There are many low cost artificial light sources used for illumination or for industrial applications: tungsten or halogen bulbs, fluorescent lamps, LED lamps etc. Some of these sources have regulated intensity of emitted light. However, there very few calibrated light sources of precisely known parameters. A light source can be considered as calibrated when its user can precisely regulate its photometric/radiometric parameters like luminance (or illuminance), radiance (or irradiance) at defined spectrum of interest. Such light sources are needed in many applications – among them, in systems for testing night vision devices, VIS-NIR cameras and SWIR imagers. Inframet offers a series of calibrated light sources that can be divides into three groups:
1. Multi-channel halogen/LED
2. Single channel halogen light sources,
3. Large emitter light sources.
The light sources from the first group are optimized to be used in systems for testing VIS-NIR cameras and SWIR cameras. These sources are expected to simulate illuminance conditions met in real life scenarios: from dark nights in Afghanistan mountains to ultra bright days in Arabian deserts. Such realistic simulation is possible only using light sources of following features:
- Ultra high dynamic (ratio of maximal luminance to minimal luminance) of light source at level over 109 (needed to simulate both bright days and dark nights),
- Variable light spectrum in VIS-NIR range (needed to simulate variation of spectrum of incoming light and variation of reflecting properties of targets under surveillance),
- Ability to emit high intensity UV light (needed to simulate conditions met at coastal areas or high altitude mountains with high UV light that are potentially damaging for some imaging sensors).
Two light sources codes as DAL/SAL belong to the first group. These light sources work by combining light from several channels: halogen bulb of 2856K color temperature spectrum and several LED sources. Intensity of the halogen bulb is regulated using an opto-mechanical attenuator that changes light intensity but does not change light color temperature. Intensity of the LED sources is regulated electronically using advanced electronic regulation/stabilization system. Intensity of UV LED is not regulated. Spectrum of emitted light can be regulated by switching light channels or by mixing light from halogen channel with LED channels. Regulation of spectrum of emitted light is additionally supported by a set of external spectral filters.
DAL/SAL light sources look externally the same but there are some design differences: different coating of integrating sphere, slightly different halogen source and different LED sources. DAL is optimized for testing VIS-NIR cameras sensitive up to about 1000 nm; SAL – for testing SWIR imagers sensitive up to about 1700 nm.
|Fig. 1. Photo of DAL light source||Fig. 2. Photo of SAL light source|
DAL/SAL light source are designed to cooperate with collimators in Inframet test systems. The image projectors (collimators) require from the light source Lambertian emission of light only in narrow angle (typically below 10°). Therefore DAL/SAL light source behave like a fully Lambertian source at emission angles up to 15°). This limited Lambertian performance is not acceptable in applications where the light source is used as an independent block. Next, there are applications when aperture of light emitter of DAL/SAL sources (40 mm) is too small. Therefore Inframet offers also H40 light source, L150 light source and H150 light source.
H40 is a single channel source halogen light source that emit light of 2856K spectrum. Technically it can be treated as a simplified DAL light source without LED channels and with modified light emitter. Diameter of light emitter in H40 sources is the same as in DAL/Sal sources (40 mm) but there are two important changes: a) Lambertian source at emission at angles up to 60°, b)ability to cooperate with bright collimators of low F-number (ratio of focal length to aperture can be as low as one).
The second change means that by using additional bright collimator an apparent aperture of the light source can be increased (typically up to 200mm) while still keeping sufficient emission angle. Practically it means that H40 light source cooperating with a series of bright CRAD refractive collimators makes possible calibration of VIS-NIR cameras or SWIR cameras with optics as big as 200mm.
Ability to calibrate VIS-NIR cameras and SWIR cameras of big optics can be also directly obtained by using light sources of big emitter. Two such light sources are offered: L150 and H150. L150 is a single channel, ultra compact (thin design) LED source that emits light using emitter of 150mm diameter. Emission in different spectral bands is possible: visible, NIR or in SWIR. H150 a single channel, halogen source that emits light using emitter of 150mm diameter. The source emits polychromatic light of 2856K spectrum in VIS-SWIR range.
Fig. 5. Photo of L150 light source
Fig. 6. Photo of H150 light source