Slide projector and overhead projector

Slide projectors and projectors can project teaching materials onto the screen, increase the visibility of teaching materials, and make up for the shortcomings of traditional visual media. They are widely used in classroom teaching. Therefore, we should know the basic structure and usage of slide projectors and projectors so that we can make better use of them in teaching.

1. Slide Projector

1. Working principle and structure of slide projector

There are many types of slide projectors in different shapes, but their optical system structures are basically the same.

1. The imaging process of a slide projector

The lens of the slide projector is equivalent to a convex lens. When the slide is between one and two focal lengths of the lens, the light source illuminates the slide through the lens to form an enlarged, inverted real image on the screen. A slide projector with this optical path feature is called a direct-projection slide projector, and its imaging process is shown in Figure 2-2-1.

Figure 2-2-1 Slide projector optical path diagram

2. Optical system of slide projector

The optical system of a slide projector includes a reflector bowl, a light source, a condenser, a projection lens, etc. Their functions in the slide projector and the requirements for them are as follows:

(1) Light source

Its function is to emit light and shine it onto the slide. Requirements for the light source: meet the "daylight projection" condition, with sufficient brightness; meet the "point light source" condition, with a small filament arrangement area; the temperature near the light source surface is low to avoid burning the slide; have a high color temperature to facilitate color restoration; the light source has a sensitive switching effect; and has a long life.

The light sources commonly used in slide projectors are mainly bromine tungsten lamps (halogen lamps) and dysprosium lamps. The structure of bromine tungsten lamps is shown in Figure 2-2-2. It has the advantages of small size, high color temperature (3200~3400K), long effective life (about 200 hours), sensitive switching effect, etc., but it requires transformer step-down and heat dissipation device. Bromine tungsten lamps commonly used in slide projectors have specifications such as 12 V/100 W, 12 V/150 W, 24 V/150 W and 24 V/250 W.

The structure of the Dysprosium lamp is shown in Figure 2-2-3. The Dysprosium lamp has a high luminous efficiency of 80 lm/W, while the bromine tungsten lamp is only 30 lm/W; the service life is long, from one hundred to several hundred hours, which is much longer than the service life of the bromine tungsten lamp; the color temperature is as high as 5,000 K, close to daylight, and the color reproduction is good; but the switching effect is poor, the arcing time is long, and the trigger power supply is bulky. Therefore, the Dysprosium lamp projection equipment is not suitable for classroom teaching.

Figure 2-2-2 Structure of bromine tungsten lamp Figure 2-2-3 Structure of dysprosium lamp

(2) Reflective bowl

The function of the reflector bowl is to reflect the light emitted from the light source to the condenser, thereby improving the utilization rate of the light energy of the light source. The reflector bowl is a concave mirror, which can be made of polished aluminum or glass coated with silver. The slide projector requires that the aperture of the reflector bowl should not be larger than the aperture of the condenser, and the relative position of the reflector bowl to the light source should ensure that the image of the lamp coincides with the filament (that is, the filament is at the center of the sphere of the concave mirror).

(3) Condenser

The function of the condenser is to make the light emitted by the light source shine more and more evenly on the slide, and the light passing through the slide is gathered into the focusing range of the projection lens, as shown in Figure 2-2-4. The solid angle W2 represents the solid angle (also called the inclusion angle) formed by the light emitted by the light source entering the slide when there is no condenser, and W1 represents the solid angle formed by the light emitted by the light source entering the slide with the condenser. Obviously, W1>W2. The larger the inclusion angle, the more light flux is used. There are two ways to expand the inclusion angle: one is to expand the aperture of the condenser, and the other is to shorten the distance between the condenser and the light source. The slide projector requires the aperture of the condenser to be slightly larger than the effective screen of the slide; the light source is at the focus of the condenser.

