Patent Application
20090040614
The present invention is related to a dispersive filter that emits polychromatic light and, in particular, to a dispersive filter that maintains the radiance of the input radiation.
Conventional dispersive monochromators use prisms or gratings to disperse light over a region of space and selectively remove wavelengths from the spectrum by blocking the wavelengths and passing desired wavelengths through slits or apertures. One type of dispersive monochromator is a double monochromator, which connects two monochromators in series to improve the stray light level performance. As the name suggests, dispersive monochromators emit monochromatic light.
Dispersive monochromators, including double monochromators, generally suffer from a loss of radiance. The output of the monochromator is dispersed spectrally and the area of illumination immediately inside the exit aperture of a dispersive monochromator is much greater than the area of illumination immediately inside the entrance aperture. Thus, the radiance of the light (integrated over the spectrum) must be lower at the exit aperture than it is at the entrance aperture.
Accordingly, dispersive monchromators, including double monochromators, are not suitable for applications where it is desirable to maintain the radiance (integrated over the pass-band of the instrument) from the entrance aperture to the exit aperture.
In accordance with an embodiment of the present invention, a dispersive filter uses two dispersion systems coupled in series with an intermediate slit between the two. Each dispersion system includes a dispersive element, such as a diffraction grating or a prism, and have similar but mirror image dispersion characteristics at the plane of the intermediate slit. The two dispersion systems are configured so that an entrance port of one of the dispersion systems is polychromatically imaged on the exit port of the other dispersion system. The first dispersion system disperses the wavelengths of input radiation to form a dispersed beam that is focused on the intermediate slit. The intermediate slit blocks selected wavelengths and passes the remaining wavelengths of the dispersed beam to the second dispersion system. The second dispersion system combines the remaining wavelengths of the dispersed beam to form an output beam, which is focused on the exit port. In this manner, the radiance of the input radiation is preserved ignoring losses caused by the optical elements and the blocked wavelengths.
Dispersive filter 100 is illustrated as including two dispersion systems, sometimes referred to herein as input and output portions 110 and 120, with an intermediate slit 132 and field lens 134 between them. While dispersive filter 100 is shown with the input and output portions 110 and 120 symmetrically arranged, it should be understood that symmetry is not required in accordance with the present invention as long as the input and output portions 110 and 120 have similar dispersions characteristics as measured at the plane of the intermediate slit 132 and include pupil matching so that the exit pupil of the input portion is imaged (polychromatically) on the entrance pupil of the output portion.
The input portion 110 includes an entrance port 112, which may be an aperture, such as a slit as illustrated in
In the output portion 120 of the dispersive filter 100, a collimating optical element 124 collimates the dispersed beam 151′ after it has passed through the intermediate slit 132. The output portion 120 includes a wavelength dispersing element, such as prism 126 or a diffraction grating, that is configured and positioned so that the spectral characteristics of prism 126 and prism 116 are mirror image at the plane of the intermediate slit 132. The prism 126, thus, receives the dispersed beam 151′ from collimating optical element 124 and reverses the dispersion of the wavelengths to form an output beam 123. The resulting output beam 123 is focused by an optical element 128 on the exit port 122, which may be an aperture, such as a slit as illustrated in
By way of example, the optical elements 114, 118, 124, and 128 may be achromatic doublet lenses, e.g., such as that produced by produced by Edmund Optics as part number 32-323, and the prisms 116 and 126 produced by Edmund Optics as part number 47-284.
The intermediate slit 132 acts as an port and is configured to eliminate selected wavelengths from the dispersed beam 151, by blocking those wavelengths and passing the remaining wavelengths. In one embodiment, one or more actuators 133 may be coupled to the intermediate slit 132 to vary the width, e.g., from 20 mm to 500 mm, and/or position of the slit 132 so that the wavelengths may be selectively blocked or passed. By way of example, one actuator may alter the width of the slit 132 and another actuator may vary the position, or alternatively, one actuator may adjust one side of the slit and the other actuator may adjust the other side to vary the width and/or position of the slit. Thus, for example, only the wavelengths at one or both ends of the light spectrum may be selected to be filtered while the remaining light is passed. By way of example, 20% to 99% of the wavelengths in the dispersed beam 151 are passed by the intermediate slit 132 and emitted from the dispersive filter 100. The more wavelengths that are permitted to pass the intermediate slit 132, the greater the benefit of preserved radiance for the dispersive filter 100.
In general, the spectrum of the dispersed beam 151 will be dispersed in angle as well as position at the intermediate slit 132, which if not controlled will cause a loss of radiance for the dispersive filter 100. A field lens 134, e.g., in the form of an achromatic doublet lens such as that described above, at or near the intermediate slit 132 can be used to negate the dispersion in angle.
It should be understood that if desired the input portion and the output portion may be composed of the same optical components. A system of mirrors at the intermediate slit 132 may be used to reflect the dispersed beam back to the dispersive element, which recombines the wavelengths to form the output beam. The output beam is focused on an exit port that is separate from the entrance port. Mirrors appropriately positioned in the beam path may be used to separate the entrance port and the exit port, as will be understood by those skilled in the art.
The input portion 110 produces the dispersed beam 151, which has a plurality of wavelengths, only three of which are illustrated in
If white light were to enter the exit port 122 of the output portion 120, the output portion 120 would also produce a wavelength-dispersed beam 152 with the optical element 124 (and field lens 134) focusing the wavelengths 152R, 152G, and 152B on the plane of the intermediate slit 132, wherein 152R, 152G, and 152B have the same wavelengths as 151R, 151G, and 151B, respectively. As can be seen in
As can be seen in
It should be understood that
As illustrated in
Additional optical elements may be used if desired. For example, a field lens 234 may be positioned at or near the intermediate slit 232 to negate any dispersion in angle of the wavelength-dispersed beam 216. Additional optical elements may be used, e.g., for focusing or collimating the light within the input portion 210 or output portion 220.
Although the present invention is illustrated in connection with specific embodiments for instructional purposes, the present invention is not limited thereto. Various adaptations and modifications may be made without departing from the scope of the invention. Therefore, the spirit and scope of the appended claims should not be limited to the foregoing description.
