US: Researchers at National Institute of Standards and Technology (NIST), Gaithersburg in the US, are developing a Hyperspectral Image Projector (HIP). Through HIP, they aim to effectively evaluate the performance of remote-sensing (RS) instruments.
RS instruments are designed to take images composed of many spectral bands, not just the minimum three components of red, green and blue (RGB) used by common digital cameras. These images are referred to as hyperspectral because each pixel contains information for hundreds or thousands of narrow spectral bands.
HIP’s purpose is to enable scientists to project hyperspectral images into sensors, simulating realistic scenes both spectrally and spatially, for performance testing and evaluation of the sensor instruments in the laboratory. For example, by using the HIP to test satellite-sensor performance in controlled laboratory settings, scientists can alleviate expensive field testing, allow better separation of environmental effects from instrument effects, and enable system-level performance testing and validation of space-flight instruments prior to launch.
Many realistic scenes of interest for testing defense and security sensors would be very difficult or dangerous to set up outside, but can be relatively easily simulated and projected into the sensors by the HIP. Similarly, tissue phantoms used to test medical optical and IR-imaging instruments are difficult to maintain and disseminate with known properties, whereas the HIP can present repeatable digital versions of tissue phantoms to these instruments.
The HIP system’s design is similar to commercially available digital light processing (DLP) projection systems in which the projected image is made from a composite of grayscale images representing each of the RGB colours. The individual grayscale images are generated by focusing light through a rotating multicoloured filter to obtain the spectral component and illuminating a digital micromirror device to obtain the spatial component. When the grayscale images are projected and combined at typical video frame rates, the result is a full RGB colour image. In contrast to the DLP system, the HIP system can project composites of numerous spectra. Instead of using a filter, the HIP system’s spectral components are generated with a spectral engine composed of dispersive optics and a spatial light modulator such as a digital micromirror device or a liquid-crystal spatial light modulator. The spatial engine, composed of a second spatial light modulator, then determines the spatial component for each spectral component. Synchronised operation of both engines ensures that each spectral component is projected sequentially in the correct proportions in each spatial region to create a time-averaged hyperspectral image.
The advantage of the HIP system is not only its ability to project realistic, spectrally, and spatially complex scenes, but also the user’s ability to arbitrarily define and control the spectral distributions at each spatial image pixel. For example, the HIP can alter certain spectral components to reflect changing scenes. This means that HIP can be used to test imagers under a wide range of conditions and for a variety of applications.