Flatness, in GD&T, is a measure of how closely a given surface is to a perfect 2D plane. That plane can be at any angle in 3D space and need not be a horizontal plane. Flatness is sometimes referred to as ‘planarity’.

Note that just because the surface may have a nominal angle on your CAD or drawings, the two planes need not be oriented at that exact angle, as long as they are parallel to one another. The below shows a 3D view of a surface that cannot be bounded between the two tolerance planes; this is a fail.

The largest InGaAs SWIR detectors application domain. These sensors are instrumental in industrial quality control and inspection processes. They can identify defects, contaminants, differences or inconsistencies in materials and products that may not be visible to the naked eye or traditional visible cameras. More materials used in today’s goods, although very similar in visible light, have very different signatures in SWIR, thus easy to discriminate or see through in that spectral band. For example, Silicon is transparent above 1250nm wavelength enabling see-through inspection in Photovoltaic or semiconductor manufacturing industries for crack or other defect detection or alignment. Similarly, plastics, organic material, metals or minerals have very different response in SWIR although very lookalike in visible light making SWIR imagers particularly effective in food sorting and waste recycling. In addition, most material radiate and emit photons at high temperature in the SWIR band. In steel and glass industries a great benefit can be achieved using SWIR solutions for hot process quality monitoring.

Short Wave Infraredsensor

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The flatness tolerance zone is visualised as two parallel planes, spaced apart by the number indicated in the flatness control. If all measured points on the surface lie within the tolerance band (i.e. between these two parallel planes) the part is a ‘pass’.

Short wave infraredcamera

Flatness is often described as being “the 2D version of straightness”. A straightness requirement applies to a 1D (linear) surface trace or axis in 3D space, whereas a flatness requirement applies to a 2D (planar) surface in 3D space. A flatness measurement can be approximated by a series of straightness line-measurements but note that the two will not be the same. Each straightness scan would be evaluated independently, and the resulting lines need not be co-planar with one another.

A flatness control is indicated by a feature control frame (FCF) with the symbol shown below. Note that flatness must be specified with a tolerance, which is the numerical value to the right of the symbol. A flatness control is stand-alone. It does not reference back to any other feature or datum on the part. The feature control frame does not include any other references.

Another alternative to using a flatness control is to use the generic profile of a surface constraint. Applied to a planar surface, this will achieve many of the constraints on surface form which a flatness callout can. However, it is less immediately accessible and understandable when reviewing an engineering print, so the more specific flatness control is a better choice where it applies.

SWIR detectors, like cooled LWIR or MWIR detectors, are photodetectors. However, they differ significantly in their operational principle. While LWIR and MWIR detectors detect emitted thermal radiation, SWIR detectors excel at capturing and interpreting reflected light in the shortwave infrared spectrum. The subset of the SWIR band covered by InGaAs based sensors and imagers typically spans from 900 to 1700 nm, making it ideal for various applications.

Above: each of these linear scans could report an individual ‘pass’ for straightness, but a flatness control with the same data combined could report a ‘fail’.

Short Wave infraredvs nearinfrared

Shortwave Infrared (SWIR) technology has emerged as a powerful imaging tool, offering unique capabilities that complement Longwave Infrared (LWIR) and Midwave Infrared (MWIR) systems. Unlike LWIR or MWIR detectors, which detect emitted thermal radiation, SWIR detectors primarily capture reflected light, making their imaging properties comparable to visible cameras or the human eye but in the invisible part of the spectrum.

Long-waveinfrared

Flatness can apply to any surface (or any area of a surface) that is nominally flat, whatever the orientation of that plane. A flatness requirement can apply to an entire surface, or just to a small sub-section of a face (this latter case can create some challenges for measurement with some traditional flatness gauging tools). It can also apply over several discontinuous faces, treating them as a single continuous plane (a ‘common zone’ tolerance) The images below show examples of surfaces to which a flatness control might apply.

Telops designs and manufactures high-performance hyperspectral imaging systems and infrared cameras for defense, industrial, and academic research applications.

