A strip is usually under tension; hence care has to be taken when implementing optical measurement systems on strip applications. Tension “hides” flatness problems since the tension extends the shortest parts of the strip most. If the tension is too high, the strip will look flat, and contact-based methods must be used.

An exception to this is crossbow, which is measurable also in high-tension areas, such as galvanization or hardening applications. Optical flatness measurement can also be useful in annealing lines for high yield strength or special steel as well as in hot rolling mills.