Improved Monochromatic Imaging Spectrometer

Abstract

Distortion inherent to a previously described system for acquisition of two-dimensional monochromatic spatial images is described. A solution is offered in the form of an improved instrument. The system uses a Czerny-Turner monochromator for spectral discrimination and a charge-coupled device (CCD) as the detector. A second Czerny-Turner monochromator, with identical dimensions, is added to correct the distortion, albeit with a slight loss in spatial resolution. With the earlier uncorrected arrangement, spatial resolution was on the order of 0.1 mm vertically and 0.3 mm horizontally, with a magnification of 0.52. With the same magnification, the new, corrected system offers spatial resolution of 0.1 mm vertically and 0.4 mm horizontally.

Local cooling, plasma reheating and thermal pinching induced by single aerosol droplets injected into an inductively coupled plasma

“…consequently, the final image becomes distorted horizontally to an extent dependent on the grating angle (i.e., wavelength setting). This effect is analogous to projecting a circular light beam perpendicularly onto a screen, which would cause the beam to appear narrower when viewed by an observer standing at an angle to the screen [31]. In the present study, the images were progressively narrowed as the spectrometer was set to longer wavelengths (i.e., when the angles of incidence and diffraction were increasingly dissimilar).…”

“…Distortion along the horizontal axis of the image is a typical artifact of the monochromatic imaging spectrometer. The cause has been detailed before [31], so only a brief explanation is given here. For different wavelength settings on a Czerny-Turner spectrometer, the angles of incidence (i.e., incoming) and diffraction (i.e., outgoing) beams on the grating are different;…”

“…In the present study, the images were progressively narrowed as the spectrometer was set to longer wavelengths (i.e., when the angles of incidence and diffraction were increasingly dissimilar). Methods to correct this distortion, either through hardware (e.g., dual spectrometer arrangements [31]) or software (e.g., computer algorithms [32] Even without any background correction, both OH molecular and atomic H emission from the vaporizing droplets are strong enough to be readily recognized from the raw ICCD images (cf. Figure 1).…”

“…For an ICP with an 18-mm diameter and an assumed temperature of 10,000 K, the time constants, which are the time required for the system to approach 1/e (i.e., 37%) of the asymptotic values, for thermal conduction [57] and ambipolar diffusion [58] are calculated to be 1.0 ms and 5.4 ms, respectively. 31 Therefore, it is reasonable to expect that, even after complete flushing of hydrogen from the plasma, it would still take a few milliseconds to completely restore the temperature equilibrium of the plasma.…”

Wavelength Scanning with a Tilting Interference Filter for Glow-Discharge Elemental Imaging

“…With the MIS, the variation in grating diffraction angle causes an increasing horizontal elongation of the image at longer wavelengths. 6,14,15 Although one can deal with this problem mathematically 14 or instrumentally, 15 no such geometric distortion exists with the tilting-filter spectrometer. A second distortion in the MIS arises from the slight difference in distance between the parabolic mirrors to the slits and to the grating, resulting in an out-of-focus image on the grating, which limits the achievable spatial resolution at the camera.…”