{"id":9862,"date":"2018-05-15T12:00:30","date_gmt":"2018-05-15T12:00:30","guid":{"rendered":"https:\/\/www.thermofisher.com\/blog\/metals\/?p=9862"},"modified":"2026-07-30T20:58:11","modified_gmt":"2026-07-30T20:58:11","slug":"comparison-of-direct-contact-optical-and-x-ray-metal-thickness-gauges","status":"publish","type":"post","link":"https:\/\/www.thermofisher.com\/blog\/metals\/comparison-of-direct-contact-optical-and-x-ray-metal-thickness-gauges\/","title":{"rendered":"Comparison of Direct Contact, Optical, and  X-Ray Metal Thickness Gauges"},"content":{"rendered":"<p><span><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/www.thermofisher.com\/blog\/wp-content\/uploads\/sites\/4\/2018\/05\/istock-518728421_rolledsteel.jpg\" style=\"float: right;border: 0;margin: 10px\" alt=\"rolled steel\" title=\"rolled steel\" width=\"450\" height=\"299\" class=\"alignright\" \/>As we learned in previous articles, <\/span><a href=\"https:\/\/www.thermofisher.com\/us\/en\/home\/industrial\/manufacturing-processing\/measurement-control-systems\/metals-coating-weight-thickness-measurement-gauges.html?icid=CAD_blog_metals_2018May\"><span>metals coating weight and thickness measurement gauges<\/span><\/a><span> provide precise, real-time measurements that help meet specifications of various applications while maximizing raw material use. <\/span><a href=\"https:\/\/www.thermofisher.com\/order\/catalog\/product\/10144030?icid=CAD_blog_metals_2018May\"><span>Thickness gauges<\/span><\/a><span> are primarily used to look for dramatic deviations in thickness &ndash; which could affect the mechanical properties of the finished product.<\/span><\/p>\n<p><span>There are different types of thickness gauge technologies suitable for the harsh conditions of sheet metal mills.<\/span><\/p>\n<p><span><a href=\"http:\/\/bit.ly\/2q5kdrh\" target=\"_blank\" rel=\"noopener\">Direct contact gauges<\/a> have the advantage of being insensitive to alloy<span>, however<\/span>, the measurement stylus marks the strip, and the mechanical tolerances of the frame prevent measurement near the centerline of the strip. Additionally, the small measurement spot size of the stylus translates microvariations in the strip surface into a noisy signal. While these variations may actually be in the strip, the signal needs to be filtered to reduce the noise, and the filtering will delay responses to actual longer term changes. Therefore, high speed Automatic Gauge Control (AGC) is not practical with contact gauges.<\/span><\/p>\n<p><a href=\"https:\/\/www.thermofisher.com\/order\/catalog\/product\/LASERTX?icid=CAD_blog_metals_2018May\"><span>Optical laser gauges<\/span><\/a><span> that employ triangulation for distance measurements are available, but they have significant drawbacks in the cold mill. First, they have a relatively narrow gap between the top and bottom frame arms which can turn this non-contact gauge into a contact gauge during strip tail out. Additionally, like the contact gauge, the sensor frame is designed with a short arm length to limit the effect of thermal expansion on the measurement. This in turn limits the allowable strip width, or restricts the measurement area to a few centimeters from the edge. Laser gauges can also be sensitive to the high amount of steam and mist that occurs in a rolling mill. Additionally the laser camera technology limits the resolution of measurement to a few microns. While this is acceptable in process line applications, it does not meet the needs of the high-speed cold rolling mills.<\/span><\/p>\n<p><span>Non-contact <\/span><a href=\"https:\/\/www.thermofisher.com\/order\/catalog\/product\/10144030?icid=CAD_blog_metals_2018May\"><span>radiation based thickness gauges<\/span><\/a><span> can use either radioisotope or X-ray sources. However in the case where the gauge measurement is to be used in a closed loop control or AGC system, there is really only one solution, X-rays. The number of photons emitted from an X-ray source is approximately 1000 times that of the commercially available isotopes. Due to the statistical nature of radiation detection, measurements made with more photons per unit time have a better signal to noise ratio, and consequently a more true measurement. In the case of X-ray versus isotope, the noise level for an isotope is on the order of 20 to 30 times worse than that of an X-ray based sensor when the same averaging time is used. Statistical noise at that level creates a situation where small changes in thickness are lost in the noise of the signal. While higher activity or multiple isotope pellets might be used in an effort to increase the signal, the regulatory and safety considerations make this option prohibitive. Another approach to improve the noise on isotope based systems is to increase the averaging, or response time. However, when this is done, small, and instantaneous changes in product thickness are blurred to the point of not being seen. (See illustration below.)