A clear example of this, and one of the stars of the Cognex booth, is the In-Sight SnAPP Vision Sensor, a vision-based sensor  that’s a major  step up from conventional laser-based sensors, providing superior capability to locate features and parts in any position and to detect subtle defects. It helps solve a range of inspection applications with flexible capabilities, including presence/absence, defect detection and most recently, counting. With its intuitive setup, users can easily deploy the sensor without extensive technical expertise. This ease of use reduces the time and cost associated with implementation, making it an attractive option for companies looking to enhance their quality control processes quickly.

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The efficiency also decreases the farther away from Littrow conditions in which the grating is used (e.g. α ≠ β). Holographic gratings may be designed with groove profiles that discriminate against high orders. This may be particularly effective in the UV and VIS using laminar groove profiles created by ion etching.

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If, for example, a light source emits a continuum of wavelengths from 20 nm to 1000 nm, then at the physical location of 800 nm in first order (Fig. 4) wavelengths of 400, 266.6, and 200 nm will also be present and available to the same detector. In order to monitor only light at 800 nm, filters must be used to eliminate the higher orders.

Efficiency in higher orders usually follows the first order efficiency curve. For a grating blazed in first order, the maximum efficiency for each of the subsequent higher orders decreases as the order k increases.

Light other than the wavelength of interest reaching a detector (often including one or more elements of “scattered light”) is referred to as stray light.

Relative efficiency measurements require the mirror to be coated with the same material and used in the same angular configuration as the grating.

λ = the central wavelength of the spectral line to be resolved Wg= the illuminated width of the grating N = the total number of grooves on the grating

Table 1 shows how α and β vary depending on the deviation angle for a 1200 g/mm grating set to diffract 500 nm in a monochromator geometry, based on Fig. 2.

A grating blazed in first order is equally blazed in the higher orders. Therefore, a grating blazed at 600 nm in first order is also blazed at 300 nm in second order, and so on.

As a general approximation, for blazed gratings the strength of a signal is reduced by 50% at two-thirds the blaze wavelength, and 1.8 times the blaze wavelength.

Before introducing the basic equations, a brief explanation on monochromatic light and continuous spectra must first be presented.

Remember, ghosts and subsequent stray light intensity are proportional to the square of order and groove density (n2 and k2 from Equation (17). Beware of using ruled gratings in high order or with high groove density.

LA = Entrance arm length LB = Exit arm length at λn βH = Angle between the perpendicular to the spectral plane and the grating normal LH = Perpendicular distance from the spectral plane to grating.

In most monochromators, the location of the entrance and exit slits are fixed, and the grating rotates around a plane through the center of its face. The angle, DV, is, therefore, a constant.

In most monochromators, the location of the entrance and exit slits are fixed, and the grating rotates around a plane through the center of its face. The angle, DV, is, therefore, a constant determined by:

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A blazed grating is one in which the grooves of the diffraction grating are controlled to form right triangles with a “blaze angle, ω,” as shown in Fig. 5. However, apex angles up to 110° may be present especially in blazed holographic gratings. The selection of the peak angle of the triangular groove offers opportunity to optimize the overall efficiency profile of the grating.

Note: Just because a grating is “non-blazed” does not necessarily mean that it is less efficient! See Fig. 7 showing the efficiency curve for an 1800 g/mm sinusoidal grooved holographic grating.

Ghosts are focused and imaged in the dispersion plane of the monochromator.Stray light of a holographic grating is usually up to a factor of ten times less than that of a classically ruled grating, is typically non-focused, and when present, radiates through 2π steradians.

In a monochromator, LB is the arm length from the focusing mirror to the exit slit, or if the grating is concave, from the grating to the exit slit. Linear dispersion, therefore, varies directly with cos β, and inversely with the exit path length, LB, diffraction order (k), and groove density, n.

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Unless otherwise indicated, the efficiency of a diffraction grating is measured in the Littrow configuration at a given wavelength.

Monochromatic light has infinitely narrow spectral width. Good sources which approximate such light include single mode lasers and very low pressure, cooled spectral calibration lamps. These are also variously known as “line” or “discrete line” sources.

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The other star of the booth, the In-Sight L38 3D Vision System, is designed to tackle increasing demand for high-precision inspection and measurement in manufacturing. This system leverages advanced 3D imaging technology to deliver highly accurate, real-time for applications that fall outside the range of what 2D visions systems are designed for. The L38 system’s ability to capture detailed 3D data enables it to detect minute defects and variations in product dimensions, ensuring optimal quality control.

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It is easy to imagine that fine spectral detail would be more easily identified in the first instrument than the second. The second instrument demonstrates “low” dispersion compared to the “higher” dispersion of the first. Linear dispersion is associated with an instrument’s ability to resolve fine spectral detail.

where, dλ is the difference in wavelength between two spectral lines of equal intensity. Resolution is then the ability of the instrument to separate adjacent spectral lines. Two peaks are considered resolved if the distance between them is such that the maximum of one falls on the first minimum of the other. This is called the Rayleigh criterion.

First order wavelengths between 200 and 380 nm may be monitored without filters because wavelengths below 190 nm are absorbed by air. If, however, the instrument is evacuated or N2 purged, higher order filters would again be required.

