The translation is a bit awkward, but it first discusses that it is not necessary to detect all flaws, but only those from a defined size (i.e. Indeed, Flaw Detectability is discussed as the last topic on day 1. John, thanks for the memory jog! I have an old copy of some course notes given at the Krautkramer school in Koln (circa 1960s.just before my time in NDT). is it 1 or 2 things in the "spot" or in visual terms, is the spot the letter "c" or "e"). ![]() Sometimes we just can not separate the details (e.g. With enough visual magnification we can see things not normally seen. I think Rick's comments on the different acuity options relate to the issue. Re: Half wavelength limit In Reply to John Brunk at 17:39 Nov-17-2012. Some wikipedia info related to the Abbe limit is here: There are likely UT-specific versions of these related, but somewhat independent classes.Ī neat demonstration of vernier versus letter recognition (a 5-10 times difference is expected) can be found here: 1951 USAF resolution target), acuity based on recognition of the relative positions of vernier lines (hyperacuity), and acuity based on recognizing letters or reading text (e.g. Also, I believe that there are four types of visual system acuity: the smallest feature detectable (visible minimum), the smallest separation between two lines (e.g. Wouldn't optical microscopy also be working with a bandwidth - although a much narrower one? Visible light would be all wavelengths between 380 and 780 nm. This has been an interesting discussion (including the chain of comments that you refer to). Re: Half wavelength limit In Reply to Ed Ginzel at 15:29 Nov-17-2012 (Opening). ![]() R & D, John Deere - Tech Works Ames, USA, Joined Jul 2011, Is the rule of thumb given for UT detection limit actually a misinterpretation of the resolution limit? But the limit is more of a resolution limit not a detection limit. Due to bandwidth this would be closer to the nominal wavelength. The equivalent limit in ultrasonics would be the smallest focal spot we can make. But in acoustics we are dealing with bandwidth, not monochromatic photons. In optics the Abbe limit is roughly d=λ/2. Light with wavelength λ, traveling in a medium with refractive index n and converging to a spot with angle theta will make a spot with radius d=λ/(2nsin(theta)). There is something called the Abby limit where observation of sub-wavelength structures with microscopes is difficult due to the Abbe diffraction limit. I have recently had a need to deal with optical limits of resolution. But perhaps the origins are not NDT related. I speculated that this was approximately the point that Krautkramer notes that reflection changes to diffraction. ![]() But I have not actually read it in any NDT reference book. Many of us (including myself) have heard this "rule of thumb" quoted. In a recent post, Massimo Carminati raised the point of the half wavelength detection limit.
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