DE3400562A1 - Autofocus circuit for slide projectors - Google Patents

Abstract

An autofocus circuit for slide projectors, having a film gate or lens which can be adjusted by an electric motor, is described, which is characterised in that glass-mounted slides and non-glass-mounted slides are detected by it and the lens or the film gate experiences a corresponding presetting by the electric motor. Apart from the voltage of the differential photoresistor, the circuit additionally also measures the photocurrent, which is substantially stronger in the case of glass-mounted slides than in the case of non-glass-mounted slides. By making available reference currents and comparing the photocurrent with these reference currents, a voltage signal is obtained which is fed to a controller, preferably in the form of an operational amplifier, which conducts a corresponding control signal to the motor control and thus brings about the presetting appropriate for the slide being projected.

Description

Autofocus circuit for slide projectors The invention relates to a circuit for the drive motor for the independent adjustment of the Film stage or lens for maximum image sharpness with slide projectors.

It is known in slide projectors to provide a device by means of which automatically determines the distance between the film to be projected and the projection lens adjusted to the maximum sharpness of the projected image.

Either the film stage or the projection lens are used for this purpose displaceable along the optical axis and driven by an electric motor.

The electric motor resides in an electrical circuit, its essential Part is a differential photoresistor.

The latter is acted upon with a light beam from one side The light source arranged by the slide is thrown onto the slide and from there onto the photoresistor on the other side of the slide.

Both the light source and the differential photoresistor are usually arranged in a stationary manner. But it can also be either the light source or the photoresistor connected either to the film stage or to the projection lens and be displaceable together with these components. As light can be preferred infrared light can be used.

The control voltage for the electric motor is between the both Parts of the differential photoresistor tapped, this voltage is only then equal to the two poles of the supply voltage if the reflected one The light beam falls or falls on the gap between the two partial photoresistors. both partial resistors are evenly exposed to light. Becomes one of the Partial resistors are exposed to more light than the other, so both will Partial voltage of the center tap unequal to the poles of the supply voltage large, and this is used to start the motor and adjust the position readjust the film stage or the lens until the tension is equal again prevails and thus again achieves the maximum image sharpness on the projection wall is.

These known autofocus devices work well and regulate the image sharpness reliably after, e.g. if the slide "jumps" in a known way, after it has stood in the projection beam for some time and has become warm.

However, the manually preselected object width is always used set between the reflection point on the slide and the projection lens, the depends in a known manner on the projection distance. Therefore, maximum Image sharpness is only achieved if the reflection point is directly on the slide film i.e. with unglazed slides.

In the case of glazed slides, however, the point of reflection is not on the slide film but on the front of the covering glass. The object width is thus adjusted to the distance between the glass pane and the projection lens, and the slide film that is seen from the projection lens behind the pane of glass is outside the object distance and is consequently on the projection screen out of focus. (On the other hand, the dust and fingerprints are sharply depicted and other imperfections that may be on the glass pane, which is understandably highly undesirable).

In itself this difference would be in the position of the reflection point and of the actual slide film is not particularly problematic, because if it is ensured if only glazed slides were projected, this difference could entered into the circuit as a fixed value and thus taken into account accordingly.

However, it is by no means always guaranteed that only glazed ones are used Slides, or always only unglazed slides, can be projected, but it is in many It is almost the rule that unglazed and glazed slides are projected into one another will. It must therefore be the position of the film stage or the lens in the one Case to be settled in a different place than in the other case.

The invention is therefore based on the object of an autofocus circuit which automatically detects whether a glazed or unglazed slide is being projected is, and automatically the film carrier or the lens to the one or the shifts to another location, depending on what kind of slide is in the projection beam path is located.

According to the invention, this object is achieved by a circuit, which has the features specified in the claims.

The circuit is characterized in that in addition to the measurement the differential voltage at the photoresistor also measures the current that flows through the differential photoresistor. The invention is based on the idea that - regardless of the voltage distribution on the two partial photoresistors - the flowing photostron depends on the intensity of the incident on the photoresistor Light depends. And this is the light intensity and thus the flowing photocurrent higher when the light is reflected off a pane of glass (glazed slide) than when it is reflected from the film itself (unglazed slide).

The strength of the photocurrent is therefore a direct criterion for whether a glazed or an unglazed slide is in the beam path. Besides, lets recognize each other even if there is no slide at all in the beam path, because then meets no light whatsoever on the photoresistor, and this remains high-resistance, so that at all no electricity or only a minimum amount of electricity flows.

The current flow with a glazed slide, with an unglazed slide and with even no slide in the beam path can be determined empirically, and it can be in the circuit a reference current source can be provided which emits corresponding reference currents. The photocurrent and the reference currents are fed to a comparator, which accordingly transmits a voltage signal to an encoder based on the agreement determined by it, which is also supplied with the voltage tapped at the phto resistor, and the in turn gives a corresponding control signal to the controller of the electric motor. This control signal is used to preset the film stage or the lens which is matched to the type of slide used in each case. There is no Slide in the beam path, so the control signal can be used to control the autofocus shut down altogether.