<\/span><\/p>\n<p><span>&nbsp;<img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/www.thermofisher.com\/blog\/wp-content\/uploads\/sites\/4\/2018\/05\/thicknessgaugesensorresponse.jpg\" style=\"vertical-align: middle;border: 0px;margin: 10px auto\" alt=\"metal gauging sensor chart\" title=\"metal gauging sensor chart\" width=\"700\" height=\"327\" \/><\/span><\/p>\n<p style=\"padding-left: 30px\"><em><span>Image above: Sensor response as a function of steel thickness for different photon energies<\/span><\/em><\/p>\n<p>Just as low signal to noise ratio is a serious factor in source selection, one must take care to select a source of a proper energy as to not have too much signal. While there may be a small spares inventory savings to use a single source type across a number of rolling mills, there are serious performance drawbacks. If an X-ray gauge is operated at too high of an energy, the dynamic range of the detector output is reduced, limiting the measurement resolution and precision. In the case of thin strip production at around 250 microns, a 10 micron change in thickness results in a signal change of less than 0.2% at a photon energy of 150 keV, whereas the same thickness change at a photon energy of 40 keV will produce a signal change of over 7%.<\/p>\n<p><em><span><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/www.thermofisher.com\/blog\/wp-content\/uploads\/sites\/4\/2018\/05\/thicknessgaugesensorresponse2.jpg\" style=\"vertical-align: middle;border: 0px;margin: 10px auto\" alt=\"steel thickness sensor measurement\" title=\"steel thickness sensor measurement\" width=\"500\" height=\"346\" \/><\/span><\/em><\/p>\n<p style=\"padding-left: 30px\"><em><span>Image above: Sensor response as a function of steel thickness for different photon energies<\/span><\/em><\/p>\n<p>When the statistical noise on the measurement is +\/- 0.1%, it is easy to see that the 150 keV source is just too much for the thin strip application.<\/p>\n<p><em><span>NOTE: To read additional details, including illustrations of simulated sensor responses to a 250 ms, 1.0% deviation from target, read the white paper, <\/span><\/em><a href=\"http:\/\/tools.thermofisher.com\/content\/sfs\/brochures\/Optimization-of-a-Cold-Rolling-Mill.pdf?icid=CAD_blog_metals_2018May\"><em><span>Optimization of a Cold Rolling Mill with a High Speed X-ray Thickness Gauge<\/span><\/em><\/a><em><span>.<\/span><\/em><\/p>\n<p>&nbsp;<\/p>\n","protected":false},"excerpt":{"rendered":"<p>As we learned in previous articles, metals coating weight and thickness measurement gauges provide precise, real-time measurements that help meet specifications of various applications while maximizing raw material use. Thickness gauges are primarily used to look for dramatic deviations in thickness &ndash; which could affect the mechanical properties of the finished product. There are different<\/p>\n","protected":false},"author":71,"featured_media":9925,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"_kad_blocks_custom_css":"","_kad_blocks_head_custom_js":"","_kad_blocks_body_custom_js":"","_kad_blocks_footer_custom_js":"","_monsterinsights_skip_tracking":false,"_genesis_hide_title":false,"_genesis_hide_breadcrumbs":false,"_genesis_hide_singular_image":false,"_genesis_hide_footer_widgets":false,"_genesis_custom_body_class":"","_genesis_custom_post_class":"","_genesis_layout":"","_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":""},"categories":[3,7,105,206],"tags":[],"division":[],"class_list":{"0":"post-9862","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-manufacturing-quality-assurance","8":"category-metals-and-alloys","9":"category-metals-measurement","10":"category-steel","11":"entry"},"_selected_authors":[807],"_selected_reviewers":[],"acf":[],"yoast_head":"<!