Spectral resolution is inversely proportional to the linear dispersion of a monochromator. The resolution requirement of the experiment is often the key performance requirement of any application. For narrow structure analysis (resolution better than 0.1 nm in the visible range), large monochromators are the best choice because they offer increased spectral dispersion and thus a higher spectral resolution.

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Most of the time, small monochromators and spectrographs have better throughput than large ones because of their larger numerical apertures (f numbers) and simpler design (often with fewer optical components).

Because of their slit/slit configuration, monochromators have less stray light or reentrant light than do spectrographs, which have no exit slit. However, when the stray light is important in an application, large-focal-length instruments, or double monochromators, are the best choice. Small devices present more risk of stray light than larger ones.

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The dispersion shows the capability to disperse light. It gives the usable bandpass of a monochromator or indicates the spectral range of a spectrograph equipped with a multichannel array detector such as a CCD camera or InGaAs array. Changing the width of the slit aperture can adjust the bandpass.

Because of field aberrations (such as field curvature), the value of the limiting resolution and acutance depends on the field position.

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Stray light relates mainly to the quality of the device’s optical components (mirrors and grating). The user is generally not aware of stray light or improper internal reflection, which can produce poor results.

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Laboratory researchers who don’t need high resolution and wide range at the same time, however, should choose larger spectrographs. These users can change gratings (i.e., to a higher or lower groove density) or acquire multiple spectra to achieve complete results.

The resolution of a camera is defined by the size of the smallest detail that the camera can optically discriminate. ISO standard 12233 defines the limiting resolution as the first frequency such that the contrast attenuation is 95% (hence a MTF value of 5%). In practice, this value is very small and is also sensitive to measurement noise

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DSLR lenses usually have a radial symmetry, which means that field dependency can be summarized by the MTF at different distances to the image center.

The MTF precisely describes the contrast attenuation: for each spatial frequency, the MTF plots the ratio of the output modulation versus the test object modulation.

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We map the acutance on a color scale as a function of focal length and aperture value. We make measurements for several field positions.

Select the camera model you are using. Type in the horizontal and vertical resolution. Select a measurement unit. Then, enter the distance from the subject and ...

Producing a larger image, especially across a large spectral range, is very difficult because of the spatial corrections required.

The value of the MTF at frequency 0 is always 100% since a flat field is considered to have been reproduced perfectly, with no intensity loss. Attenuation due to lens transmission is measured separately.

Well-corrected, fixed, compact spectrographs can provide excellent image quality. Some can discrimi-nate three or four spectral channels over a 6-mm-tall focal plane. Interesting axial spectrograph con-figurations using prisms and lenses also offer excellent image quality over a few hundred nanometers in the visible range.

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If you need to analyze a short spectral range at low resolution, you can probably use an inexpensive, compact monochromator or spectrograph. Even if these devices have stray light, chemometric calibration methods can correct it without influencing the results. However, if you need high resolution, accuracy or versatility, large monochromators and spectrographs are often the safest buy. That is why these are generally the best instruments for research or hightechnology industries.

If the application’s most important requirement is to acquire a large spectral range in one shot, small spectrographs are better, as in a process application. In fact, single timonochromators with a focal length below 0.3 m are suitable where most stray light is not considered a major problem.

We plot the acutance as a function of the radial field position, taking into consideration horizontal and vertical directions. We make measurements for several focal lengths and aperture values.

In terms of optical design, most large monochromators/ spectrographs use the asymmetric Czerny-Turner configuration. Smaller instruments tend to use an asymmetric “V” con-figuration as a compromise.

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Sharpness is a subjective quality attribute of an image or a lens. Sharpness indicates the visually perceived quality of details of an image or details reproduced by a lens. It is associated with both resolution and contrast of reproduced details (within an image or by a lens).

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For the DxOMark sharpness score, we compute MTF in horizontal and vertical directions by examining the spatial response of the camera to a perfect edge. This measurement follows ISO standard 12233.

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An objective measure of sharpness which takes into account the sensitivity of the human visual system to specific spatial frequencies and the viewing distance of an image. Edge acutance refers to the ability of a photographic system to show a sharp edge between contiguous areas of low and high illuminance. Texture acutance refers to the ability of a photographic system to show details without noticeable degradations.

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Note: f/value is not always the controlling factor of throughput. For example, light may be collected from a source at f/1 and projected onto the entrance slit of an f/6 monochromator so that the entire image is contained within the slit. Then the system will operate on the basis of the photon collection in the f/l cone and not the f/6 cone of the monochromator.

Sharpness is based on the MTF measurement. Let’s start with a practical measurement experiment to help describe what MTF is. If we look at a series of black and white stripes with varying spatial frequency and fixed contrast (same reflectance ratio between black and white, independent of the spatial frequency), we can see that the image of high-frequency stripes has a lower contrast than that for low-frequency stripes. For very high frequencies, the image eventually becomes (almost) uniform, with an intermediate level between black and white, which our eyes perceive as a shade of gray.

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The DxOMark score for Sharpness is based on the Perceptual Megapixel (P-Mpix) concept that weights the Modulation Transfer Function (MTF) of the lens with the human visual acuity. Read more about Perceptual Megapixels.

Monochromators work by scanning through the spectral features of the optical signals, step by step. As a result, the measurement process is generally slower than that of spectrographs with multichannel array detectors that operate in a fixed grating position and directly acquire a full spectrum according to their dispend mainly on the grating motor drive. Instruments with 0.5-m or higher focal length are usually equipped with sine bar mechanisms that give excellent accuracy (better than 0.1 nm) and repeatability (better than 0.01 nm). But the trade-off is speed: It can take several minutes to scan a large spectral range with high spectral resolution in monochromator mode. Smaller devices commonly use direct, or worm drives, because their resolution specifications are lower. In this case, they set the grating position within a few seconds.

A more useful figure of merit for comparing monochromators for imaging applications is light-gathering capability. Manufacturers of small instruments usually find that to preserve spectral resolution, they can produce only a small image.

However, the numerical aperture is not the final consideration in optical throughput. The linear dispersion is also important because it defines the input aperture sizes for a particular spectral resolution.

The commercial introduction of imaging spectrographs has partially corrected the spatial imaging quality issues. These instruments use toroidal gratings or toroidal mirrors to correct for astigmatism in the image plane and to improve image quality, while keeping the numerical aperture at the same level as non-imaging devices. This correction requires complex calculations. The choice of the toroid, the optical incident angles of the device and wavelength optimizations shows the expertise of the manufacturer.

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No one device can cover all spectroscopic applications. However, a user who carefully analyzes the spectral and performance requirements of an application can weigh the tradeoffs involved in choosing between small- and large-focal-length monochromators and spectrographs.

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