Non-contact measurement & inspection
Glenn Wedgbrow, business development manager at Micro-Epsilon UK, highlights the benefits and limitations of red and blue laser light sensor technologies, as well as those applications where blue lasers are a better choice over traditional red lasers.
Principles of laser point triangulation (left) and laser line triangulation sensors (right)
and dynamic dimensional measurements. Most often, the laser used in this application is red in colour, and this type of laser suits the vast majority of industrial applications. In addition, because of the red laser’s ubiquity and lengthy development history, a wide variety of cost effective red laser measurement and sensing systems are available. Red lasers perform best when reflected from
matt or low reflectivity surfaces, or what are called “non-specular” surfaces. Specular surfaces are typically glossy, polished or highly reflective. Such materials introduce distortion to the red reflected light, adding ‘noise’ to the image detector readings and decreasing measurement accuracy. Red laser light is also preferable for fast
signal from the linear displacement sensor takes measurements in units of millimetres or inches, or even in micrometers. Laser triangulation sensors pinpoint a
narrow beam of laser light onto an object or sample, and light reflected from the object’s surface is focused by a high-quality optical lens onto a charge-coupled device (CCD) or complementary metal-oxide semiconductor (CMOS) image sensor. As the object moves, the reflected light also changes position on the image sensor, which is computed to detect the position of the object or sample. The triangulation principle involves the measurement of distance on a wide range of material surfaces. To measure displacement, distance and position, laser point triangulation sensors are used. To measure profile and gap, laser line triangulation sensors or ‘laser scanners’ are used.
REd vs. blUE: WhEn to UsE Which Laser triangulation technology works best in automated position sensing, as well as for passive
Instrumentation Monthly March 2022
Distance measurement on anti-reflective coated glass
moving objects, due to its long wavelength and intensity. Red lasers typically operate at 670 nm, close to the infrared spectrum and are perfect for use with CCD sensors. The high intensity of the red laser can also be a disadvantage, as it is more likely to penetrate and be diffused on the target material, especially on organic and transparent materials, further decreasing measurement accuracy. The same is also true for metal forging. It was formerly a challenge to use red laser
measuring techniques in the steel industry, where red-hot glowing metals, typically above 700°C, emit sufficient infrared light to skew the image sensor readings. To solve these challenges, blue light laser
triangulation technology was developed. Blue lasers operate at approximately 405 nm of the visible light spectrum, a shorter wavelength with lower intensity than red, and close to the ultraviolet spectrum. This creates a more focused laser point with less surface diffusion, resulting in less noise in the sensor, thereby increasing measurement accuracy, sometimes by a scale of two or three times when compared to red lasers. Blue laser technology is more than just a
light colour change. It also requires a different lens that is optimised for short wavelength radiation, different optical filters for the reflected light, a different light sensor and innovative analysis algorithms. Blue light laser triangulation technology has
proved so valuable that the applications for it have expanded over time. It is especially prevalent in automotive testing, for example in engine manifolds or brake discs. It has also been used in paper production because a red light laser shines through damp paper pulp and does not return usable measurements. Blue light is preferable to red light laser
triangulation for measuring transparent or translucent materials, reflective or specular surfaces, and organic materials, including wood and food.
PatEntEd tEchnoloGy Blue laser measurements on red-hot objects, as well as transparent objects such as glass or plastic, is a proprietary and patented technology from Micro-Epsilon. The company plays a leading role in measurement technology. For more than 50 years, the company has offered reliable, high performance, unique solutions whenever high precision measurement or inspection is required. Its product range covers sensors to measure distance and displacement, sensors for infrared temperature measurement and colour detection, as well as systems for dimensional measurement and defect detection.
Micro-Epsilon UK
www.micro-epsilon.co.uk
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