Abstract: A thermochromic sensor for locating the area of contact between two surfaces moving in relation to each other, has applications in boundary line officiating and other situations where the location of the area of contact is important. For tennis and similar sports, thermochromic sensors are placed along boundary lines. The thermochromic sensors can be formed with thermochromic liquid crystals, which change color in the area of contact, thereby accurately indicating where a ball contacted the court. The color change can be used to characterize the speed of the impact. A temperature controller can be built into the sensor or placed near the sensor to maintain the temperature of the sensor near the sensor trigger point--the point at which a color change will occur on contact.