Figure 2-2-4 Function of condenser

(4) Projection lens

The function of the projection lens is to form an enlarged, clear, inverted real image of the slide on the screen. The slide projector requires the projection lens to have a strong ability to overcome aberrations; it should have a suitable aperture so that the projection lens can move within the range and allow all the light to pass through the projection lens.

At present, slide projectors all use composite lenses composed of several coated lenses to reduce or even eliminate aberrations. The method of identifying coated lenses is: when observing the coated lens in the reflection direction (from the side), the lens appears bluish purple or golden yellow, but when observing through the lens (from the front), it has no color.

The projection lens is marked with focal length F and relative aperture O, and their relationship is:

Where D is the effective diameter of the projection lens. The relative aperture O is usually expressed as a value of 1: (F/D), such as 1:1.2, 1:1.3, etc., indicating that the smaller the value after the “:”, the greater the light transmittance per unit time of the lens.

3. The body of the slide projector

It is a carrier used to support various optical components, slides and other accessories. It includes a light box, film gate, film rack, lens barrel, chassis, motor, electric fan, etc. The light box is usually made of iron or hard plastic, and the box wall has ventilation and heat dissipation windows.

(II) Several Commonly Used Slide Projectors

According to the film changing method, there are simple slide projectors and automatic slide projectors; according to the imaging principle, there are direct-type and reflective-type slide projectors; according to the form of the slide, there are single-slide projectors, roll-type slide projectors and microscopic slide projectors; according to the shape of the film box, single-slide projectors are divided into straight-box push-pull type and disc lift-and-fall type. The most commonly used slide projector at present is the single-slide direct-type automatic slide projector (abbreviated as automatic slide projector), which is introduced as a typical machine here, and other slide projectors are briefly introduced.

1. Automatic slide projector

The appearance of the automatic slide projector is shown in Figure 2-2-5. In addition to the general slide projector structure, it also has the electrical part and mechanical transmission part of automatic film change and automatic focusing. Only the knowledge related to its use is introduced here.

(1) Slides

The slides of the automatic slide projector have the same specifications, as shown in Figure 2-2-6. The outer size of the film frame is 50 mm × 50 mm, and the frame size is 18 mm × 24 mm, which is the same as the frame size of the 135 film used for photography, so slides of this specification are collectively called 135 slides. The film frame is divided into two types: paper frame and plastic frame. The paper frame is cheap, but it is easy to deform during use, resulting in poor film transport and jams; the plastic frame is not easy to deform, strong and durable, but the price is relatively high.

Figure 2-2-5 Physical diagram of automatic slide projector Figure 2-2-6 Slide frame

(2) General method of changing slides in an automatic slide projector

① Automatic timing film change. That is, the film is changed forward once every certain time. The interval time can be adjusted and set manually.

② Remote control for changing films. It can be radio remote control, ultrasonic remote control or infrared remote control. When using it, first set the machine function selection switch to the remote control state, and then use the remote control to control the slide projector to perform forward film change, reverse film change, focus or power on and off within a distance of no more than 10 meters.

③ Synchronous switching of audio and video, also known as signal-controlled switching. It means that when the recorder finishes playing the slide commentary, it sends a switching signal to the slide projector at the location where the slide should be switched, and controls the slide projector to perform a forward switching action. Note that it cannot switch the slide in the reverse direction. There are generally two ways to switch the slide signal. One is to record the switching signal at the location where the slide is to be switched, such as a 1,000 Hz audio signal. Usually, the switching signal and the commentary are recorded on different channels. When playing, the commentary channel is connected to the speaker, and the switching signal channel is sent to the control signal input terminal of the slide projector through a transmission line, so that there is only the commentary sound but no switching signal in the speaker; the other is to leave a blank on the tape at the location where the slide needs to be switched, which should generally be greater than 5 seconds, that is, the commentary is interrupted for 5 seconds. At the same time, it is required that there should be no pause greater than 3 seconds in the middle of each commentary. When using this audio-visual synchronization method, the commentary signal of the recorder is directly sent to the slide projector through a transmission line. When the slide projector encounters a blank of 5 seconds, the slide is switched once.

