Distance measurement by using light. Classical methods use the fact that light travels in a straight line (except for the effect of refraction) by making the light rays the two sides of a isosceles triangle in which the observer is either at the vertex of the triangle and uses the measured angle there together with the known length of the base, or he is at the base and uses the two opposite angles there together with the known length of the base. This is actually distance determination if the observer actually calculates the distance from the measured angle or angles; it is distance measurement if the instrument used does the calculating and displays the corresponding distance. The stadia method and the subtense-bar method are examples. Newer methods do not use the properties of a triangle but measure the distance directly. The instrument measures the difference between the time or phase at which light is emitted and the time or phase at which the light returns from the distant point and converts this quantity to a distance. The term is also applied to methods in which infrared radiation is used instead of light.