Theoretically, a 1200 g/mm grating with a width of 110 mm that is used in first order has a numerical resolving power R = 1200 × 110 = 132,000. Therefore, at 500 nm, the bandpass is equal to:

Holographic gratings show no ghosts because there are no periodic ruling errors and, therefore, often represent the best solution to ghost problems.

Blazed grating groove profiles are calculated for the Littrow condition where the incident and diffracted rays are in auto collimation.

In a spectrograph, the linear dispersion for any wavelength other than the wavelength which is normal to the spectral plane will be modified by the cosine of the angle of inclination or tilt angle (γ) at wavelength λn. Fig. 3 shows a “flat field” spectrograph as used with a linear diode array.

Blaze is defined as the concentration of a limited region of the spectrum into any order other than the zero order. Blazed gratings are manufactured to produce maximum efficiency at designated wavelengths. A grating may, therefore, be described as “blazed at 250 nm” or “blazed at 1 micron,” etc. by appropriate selection of groove geometry.

Table 1 shows how the angle of incidence and angle of diffraction vary depending on the deviation angle for a 1200 g/mm grating set to diffract 500 nm in a monochromator geometry.

Additionally, the In-Sight SnAPP sensor is designed with robustness in mind, capable of operating in harsh industrial environments. This resilience, combined with its advanced vision capabilities, positions the In-Sight SnAPP as a critical tool for manufacturers aiming to achieve higher levels of accuracy and efficiency in their operations than what’s possible with conventional laser-based sensors

Holographic gratings show no ghosts because there are no periodic ruling errors and, therefore, often represent the best solution to ghost problems.

Diffraction gratings are manufactured either classically with the use of a ruling engine by burnishing grooves with a diamond stylus, or holographically with the use of interference fringes generated at the intersection of two laser beams1.

With their reputation for machine vision and barcode scanning technologies, Cognex boasted an impressive showcase of scanners, machine vision systems and vision sensors at their 2024 Automate booth in Chicago.

Classically ruled gratings may be planar or concave and possess grooves, each parallel with the next. Holographic grating grooves may be either parallel or of unequal distribution in order to optimize system performance. Holographic gratings are generated on planar, spherical, toroidal, and many other surfaces.

As a whole, the manufacturing industry has begun embracing artificial intelligence (AI) as a means to increase automation and lessen labor shortage pain points. Cognex specifically believes that AI will help companies simplify their automation with new vision systems and sensing technologies.

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Regardless of the shape of the surface or whether classically ruled or holographic, the text that follows is applicable to each; explanations are provided where there are differences.

See Fig. 6 and Fig. 7 for typical efficiency curves of a blazed, ruled grating, and a non-blazed, holographic grating, respectively.

Fig. 4 shows a first order spectrum from 200 to 1000 nm spread over a focal field in spectrograph configuration. From Equation (1) with a grating of given groove density and for a given value of α and β:

Diffraction gratings are manufactured either classically with the use of a ruling engine, or holographically with the use of interference fringes generated at the intersection of two laser beams.

These solutions demonstrate the company’s commitment to using AI to help end users and manufacturers. Their deep learning and edge learning offerings can take difficult and complex systems and make it accessible and usable on a wide scale. As the manufacturing industry faces labor and skill shortages, AI and automation solutions like this are increasingly necessary and popular.

If the diffraction grating has periodic ruling errors, a ghost, which is not scattered light, will be focused in the dispersion plane. Ghost intensity is given by:

One of the L38’s standout features is its high-speed processing capabilities, allowing for rapid inspection without compromising accuracy. This particularly benefits industries where maintaining high production speeds is crucial, such as automotive, electronics and consumer goods manufacturing. The L38 system also boasts an easy-to-use interface, eliminating set-up barriers. This combination of speed, precision and usability makes the In-Sight L38 a valuable addition to any manufacturing process and enhancing overall productivity and product quality.

Blazed grating groove profiles are calculated for the Littrow condition where the incident and diffracted rays are in auto collimation (i.e., α = β). The input and output rays, therefore, propagate along the same axis. In this case, at the “blaze” wavelength λB.

Because bandpass is also determined by the slit width of the spectrometer and residual system aberrations, an achieved bandpass at this level is only possible in diffraction limited instruments, assuming an unlikely 100% of theoretical.

A continuous spectrum has finite spectral width, e.g. “white light.” In principle, all wavelengths are present, but in practice a “continuum” is almost always a segment of a spectrum. Sometimes a continuous spectral segment may be only a few parts of a nanometer wide and resemble a line spectrum.

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Cognex continues solidifying its position as a leader in the automation industry with its innovative vision systems such as In-Sight SnAPP and In-Sight L38. By integrating advanced AI capabilities and user-friendly interfaces, these technologies address the critical needs of modern manufacturing, from enhancing quality control to optimizing production speeds. Cognex’s cutting-edge solutions offer a glimpse into the future of smart manufacturing.

Linear dispersion defines the extent to which a spectral interval is spread out across the focal field of a spectrometer and is expressed in nm/mm, Å/mm, cm-1/mm, etc. For example, consider two spectrometers: one instrument disperses a 0.1 nm spectral segment over 1 mm while the other takes a 10 nm spectral segment and spreads it over 1 mm.

Diffraction gratings are manufactured either classically with the use of a ruling engine, or holographically with the use of interference fringes generated at the intersection of two laser beams.