The structure and working principle of the projector

Like a slide projector, a projector also uses the convex lens imaging principle to project transparent objects onto the screen to form an enlarged image. The structure is shown in Figure 2-2-8. The projector is mainly composed of an optical system, ventilation equipment and circuits.

Figure 2-2-7 Structure of a reflective slide projector Figure 2-2-8 Structure of a projector

1. Optical system of projector

The optical system of the projector mainly includes the light source, auxiliary condenser, threaded lens, projection lens, reflector and reflective bowl. The requirements and functions of the light source, projection lens and reflective bowl are similar to those of the slide projector, so they will not be discussed here.

(1) Projector focusing system

Most of the condensers of projectors are threaded lenses, also called Fresnel lenses, which are made of two thin flat organic glass plates with concentric threads, and act like a large-diameter convex lens. The advantages of threaded lenses are large diameter, light weight, and good light transmittance. The main disadvantage is that they will deform when the temperature rises above 70°C.

In order to reduce the surface temperature of the spiral lens, a meniscus mirror, also called an auxiliary condenser, is placed between the light source and the condenser. It can not only block the radiation of the light source's heat energy to the spiral lens, but also expand the inclusion angle of the spiral lens. There is a carrier glass on the spiral lens, which can not only project slides, but also be used for writing, and can also be used for demonstrations of certain experiments or teaching aids.

(2) Plane mirror

The projector is equipped with an adjustable plane reflector to convert the vertical optical axis from the condenser into a horizontal optical axis. Therefore, the slide is changed from being placed upside down vertically to being placed upright horizontally, and a magnified upright image is obtained on the screen, so that the teacher can explain and demonstrate to the students, which is very convenient. The plane reflector must be turned on before the projector is turned on to ensure that the light emitted by the light source is directed to the screen after the power is turned on, so that the light spot appears on the screen; if the plane reflector is not turned on, it will reflect the light emitted by the light source back to the light source after the power is turned on, increasing the temperature inside the machine and on the surface of the light source, and affecting the life of the projector.

2. Other components and circuits of the projector

There are also some important electrical equipment and ventilation equipment in the projector, as shown in Figure 2-2-8. The transformer converts 220 V mains electricity into 24 V AC voltage for projection bulbs. Projection bulbs are mostly 24 V/300 W bromine tungsten lamps, which generate a lot of heat when working. If it is allowed to accumulate, the temperature inside the machine will inevitably rise, which will damage the threaded lens and electrical equipment. Therefore, a squirrel cage fan is installed in the machine to discharge the heat inside the machine in time and reduce the temperature inside the machine. Rotating the focus knob will drive the projection lens and the plane reflector to move up and down in the vertical direction, changing the distance between the projection lens and the object (projection film) to achieve the focusing process. In addition, the elevation angle of the plane reflector is adjustable. Changing the elevation angle can adjust the height of the image on the screen. The reflector can also rotate 360 ​​degrees in the horizontal plane, so that the image can be projected on any wall of the projection room without moving the machine body.

The circuit of the projector is shown in Figure 2-2-9, where the K1 power switch is linked to ensure that the fan and the bulb are turned on or off at the same time. The K2 safety switch is also called the film gate switch, which is located below the condenser plate (projector film gate). When the film gate is lifted, the film gate switch is disconnected, making the working switch invalid, and the bulb and fan do not work; when the film gate falls, the switch is turned on, and the working switch resumes function.

Figure 2-2-9 Electrical schematic diagram of projector

At present, there is also a high-brightness projector, whose structure is basically the same as that of an ordinary projector, except that the light source is not a bromine tungsten lamp but a dysprosium lamp whose light efficiency is 2 to 3 times that of a bromine tungsten lamp. For usage instructions, please refer to the relevant parts of the reflective slide projector.

Use of projector

1. General adjustment of projection equipment

① Adjust the light spot, that is, make the light spot evenly projected onto the screen. If it is uneven, adjust the position of the bulb and the reflector bowl or condenser slowly until it is evenly adjusted.

② Adjust the optical path, that is, make the optical paths of the symmetrical points on the screen relative to the optical axis equal. For example, move the position and angle of the slide projector and the position and inclination of the screen, so that the screen is perpendicular to the axis of the incident light, and the picture will not produce trapezoidal distortion.

③ Adjust the distance, that is, adjust the distance between the slide projector and the screen to change the size of the image. Generally speaking, the width or height of the image should be 1/5 of the length of the classroom.

④ Adjust the focal length, that is, adjust the distance between the lens and the object being projected to make the picture appear as clear as possible.

2. Usage of special operating parts of the projector

(1) How to use the hue adjustment knob

During the focusing process, if you find that the center and edge of the picture cannot be adjusted to be clear at the same time, or color fringing appears on the image lines (note that this does not refer to the color fringing on the edge of the projection spot), you should adjust the color tone adjustment knob to make it normal.

(2) Usage of strong and weak light switch

If the indoor light is too strong and dilutes the image on the screen, the projector can use strong light, that is, turn the strong and weak light switch to the strong light position. When working in the strong light position, the bulb voltage is higher than normal, which will shorten the life of the bulb, so try not to use strong light when it is available. But when shutting down, first turn the strong and weak light switch to the weak light position.

(3) How to use the light switch

If the bulb of the projector is damaged normally during projection, the lamp replacement switch can be placed in another position, and the circuit will be connected to the spare bulb, so as to achieve the purpose of quick replacement of the bulb.