Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Reexamination Certificate
2001-03-02
2003-07-01
Smith, Zandra V. (Department: 2877)
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
Reexamination Certificate
active
06587198
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
REFERENCE TO A MICROFICHE APPENDIX
Not Applicable
BACKGROUND OF THE INVENTION
Grating spectrometers broadly define the optical art to which the present invention relates, particularly compact spectrometers adapted for examining visually the spectrum of the sun. High dispersion, that is, the ability of a spectrometer to show closely-spaced detail as separate, is only advantageous visually if the image of the spectrum conveyed to the eye is bright enough for the eye to perceive the detail. Sunlight can be so highly dispersed and, subsequently, magnified that it appears faint, the fine detail murky. The present invention extends the eye's dynamic range by ensuring that high, spectral resolution remains matched to good, apparent contrast beyond the usual limits of the dynamic range.
The sun's visible spectrum may be roughly differentiated into the spectrum of the photosphere and the spectra, quite different, of sunspots. This difference may, in principle, be seen whenever the image of a sunspot is projected onto the entrance slit of a high-dispersion spectrometer, provided only that the sunspot is wider than the slit. It is best seen, however, when the image of the sunspot is made as large as possible relative to the entrance slit's length, provided only that good contrast is maintained.
In sum, then, visually to exploit high dispersion, as well as to distinguish sunspot spectra, the spectrometer's light-input device must, of necessity, be a telescope. Telescopic means for intensifying and imaging light, however, particularly if movably mounted and modestly sized, do not couple easily to laboratory, bench spectrometers, which may weigh several kilograms. The present invention, by comparison, is light in weight and very small. Nonetheless, it discloses detail of haunting subtlety throughout the visual range. Its resolution closely approaches the practical limit of its diffracting grating. Solar absorption lines having peaks spaced apart by ≈0.31 Å (0.031 nm) are easily split. For a spectrometer that fits comfortably into the palm of a hand, that is rather good.
The perception of faint, solar absorption lines is significantly enhanced by the elimination of glare, that is, by avoiding halation of the retina. If brightness is optimized for those wavelengths at which the eye is most sensitive, then the eye's light-adaptive ability will automatically compensate for reduced brightness throughout the remainder of the eye's nearly 260 nm, dynamic range. Shown in
FIG. 1
(see Warren J. Smith,
Modern Optical Engineering, ©
1990 McGraw Hill), the human eye's photopic (color) sensitivity peaks somewhere between 550 nm and 560 nm, before falling off precipitously below about 435 nm and above about 695 nm (where sensitivity drops to approximately 1% of peak sensitivity), depending on the individual. These “fall-off” wavelengths, however, are significant for a solar-related reason, too: they are where many of the most important features of the sun's spectrum, “must-see” items on any first, educational tour, are to be found. The broadest of all the solar, absorption lines, the resonance, K, line of singly ionized calcium (CaII), lies at 393.4 nm, followed closely by the H line of CaII at 396.8 nm. The most important solar line of all, certainly historically, is H-alpha, written H&agr;, the first line in the Balmer series of hydrogen, at 656.3 nm. Redward of H&agr; are the stunning, telluric lines of O
2
. Their band head, Fraunhofer's B line, lies at 686.7 nm.
The human eye's greatly diminished sensitivity below 400 nm must be compensated, if the H and K lines are to be observed distinctly. If brightness has already been adjusted so that the continuum is free of glare at 560 nm, then below 410 nm the continuum will simply have become too faint for the human visual system dynamically to compensate contrast on its own. Depending on the time of day (i.e. on solar altitude) and on the humidity (terrestrial water vapor absorption), the H and K lines may either not be seen at all, or they may appear as black phantoms against an only-very-faintly luminous, deep-violet background. Typically, to compensate, a spectrometer's slit will be widened, that is, a tradeoff is made for increased brightness at the expense of diminished resolution. However, a ten-fold increase in perceived intensity requires a roughly ten-fold increase in slit width, and with this widening many fine lines and features near, between, and in the H and K lines are lost to view. The lines' extraordinary, natural broadening appears narrower than it actually is, as the lines' albatross-wide, feather-fine wings get merged evermore coarsely into the expediently-brightened continuum. Then, too, the necessary, adjustable slit, will be expensive, especially if it is to be capable of reliably repeating widths <10&mgr; (1 micron=10
−6
m≈0.00004″) while maintaining slit-jaw parallelism.
The present invention eliminates all of these disadvantages.
The present invention, especially in its exemplary embodiment, combines high-resolution with low-cost, small size, and low weight, and in such a manner that any notable improvement in contrast and/or in resolution will require a disproportionate increase in expense and/or in bulk. The present invention's single, spherical mirror could, for example, be replaced by independently mounted, but far-more costly, toroidal mirrors. The present invention is thus intended to satisfy an unmet, instrumental need among educators, and to supply an IR/UV-shielded, solar spectrometer to the high-end, amateur, astronomical community.
BRIEF SUMMARY OF THE INVENTION
Small, grating monochromators, off-the-shelf units built, for example, by Acton Research or Optometrics, usually are rather fast, commonly ≦f/4, which allows them to accommodate, with appropriate, internal baffling, light input from fiber-optic light guides, which ordinarily have high, numerical apertures (0.2 to 0.5) as well as fiber diameters of around 200 microns. U.S. Pat. No. 5,231,461 to Silvergate et al. (1993) shows a collimating mirror illuminated by sunlight input through a fiber-optic “slit”.
Numerical aperture, we recall, for an imaging mirror or conventional lens is just one half the reciprocal of its paraxial focal length. Results discussed below first in terms of f/ratio will frequently be converted for ease of reference into the equivalent, numerical-aperture (n.a.) formulation.
In contrast to the typical, small monochromator, a telescope will usually have an f/ratio substantially higher than f/4, say f/8. Such a telescope, if used as the light-input device for an f/4 monochromator, will not illuminate fully the monochromator's grating, that is, the telescope's ray cone will be excessively narrow, and so the telescope will fail fully to exploit the grating's resolution.
In the solar case, where the desire is understandably great to project onto the spectrometer's entrance slit as large an image of the sun as possible, in order better to isolate sunspot spectra, the apparent mismatch of high-f/ratio telescope as small-monochromator, light-input device is only aggravated. The larger the desired solar image, the greater must be the telescope's effective focal length. Given the high cost of large-diameter optics, the greater the effective focal length, the higher will be, as a practical matter, the f/ratio.
A monochromator, we note, can easily be turned into a visual spectrometer by first removing the exit-slit and by then installing magnifying optics with which to view the imaged spectrum, normally hidden behind the exit-slit assembly. The one, obvious exception, of course, is the true Littrow mount.
There is yet another reason, why, for a high-dispersion spectrometer in which the image of the spectrum is magnified for viewing, i.e. in which the input light is very-greatly spread out
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