What Is a Prism?

Prism, a transparent object surrounded by two planes that intersect but are not parallel to each other. A prism is a polyhedron made of transparent materials (such as glass, crystal, etc.). It is widely used in optical instruments. Prisms can be divided into several types according to their nature and use. For example, in a spectroscopic instrument, the composite light is decomposed into a "dispersion prism" of the spectrum. An equilateral triangular prism is more commonly used. In periscopes, binoculars and other instruments, the direction of light is changed to adjust its imaging position. "Reflecting prisms" generally use right-angle prisms.

Prisms are transparent materials (such as

Newton discovered the phenomenon of light dispersion in 1666, and the Chinese were ahead of foreigners in this regard. In the 10th century, the Chinese called the natural transparent crystals that were exposed to sunlight called "five-light stones" or "light-emitting stones", and realized that "it's just like the sun, and they become five colors like a rainbow". This is the earliest recognition of the dispersion phenomenon of light in the world. It shows that people have liberated the phenomenon of light dispersion from mystery, knowing that it is a natural phenomenon, which is a great progress in understanding light. It was seven hundred years before Newton divided the daylight into seven colors through a prism, indicating that white light is a composite of these seven colors. ^[1]

Polyhedrons made of transparent materials are important optical components. The plane where the light enters and exits is called the side, and the plane perpendicular to the side is called the main section. According to the shape of the main section, it can be divided into triangular prisms, right-angle prisms, and pentagonal prisms. The main cross section of a triangular prism is a triangle with two refracting surfaces. The angle between them is called the top angle, and the plane opposite to the top angle is the bottom surface. According to the law of refraction, light passes through a prism and deflects twice toward the bottom. The angle q between the outgoing light and the incident light is called

In modern life, prisms are widely used in digital equipment, science and technology, medical equipment and other fields.

Commonly used digital equipment: cameras, closed-circuit televisions, projectors, digital cameras, digital video cameras, CCD lenses, and various optical equipment;

Science and technology: telescope, microscope, level, fingerprint, gun sight, solar energy converter and various measuring instruments;

Medical equipment: cystoscope, gastroscope and various laser treatment equipment. ^[1]

Prism concept

The excess time it takes for light to propagate through a reflective prism will increase the measured distance by a certain value, which means that the speed of light traveling through glass is slower than that in air. We usually call this increased value the prism constant.

There are two kinds of prism constants. Usually, the domestic prism we use is -30mm, and the imported prism is 0mm. As for how to distinguish the prism constant, you can look at the bottom of the prism. If the prism's anchor bolt is flat with the plastic shell, it is -30mm. If it is not, it is 0mm; another tip is to teach you the direction or angle of the coordinate point as far as possible before the backsight is determined. You cannot take the center of the prism, because you cannot see the center of the prism if it is far away. In addition, the distance measurement setting does not need to be aligned with the center of the prism, as long as you look at the reflective surface of the prism at any point. ^[2]

Basic knowledge of prism constants

The role of the reflective prism: When using a reflective prism (or a reflective sheet) as a reflective object for distance measurement, the reflective prism receives the optical signal from the total station and reflects it back. The total station sends out an optical signal, receives the optical signal reflected from the reflecting prism, calculates the phase shift of the optical signal, etc., thereby indirectly obtaining the time for the light to pass, and finally measures the distance from the total station to the reflecting prism.

Principle of reflective prism: The working principle of reflective prism is actually the law of reflection and refraction of light. When light is reflected in the same medium, its reflection angle and incident angle are equal; when light is incident from another medium perpendicular to the plane of the two mediums and enters another medium, it will not be refracted. ^[2]

Prism prism constant setting

1. Find a known baseline. Set the instrument and the prism at both ends of the baseline to set the prism constant in the instrument to 0 (note that when some total stations set the prism number to 0, a matching prism constant is actually defaulted. At this time, you should set this parameter to other appropriate values.) The difference between the horizontal distance observation and the constant value under the measurement is the prism constant. Of course, this result includes the measurement error, so you should observe it, and take the average as an approximation. In fact, the prism constants of general manufacturers are not accurate to mm, so the decimals below millimeters are rounded up.

2. Set up an instrument at two points and a prism with a prism constant to measure the distance between the two points, and then use the first method to operate, it is necessary to pay special attention to: make sure to set the prism constant What is the implied prism constant when it is 0? ^[2]