# Full reflection prism

## Principle

Figure 19-26 Isothermal Corner Triangle ABC represents a cross section of a total reflection prism, two straight angles AB and BC represent two mutually perpendicularity on the prism Side. If the light is vertically smashed on the AB surface, it will be incident in the original direction, shot on the AC surface, due to the incident angle (45 °), the critical angle (42 °) injection of the air from the glass, the light A total reflection is generated on the AC surface, and it is emitted from the prism along a direction perpendicular to the BC. If the vertical shot AC face (as shown in Figures 19-26 B), after the prism is incident in the original direction, all reflection occurs on both sides of the AC and BC, and finally, in contrast, in the opposite direction of incidence. The direction is used to shoot in a lot of lives from the AC face, such as bicycle taillights (Fig. 19-27), use this principle

in optical instruments often use full reflection prism replacement plane mirror , Change the direction of communication of the light. Figure 19-28 is a light path diagram of a total reflection prism application in a percentage mirror. In the telescope, in order to obtain a large magnification, the lens barrel is long, using a total reflection prism, can shorten the length of the lens barrel (Fig. 19-29).

## principle

The working principle of the reflection prism is actually the reflection law and refractive law of light. When the light is reflected in the same medium, the reflection angle and the incident angle are equal; when the light is incident in the diaphragm of the medium, it is not refracted when the dielectric plane is incident to another medium.

The prism in the actual application is shown in Figure 1; the structure of the prism tail is three sides (A, B, C surface shown in Fig. 2), which is as shown in Figure 2; principle is shown in FIG. Figure 3 is a cross section of the tail of the reflection prism in Fig. 3, wherein the angle is 90 degrees, and the a plane and the B are perpendicular. The incident light R1 is incident on the surface B surface, and its reflected light is reflected on the surface A, and finally the reflected light R2 returns, and its direction is reversed in the direction of R1. The following relationship can be known from the reflection law of light:

, R1 and R2 are parallel. That is to say, the reflected prism can be launched back in accordance with the original road.

Initially, the total reflection prism is only equivalent to a plane mirror, and it can be replaced by plane mirror using a total reflection prism, but it is actually not the case. The general plane mirror is silver plated in the rear surface of the glass, that is, the front surface of the plane is also reflected in the glass surface, and the light should be reflected by the glass surface and the silver surface, so it will become a plurality of images (Fig. 19-30) . The first time the image (main image) formed by silver surface reflection is brighter, and other images are getting darker, generally not paying attention, but for precision optical instruments such as cameras, telescope, microscope, etc. These excess images must be removed, so a total reflection prism is often used. Of course, if silver is plated on the front surface of the glass, there will be no more images, but the front surface is silver, and the silver surface is easy to fall off.

## Effects

When the ranging with a reflective prism (or reflection sheet) is used as a reflective, the reflected prism receives the optical signal emitted by the full station and reflects it back. The full station is transmitted, and the optical signal reflected from the reflective prism is received, the phase movement of the optical signal, etc., thereby indirect the time, and finally measuring the distance to the reflection prism.

## Prism constant

Since the air refractive index is approximately equal to 1.0, the refractive index of the glass is approximately equal to 1.5; according to the formula, the speed is smaller than when the light is passed through the air. .

When measuring the distance between the instrument to the reflective prism, the instrument is longer than the actual distance based on the measured distance. Therefore, the prism constant depends on the refractive index of the glass and the thickness of the prism (the length of the light). It is assumed that the reflective prism top is on the vertical line of the measuring point, then the correction value of the refractive index of the prism (glass material) is the prism constant. However, due to the need for fixing, the vertex position of the prism is not at the vertical line of the test point.

The calculation of the prism constant of the actual application (Figure 4) is as follows:

in the above formula:

, for example, a factory manufactured by reflection prism is in the application It will be found that the prism constant nominally nominally nominally -40 and -30; this is to change the prism flow by increasing or decreasing an outer frame. In fact, it is changed the value of H.

## Type

The ranging beam emitted from the instrument will increase the beam with its passing distance increases. When using a reflection prism, the return light received by the instrument will be reduced. Multiple reflection prisms are used in actual applications when performing long distance measurements. Common prisms are: single prism; 3 prisms; 9 prisms; simple prisms; benchmark single prisms, etc.

About the reflective prism, when manufacturing, according to the requirements of the supporting whole station, the range of range findings, "taper" has a negative a few seconds to positive for dozens of seconds. Positive good processing, the cost is also relatively low; and the larger value, the greater the cost, there is a lot of cost. The coating is also carried out as required in the manufacture. Coatings also have film materials and process requirements, and there are many different costs. When users choose to use, they don't necessarily consider these squares; to meet their own engineering requirements for the selected attachments.

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