-- 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One of the most widely used methods for determining gold content is X-ray fluorescence (XRF) analysis, a non-destructive testing technique that quickly evaluates metal composition. However, rhodium plating\u2014a common surface treatment used on white\u2026","rel":"","context":"In &quot;Metals &amp; Alloys&quot;","block_context":{"text":"Metals &amp; Alloys","link":"https:\/\/admin.acceleratingscience.com\/metals\/metals-and-alloys\/"},"img":{"alt_text":"Banner of a cube symbolizing element number 45 of the periodic table. Rhodium. (Rh)","src":"https:\/\/i0.wp.com\/admin.acceleratingscience.com\/metals\/wp-content\/uploads\/sites\/4\/2026\/07\/iStock-2281505711.jpg?resize=350%2C200&ssl=1","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/admin.acceleratingscience.com\/metals\/wp-content\/uploads\/sites\/4\/2026\/07\/iStock-2281505711.jpg?resize=350%2C200&ssl=1 1x, https:\/\/i0.wp.com\/admin.acceleratingscience.com\/metals\/wp-content\/uploads\/sites\/4\/2026\/07\/iStock-2281505711.jpg?resize=525%2C300&ssl=1 1.5x, https:\/\/i0.wp.com\/admin.acceleratingscience.com\/metals\/wp-content\/uploads\/sites\/4\/2026\/07\/iStock-2281505711.jpg?resize=700%2C400&ssl=1 2x, https:\/\/i0.wp.com\/admin.acceleratingscience.com\/metals\/wp-content\/uploads\/sites\/4\/2026\/07\/iStock-2281505711.jpg?resize=1050%2C600&ssl=1 3x, https:\/\/i0.wp.com\/admin.acceleratingscience.com\/metals\/wp-content\/uploads\/sites\/4\/2026\/07\/iStock-2281505711.jpg?resize=1400%2C800&ssl=1 4x"},"classes":[]},{"id":13418,"url":"https:\/\/www.thermofisher.com\/blog\/metals\/buyer-and-seller-beware-how-to-detect-fake-gold-in-current-times\/","url_meta":{"origin":9862,"position":1},"title":"Buyer and Seller Beware: How to Detect Fake Gold in Current Times","author":"Marlene Gasdia-Cochrane","date":"April 7, 2026","format":false,"excerpt":"A Practical Guide for Jewelers and Pawn Shop Owners The rise in gold prices and continued market volatility have made precious metals more attractive\u2014not just to legitimate sellers, but also to increasingly sophisticated counterfeiters. 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The Issue: Radiation Contamination of a Food Supply Last Fall, it was reported by\u2026","rel":"","context":"In &quot;Scrap Metal Recycling&quot;","block_context":{"text":"Scrap Metal Recycling","link":"https:\/\/admin.acceleratingscience.com\/metals\/scrap-metal-recycling\/"},"img":{"alt_text":"Caesium, Cesium, on periodic table of the elements. Alkali metal named after Latin caesius, sky-blue. Symbol Cs, atomic number 55. Fission product caesium-137 is extracted from nuclear reactor waste.","src":"https:\/\/i0.wp.com\/admin.acceleratingscience.com\/metals\/wp-content\/uploads\/sites\/4\/2026\/03\/iStock-1413396579_caesium.jpg?resize=350%2C200&ssl=1","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/admin.acceleratingscience.com\/metals\/wp-content\/uploads\/sites\/4\/2026\/03\/iStock-1413396579_caesium.jpg?resize=350%2C200&ssl=1 1x, https:\/\/i0.wp.com\/admin.acceleratingscience.com\/metals\/wp-content\/uploads\/sites\/4\/2026\/03\/iStock-1413396579_caesium.jpg?resize=525%2C300&ssl=1 1.5x, https:\/\/i0.wp.com\/admin.acceleratingscience.com\/metals\/wp-content\/uploads\/sites\/4\/2026\/03\/iStock-1413396579_caesium.jpg?resize=700%2C400&ssl=1 2x, https:\/\/i0.wp.com\/admin.acceleratingscience.com\/metals\/wp-content\/uploads\/sites\/4\/2026\/03\/iStock-1413396579_caesium.jpg?resize=1050%2C600&ssl=1 3x, https:\/\/i0.wp.com\/admin.acceleratingscience.com\/metals\/wp-content\/uploads\/sites\/4\/2026\/03\/iStock-1413396579_caesium.jpg?resize=1400%2C800&ssl=1 4x"},"classes":[]},{"id":13545,"url":"https:\/\/www.thermofisher.com\/blog\/metals\/how-to-maximize-productivity-in-lightweight-metal-scrap-sorting-at-recycling-facilities\/","url_meta":{"origin":9862,"position":3},"title":"How to Maximize Productivity in Lightweight Metal Scrap Sorting at Recycling Facilities","author":"Marlene Gasdia-Cochrane","date":"May 18, 2026","format":false,"excerpt":"Addressing the Challenge of Lightweight Alloy Identification Lightweight metal scrap sorting\u2014especially for aluminum and magnesium alloys\u2014presents a unique operational challenge. 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