New standards institute method can test optical imaging sensors

Detecting climate change with remote-sensing instruments, providing state-of-the-art surveillance imagery for security and defense, or getting the most out of medical imagers requires excellent knowledge of the sensor's performance. One of the challenges scientists and engineers developing these instruments face is effectively evaluating their performance. To address this, we are developing the Hyperspectral Image Projector, or HIP (see Figure 1), which will enable performance evaluation of cameras and other imaging instruments using realistic scenes.

Remote-sensing instruments and medical imagers 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.¹

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.^{2, 3}

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 colors. The individual grayscale images are generated by focusing light through a rotating multicolored 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 color 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. Synchronized 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.⁴

Figure 1. Schematic of the Hyperspectral Image Projector (HIP). A spectral engine optically in series with a spatial engine projects a time-integrated 2D image to the sensor. Each spatial pixel is presented with a spectrum that can be programmed to simulate those that occur in realistic scenes. DMD: Digital micromirror device.

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.

We recently demonstrated the HIP system's capabilities by projecting a hyperspectral image of a coral reef (see Figure 2).⁵ The original image, acquired by an airborne hyperspectral sensor, was deconvolved into six spectral components, and then re-projected using the HIP into a laboratory imaging spectrometer. The pictures below present an RGB version of the original image and the HIP-projected image.

Figure 2. Original image of a coral reef and the HIP-projected image measured by a remote-sensing imager in lab testing.

Our research, performed with the HIP operating in the visible spectrum, served to prove the concept. We and our collaborators are continuing to develop the HIP by extending the spectral range into the IR and UV, increasing the spectral resolution and brightness, and enabling the showing of dynamic scenes such as hyperspectral image movies. In addition, portable, rack-mounted prototypes being built will allow the HIP to be used by other scientists and test engineers in their own labs.

This work was funded in part by the Department of Defense Test Resource Management Center's Test and Evaluation/Science and Technology Program and in part by the National Institute of Standards and Technology's Office of Law Enforcement Standards.