Imaging spectrometers are important instruments in remote sensing.They allow researchers to determine the amount of light an area is reflecting as a function of wavelength. This enables many types of analysis that simple color filters can’t.
The fundamental ability of imaging spectrometers is their capacity to capture a nearly continuous spectrum for every point in an image. They do this by mapping a strip of land onto a line of pixels on the image sensor, with the spectral information captured by a line in the orthogonal direction, then building up a spectrally resolved image one strip at a time.
Performance of an imaging spectrometer can be broken down into two categories: spatial and spectral. Spatial performance includes the field of view (how wide the strip of land is) and the spatial resolution (see our Ground Sampling Distance page). The spectral range and resolution specifications describe how broad a waveband the spectrometer covers and how narrow a wavelength band can be resolved.
Diffraction gratings are an essential part of imaging spectrometers. They come in two varieties, transmission gratings and reflection gratings. Transmission gratings are rarely used. Reflection gratings are glass coated with a metal that provides high reflectivity across a broad range of wavelengths, from the ultraviolet to the infrared. They can be flat or curved; two examples of each are provided below.
Commercial convex gratings for imaging spectrometers are mostly made by companies such as Jobin-Yvon Inc. in France; Carl Zeiss Inc. in Germany; NASA and Richardson Grating Laboratory Inc. in the United States; and Headwall Photonics Inc., Spectrogon Inc., and Hitachi in Japan.
Below are four different types of spectrometers. Each come with their own advantages and disadvantages.
Transmissive Spectrometers
A transmissive imaging spectrometer may be worth considering for systems with sufficiently narrow wavebands. It consists of a slit, a collimating lens group, a plane grating and a focusing lens group. In this configuration, the light is collimated by the first lens group, diffracted and reflected by the grating, then focused onto the detector by the second lens group.
An advantage of this configuration is that the lenses can be made quite fast; F/1.4 is certainly achievable. It is also possible to image a wide field of view. The main disadvantage of this configuration is the difficulty of achieving athermalization. To achieve it, special glass is required, which can be scarce and costly. Choosing special glasses for athermalization makes it difficult to simultaneously achromatize the lenses over a wide spectrum.
Offner Spectrometer
An Offner spectrometer consists of three concentric mirrors: a primary concave mirror, a secondary convex mirror, and a tertiary concave mirror. The term “concentric” here signifies that all three mirrors share a common center of curvature. Typically, the diffraction grating, which serves as the dispersive element, is located on the convex secondary mirror. It is also common for the primary and tertiary mirrors to be one piece.
Light entering the Offner spectrometer through an entrance slit is first reflected by the primary concave mirror, then directed to the convex secondary mirror (the grating), where it is both diffracted and reflected. The dispersed light then travels to the tertiary concave mirror, which focuses the different wavelengths onto a detector array.
The fact that Offner spectrometers consist only of mirrors gives them the ability to cover extremely wide wavelength ranges. Manufacturing the mirrors from low expansion materials and building the structure from carbon-fiber makes this spectrometer athermal over a wide temperature range. The major limitation of the Offner configuration is that it is limited to roughly F/3 or slower. Convex gratings can also be expensive and take a long time to procure.
Dyson Spectrometers
While Dyson spectrometers also use concentricity to minimize aberrations, they consist of a thick lens and a mirror. For the curious, the design equation is: n*RL = (n-1)*RM, where RL and RM are the radii of the lens and mirror and n is the refractive index of the lens. The thickness of the lens is, of course, RL. In this spectrometer, the entrance slit is far enough off-axis so there is room for the slit and the image sensor. Light from the slit passes through the lens, is reflected by the mirror, which is also the grating, passes back through the lens and is focused onto the image sensor.
The primary advantages of the Dyson spectrometer are compactness and speed. This spectrometer can be as fast as F/1. There is, of course, a tradeoff with resolution, but the design space is sufficient for many purposes. Athermalization is also readily achievable by making the lens of fused silica and the structure of carbon-fiber. If weight is a concern, the lens of the Dyson spectrometer can be a problem. As with the Offner spectrometer, the curved grating can be a cost and schedule issue.
Czerny-Turner Spectrometers
Czerny-Turner spectrometers are a workhorse in the lab, but they can also be used for remote sensing. They consist of two concave mirrors and a plane grating. Light from the slit is collimated by the first mirror, reflected and diffracted by the grating, and then focused onto the image sensor by the second mirror.
These spectrometers are widely used because the plane grating and simple spherical mirrors keep the cost to a minimum. Although performance is adequate for many applications, astigmatism limits resolution as F/# decreases; F/2.8 is about the limit. This is comparable to the Offner spectrometer, but the Offner design permits a much wider field of view.
Have a project you’re working on, please give us a call at (651)315-8249, or fill out our contact us form.