a luminance level reference for the adjustment of the light level sensor. Similarly, the radiance level reference for the adjustment of the rain sensor is provided by an NIR sensitive monitor detector. The halogen lamp is located outside the sphere enabling the radiance of the uniform source to be adjusted as required by a variable aperture. The integrating sphere light source is calibrated in luminance and radiance. The modular sphere concept from Gigahertz-Optik GmbH can be configured to provide a suitable reference source for many types and sizes of optoelectronic sensors.
Quantifying the veiling glare of imaging systems
Veiling glare is most apparent when bright light spots or bright scenes are within the field of view of an imaging system. It is produced by unwanted scattered and reflected light from the image forming beam within the camera system that eventually impinges the image plane. It doesn’t form an image, but may be quite uniformly distributed resulting in image fogging and a reduction in image contrast. There are numerous causes of veiling glare including internal multiple reflections between the surfaces of lenses; scatter from lens imperfections; scatter and reflections from components and internal surfaces of the camera body; fluorescence; bulk scatter within lenses.
By quantifying the veiling glare, also referred to as flare, the usable dynamic range of a camera can be increased. This requires a light source that can provide sufficiently high light-dark contrast.
The standard ISO 9358  defines veiling glare index (VGI) as the “ratio of the irradiance at the centre of the image of a small, circular, perfectly black area superimposed on an extended field of uniform radiance, to the irradiance at the same point of the image plane when the black area is removed.” VGI is usually expressed as a percentage. A good method of quantifying veiling glare is therefore based on an integrating sphere light source. This generates at its light exit port a uniform light field with diffuse, quasi-Lambertian radiation characteristics. Since the light field is larger than the lens diameter of the camera, the entire optics and thus the entire camera sensor are uniformly illuminated.
In order to generate a contrast, a light trap is positioned on the opposite side of the sphere to the camera which is imaged by the camera sensor as a black area. Ideally, the signal from the ‘black’ area would be zero. However, this is not possible for two reasons:
1. Due to veiling glare, stray light falls on the black image field;
2. The light trap is not perfect i.e. does not absorb all incident light completely.
The quality of a light trap is determined by the complexity of its construction. In the simplest case, it is an area of matt black paint. In this case, the light absorption of the paint determines the degree of blackness. An improvement is offered by light traps that are designed from matt black coated tunnels with conical shaped internal surfaces. An even greater degree of absorption can be achieved with an Integrating sphere which is matt black coated on the inside.
The modular integrating sphere system from Gigahertz-Optik GmbH enables the configuration of custom solutions to meet even the most specific requirements. For example, a system for quantifying the veiling glare of space telescopes comprising a 100cm diameter integrating sphere light source and a matching 50cm diameter integrating sphere light trap was designed and configured to meet the application requirements using standard and special modules.
 ISO 9358 (1994): Optics and optical instruments– Veiling glare of image forming systems – Definitions and methods of measurementRead less