Flatness can also be applied to certain specific ‘virtual’ features. For instance, where two flat surfaces are parallel but offset, the derived median plane between the two faces can be controlled with a flatness callout. The images below show this with the two lighter blue planes (actual part surfaces) forming the derived median plane (the ‘virtual’ plane shown in darker blue):

SWIR technology has proven invaluable in agriculture and crop monitoring. By assessing plant health and detecting diseases in vegetation, farmers can optimize crop yield and reduce the use of harmful pesticides.

SWIR sensor

SWIR wavelength range

A flatness callout with no other information should be interpreted to mean that the flatness control applies to the entire surface. Flatness may be expressed as a local rate over a larger area (e.g. a zonal flatness of ‘x’ microns per millimetre).

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Xenics is a designer and manufacturer of infrared sensors, cores and cameras that deliver unparalleled Electro-Optical performance and functionalities.

Note that you might also see flatness applied as a ‘per unit’ control, in which case an extra number will appear on the right. Think of this as a smaller, moving tolerance area which must be satisfied anywhere on the surface locally. This will normally be a tighter tolerance than the overall flatness on the surface.

In addition to InGaAs SWIR cameras, there are VisNIR (Visible to Near-Infrared) cameras that offer sensitivity in a broader wavelength range, spanning from 400 to 1700 nm. VisNIR cameras provide valuable information in both visible and near-infrared regions, making them versatile tools in various scientific and industrial applications.

Thermalinfraredwavelength

InGaAs SWIR detectors enhance surveillance and security systems by providing clear images even in low light or challenging environmental conditions.

SWIR imaging aids in the preservation and restoration of artwork and cultural artifacts. It can uncover hidden details, pigments, and features that may not be apparent with visible light imaging.

See how an optical CMM measures critical quality parameters on an artificial joint or create highly detailed wear maps for R&D.

In this article, we delve into the world of SWIR technology, focusing on the recent breakthroughs enabled by InGaAs material, which has facilitated the development of not-cryogenically cooled SWIR imagers. Other materials sensitive in SWIR band have emerged these last years.

Interested in fast and accurate measurement of precision components with an optical CMM? Try the OmniLux range of coordinate measuring machines.

Keep in mind that a flatness measurement is almost always a 2D measurement in 3D space (it is sometimes measured using a 1D curvilinear scanning path, although this gives limited coverage). As well as on 2D prints, you may also encounter flatness callouts directly in a 3D CAD model.

Short Wave infraredwavelength

Recent advancements in SWIR technology have been driven by the development of Indium Gallium Arsenide (InGaAs) material. This cutting-edge material has revolutionized not-cryogenically cooled SWIR imagers, significantly improving their performance and affordability. InGaAs SWIR detectors, in particular, demonstrate exceptional sensitivity in the SWIR band from 900 to 1700 nm, capturing critical information within this range.

In the medical field, SWIR technology assists in non-invasive imaging techniques, such as fluorescence imaging, Optical Coherence Tomography (OCT) and vein visualization, enabling better diagnostics and treatment planning.

For more details, see ISO 12781, and ASME 14.5-2018, Section 8. Be aware also of the tolerancing classes for straightness and flatness specified by ISO 2768.

The applications of InGaAs SWIR detectors span across various sectors, offering a myriad of possibilities for industries and researchers. Some key areas where these detectors find extensive use include the following. The benefits provided by SWIR technology in these application segments translate into clear enhancements on today’s concerns: production lines throughput and yield increase, Green-House effect gases emission reduction, energy saving, food quality and safety, waste recycling and reuse and many more.

One fascinating aspect of SWIR technology is its similarity to visible imaging. Just as visible cameras and the human eye observe the world using reflected light, SWIR images resemble black and white visible images. This unique characteristic bridges the gap between the invisible infrared world and the familiar visible spectrum, allowing users to interpret SWIR images with ease. SWIR's impressive resolution and detail make it valuable in a wide range of industries and applications.

To maintain optimal operating temperatures, InGaAs SWIR detectors often incorporate Peltier coolers. This cooling mechanism ensures high-quality imaging without the need for cryogenic cooling, making SWIR technology more practical and cost-effective for various applications. Peltier cooling has played a pivotal role in expanding the accessibility of SWIR technology across different industries.

Use the OmniLux family of optical CMMs to measure GD&T parameters faster, more accurately and without contacting your precision engineering components.