The slot is an opening that controls the amount of light that enters the spectrometer. The width of the slot affects the resolution; the narrower the slot, the higher the resolution. However, narrower slots also decrease signal strength. These two factors must be balanced when selecting the size of a slot.
The main function of the input slot is to control the resolution of the spectrometer. This function has two components: restricting the angle of light entering the train of optical components within the spectrometer and controlling the width of the light beam that hits the DMD. The purpose of the input slot is to define a sharp object for the optical bench. The size (width (Ws) and height (Hs)) of the input slot is one of the main factors affecting spectrograph performance.
The image width of the input slot is a key factor in determining the spectral resolution of the spectrometer when it is greater than the pixel width of the detector array. Both system performance and resolution must be balanced by selecting an appropriate input slot width. With different wavelengths now scattered around the outlet slot, the width of the slots controls the bandwidth of monochromatic light. By turning the grille slightly, different wavelengths will pass through the narrow slots.
With that background, we can now discuss your second point. As the width of the slot decreases, the slot increasingly blocks light traveling at a large angle to the optical axis (the central axis of symmetry of the optical processing system). The edges of the slot are machined with great care so that the distance between the two edges through which light passes is equidistant along the opening of the slot. This increases the importance of choosing the slot width, since it will be difficult and expensive to install a new slot once the spectrometer has been assembled.
This spreadsheet demonstrates the spectral distribution of the slit function, transmission and measured light for a simulated dispersive absorption spectrophotometer with a continuous light source, adjustable wavelength, mechanical slot width, reciprocal linear dispersion, spectral bandpass, absorber, average width spectral, concentration, path length and unabsorbed scattered light. However, the signal-to-noise ratio decreases as the slot width decreases, so it is not always practical to use the smallest possible slot width. In such applications, the high performance of the larger slot is more useful than the improved resolution of the narrower slot. For example, you can specify a narrow slot for high resolution for sharp peak absorbance measurements, but then switch to a wider slot for high performance in fluorescence and low light measurements.
Interchangeable slots allow users to adjust the width of the spectrometer slot on the fly and more easily manage measurement flexibility. With a smaller slot size, the integration time required to measure the fluorescent molecule increases rapidly because the light that can pass through the slot has been reduced considerably. The concave mirror E then focuses the light on the outlet slot F, forming a spectrum across the outlet slot. Some simple instruments, for example the common Spectronic 20, have fixed slot widths, but most research-grade instruments have user-controllable slot widths.
There is a geometric mismatch between the slot and the outlet of the lens, especially in the case of thinner slits, which are very narrow rectangles. With the interchangeable slit design of Ocean Insight spectrometers, users can adjust the optical resolution and performance of the spectrometer simply by changing the interchangeable slots. This simulation, Effect of slot width on the signal-to-noise ratio in absorption spectroscopy, shows how these opposing factors combine: decreasing the observed absorbance and the linearity of the analytical curve versus increasing light intensity and the signal-to-noise ratio as the slot width increases. .
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