These teachings relate to optical designs, which provide reduced unwanted background radiation, increased signal throughput, or a combination of both.
As detecting elements become smaller, the size of the associated slit elements in imaging spectrometer designs typically become smaller. With this decreased size, the effects of the diffraction of light become more significant, leading to the leakage of unwanted background radiation from outside the geometric rays to pass through to the detecting element, as well as leading to the loss of signal radiation that is diffracted outside of the geometric ray bundle and vignette by the optical elements or stop of the system.
This effect can be quite significant, particularly in infrared systems where unwanted background radiation that is not shielded from the detecting element can prematurely fill the electron wells of photonic detecting elements, or mask the true temperature of objects in the scene in the case of pyroelectric detecting elements. In systems where radiometric sensitivity or accuracy is concerned, the impact of these diffraction effects can be intolerable.
For example, consider some applications of hyperspectral imaging in which it is desirable to have a spectrometer that is intended to identify targets by their spectral signatures in the infrared portion of the electromagnetic spectrum, but must also be very small size and mass so that the system can be transported in an unmanned aerial vehicles (UAV) or be man-portable. These sensors may typically have detecting elements and associated slit apertures whose dimensions are on the same order as the wavelength of the light they are intended to detect, in which case the effects of diffraction are substantial, and can easily reach diffraction angles of 30 degrees or more. If accurate radiometric measurement of the targets is necessary, then this diffraction can mask or hide the target from detection or even introduce false alarms detections. Additionally, the loss of wanted signal from the target due to diffraction can reduce the sensitivity of the sensor, allowing potential targets and threats to go unidentified.
There is a need for optical designs that increasing the overall throughput, decrease the overall background radiation, or a combination thereof.