All slide projectors that require audio film-changing signals are equipped by the manufacturer with a signal generator that generates the film-changing signal. Just press the button of the signal generator to send out the film-changing signal, and then send the signal to a channel input jack of the recorder (not the commentary channel) via a transmission line.

④ Manual film change, also called electric film change. There is a film change button on the automatic slide projector. Press the advance button once to advance the film once; press the rewind button to rewind the film once. Some slide projectors only have one film change button, and the duration of pressing the switch is used to distinguish between advance and rewind.

⑤Wired manual control slide change. The control button is placed in the manual control box and connected to the slide projector by wires.

(3) Focus adjustment of automatic slide projector

Focusing, also known as focusing, refers to the process of adjusting the front and rear position of the projection lens to change the image clarity. The purpose of focusing should be to make the image clear. There are three ways to focus an automatic slide projector, namely rotating lens focusing, manual control box focusing and automatic focusing.

① Rotate the lens to focus. Rotate the lens by hand, and use the threads on the outer edge of the lens barrel to make the lens move and retract, so as to achieve focusing. It is the most basic way to focus. The specific operation is: after turning on the machine, input a slide into the slide projector, then rotate the lens and observe the changes in the clarity of the image on the screen until the image on the screen is the clearest.

②Wired manual focus. Operate the focus button on the manual control box to control the focus motor to drive the lens to achieve telescopic movement. Press the front adjustment button to extend the lens, and press the rear adjustment button to retract the lens. Some slide projectors have only one focus button. Press this button to move the lens back and forth repeatedly within a certain range, and the focus range is not large. When the manual control box cannot make the image clearest, it is necessary to rotate the lens to achieve focus.

③ Automatic focus. A slide projector with this function is equipped with a photoelectric device that can detect the position change of the slide at the film gate. Once the slide is detected to deviate from the standard position, it will automatically issue a command to make the focus motor work, drive the lens forward or backward, so as to ensure that the image on the screen is always clear. The focusing range of this function is generally very small, and it should be used on the basis of rotating the lens to focus.

2. Reflective slide projector

(1) Working principle of reflective slide projector

The reflective slide projector is also called a real object slide projector. Its structure is shown in Figure 2-2-7. The light emitted by the light source is irradiated onto the pallet through the heat-insulating glass. The opaque flat object is placed on the pallet, so the object is illuminated by the light. The light emitted by the object is reflected by the reflector and magnified by the projection lens to form an image on the vertical screen.

Since the physical reflective slide projector has a low light energy utilization rate, in order to make the screen image reach the conditions of a bright room projection, the light source brightness must be much higher than that of a direct projection slide projector, so a dysprosium lamp with high luminous efficiency is used as the light source. And because the glass shell of the dysprosium lamp has a very high temperature, about 800 ℃, it is necessary to install heat-insulating glass in front of the light source to prevent the temperature at the support plate from being too high.

(2) Use of reflective slide projector

① Check before starting. Due to the high power of the H-type lamp, the circuit should have a power-carrying capacity of more than 10 A. A small circuit capacity will burn out the incoming line fuse or cause a circuit accident. Therefore, the capacity of the power circuit should be checked before turning on the power.

② Turn on the machine. After the power is turned on, turn on the fan switch first, and then turn on the HMI lamp switch. At this time, the HMI lamp is not bright, and you need to press the trigger button again. At this time, you should be able to hear the discharge sound, which is the performance of arc light. Continue to press the trigger button. When you see a dark yellow spot in the center of the lamp tube and a relatively stable light (although very weak) appears on the lens, release the trigger button immediately. The lamp tube will gradually become brighter, and after 3 to 4 minutes, the light color can reach a white light close to daylight, indicating that it has entered the projection state. Send the physical object to be projected to the pallet for projection, and make the screen image clear by focusing. After the projection, turn off the light source (HMI lamp) switch first, and then turn off the fan switch.

③ Precautions. If the projector fails to ignite after several consecutive triggering, stop for 3 to 5 minutes before triggering again; do not press the trigger switch before installing the dysprosium lamp; ultraviolet rays are emitted when the dysprosium lamp is working, so be sure to protect your eyes; there is a 30 KV high voltage when the trigger is triggered, so the cover must not be opened to trigger; the object to be projected must be placed on the tray and close to the transparent glass surface at the bottom of the slide projector case.

2. Projector

The projector is a new type of classroom teaching equipment that integrates teaching functions such as blackboard, slides, and diagrams.

1. Structure and working principle of 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.

(II) 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.

3. Maintenance of slide projectors and overhead projectors

Correct and timely maintenance of equipment is an important guarantee for giving full play to the benefits of equipment and extending the life of equipment. For slide projectors, the maintenance items are mainly cleaning, lubrication, moisture-proofing and replacement of damaged parts.

1. Cleaning

If there is dust on the lens, use a balloon to blow it away, or use a soft brush to brush it away. If there is other dirt, use lens paper to wipe it off. The threaded lens is made of organic glass and is very easy to scratch. It should be cleaned by washing with water.

2. Lubrication

All parts where relative friction occurs between operating parts should be lubricated regularly, but rubber and plastic parts do not need to be lubricated.

3. Moisture-proof and regular power supply

When the machine is idle for a long time, it should be powered on for half an hour every quarter to drive away internal moisture. At the same time, the capacitors in the circuit will be charged and their life will be extended.

4. Replace the light bulb

Replacing projection bulbs is a frequent item in maintenance work. After replacing a new projection bulb, if there are uneven light, missing corners, poor clarity, etc. on the screen, it means that the filament of the new bulb is not on the optical axis or at the focus of the condenser. Accurate judgment method: Turn on the power switch of the projector, do not put the slide on the stage, focus directly, so that the threads of the threaded lens can be seen clearly on the screen, and then put a piece of paper with a small hole of 3 mm in diameter on the stage, adjust the position of the paper while watching the image on the screen, so that the small hole is exactly at the center of the threaded lens, fix the position of the paper, then remove the projection lens, and use white paper as a screen at the lens position. If there is only one filament image, it means that the position of the bulb is correct. If there is a light spot next to the image, it means that the position of the bulb is incorrect. When correcting, turn off the power first, and wait for the bulb to cool down before operating to prevent burns.

4. Screen Type and Installation

1. Types and selection of screens

Screens used in teaching can generally be classified into the following forms:

① According to the appearance, there are roll type, film type and frame type;

② According to the projection method, there are two types: reflective and transmissive;

③ According to the optical materials used to make the screen, there are wooden screens, white cloth screens, glass bead screens, metal screens and cloth-based white plastic screens;

④ According to the optical characteristics of the screen surface, there are full diffuse reflection screens and directional diffuse reflection screens;

⑤The selection of screen mainly considers: screen specifications, usage site, screen optical performance and other factors.

(B) Screen Installation

1. Location of the screen

The screen is usually hung on one side of the blackboard. If it must be hung in the middle of the blackboard, the screen should be able to roll up automatically.

2. The distance between the screen and the seats

The screen should be installed in a position that ensures that the distance between the student closest to the screen and the screen is greater than 1.3 times the width of the screen, and the distance between the student farthest from the screen and the screen is less than 8 times the width of the screen, preferably 5 to 6 times.

3. Screen height and orientation

The height of the screen should be such that every student in the classroom can see the entire screen. Generally, the bottom edge of the screen should be 1.0 to 1.2 m from the ground. Considering that the screen is hung on one side of the blackboard, its plane should be slightly tilted to the other side (at an angle of about 20º). Considering that the position of the projector and slide projector cannot be too high, the plane of the screen should be slightly tilted forward (at an angle of about 10º).