The voltage signal coming from the comparator can also be used for this to control display means, e.g. in the form of different colored light emitting diodes, so that the user is also visually shown which type of slide is currently in the beam path stands.

In the drawing, the invention is shown in an exemplary embodiment, where the autofocus light source and the differential photoresistor are both involved connected to the lens. It shows: Fig, la and lb the position of the lens at maximum image sharpness with differently framed slides, and FIG. 2 is a block diagram the invented autofocus circuit.

In Fig. La and lb, 1 denotes the slide film, which is in one There is a projection beam path which defines the optical axis 2. On this A projection lamp 3 and a projection lens 4 are also arranged on the axis. The latter projects an image of the slide film 1 onto an image in the direction of arrow B, projection screen not shown. The objective 4 is axial in a slide 4a adjustable, in turn by a motor 15 (Fig.2) along the optical axis is movable.

With a light source behind a slit 6 is designated, which throws a light beam 7 onto the slide.

The light beam is reflected on the slide and reaches a differential photoresistor 8, which is an electrical part of the control circuit of the electric motor 15. The source of light 5 and the photoresistor 8 are firmly connected to the carriage 4a.

In Fig.la the slide film 1 is held in a cardboard frame 9, namely without a covering glass pane and is in the plane of the film carrier FB. Suppose that for a sharp image of the slide film 1 on the projection screen an object distance A between the lens 4 and the slide film 1 must be observed. In this position of the slide film 1, the light beam 7 is centered on the differential photoresist 8 reflected. The position of the slide film 1 along the optical axis 2 changed from the distance A out, one or the other part of the resistance becomes 8 more or less illuminated. This ultimately causes the electric motor 15 to work and move the lens until the slide film 1 again stands at distance A.

Instead of a cardboard frame without glass panes, the slide film 1 but e.g. also be held in a plastic frame in which the film is on both sides each covered with a pane of glass 10a, 10b, each of which is an integral part her frame half is. These glass panes generally have a thickness dA / 0.9 mm.

If such a slide is now brought into the beam path (Fig.lb) and If the slide film 1 is also in the plane FB, the reflection of the takes place Light beam 7 no longer on the slide film 1 but already on the front the Glass pane 10a. The reflected beam now does not hit the photoresistor 8 more symmetrically, but the photoresistor is illuminated asymmetrically. this has the consequence that the voltage tapped at the photoresistor for the motor control sinks. This causes the motor 15 to start up and to move the carriage 4, with the tendency to return the point of reflection symmetrically to the optical axis or to relocate to the phto resistor.

Without the invented circuit, the carriage 4a would be around the track d in the representation of Fig.lb shifted to the right up to the dashed line Position. The distance between slide film 1 and lens 4 would then be A + d, and the slide film 1 would thus lie outside the plane of focus.

The invented circuit, however, causes this shift only partially takes place. The slide would only be displaced by a distance As. The glass pane 10a brought into the beam path extends in accordance with the laws In geometrical optics, distance A by a distance #s which is about 1/3 of the thickness d of the glass pane 10a is n. The exact value of #s is = (1 - n,). D, where n is the refractive index of the glass and n is the refractive index of the surrounding Medium is. The glass pane 10b does not play a role in this consideration.

The objective 4 must be moved further away from the slide film 1 by this distance Asl / 3d so that visually the same spacing ratios are again compared to Fig.la prevail and the film 1 is sharply formed on the projection screen a.b.

However, the reflection of the light beam 7 takes place at the distance A + 6s not at the level of the optical axis, but outside it at point C, and thus the photoresistor 8 remains illuminated asymmetrically and delivers a lower voltage UphOto. The invention now consists in providing the encoder with a voltage signal to feed, which in this case has the same size as UphOto, because then remains the controller controlled by the encoder and with it the motor 15 in the idle state.

This voltage signal is obtained by measuring the photocurrent 1photo and will now be explained with reference to Fig.2. In Figure 2 is again the Differential photoresistor 8 shown, between its two partial resistances the voltage tap Uphoto takes place. In addition, the photocurrent is Iphoto at point D. tapped.

The circuit assumes that on the glass pane 10a (Fig.lb) significantly more light is reflected than on the slide film 1 itself (Fig.la). In the When projecting a glazed slide, the photoresistor 8 will therefore have an increased light intensity applied, which leads to the flow of a stronger current through this resistor. For the strength of the flowing current is the resistance value of the individual Partial resistances, i.e. the symmetrical or asymmetrical exposure of the differential photoresist irrelevant, the electricity only depends on the illuminance the total resistance.

This illuminance and thus the strength of the photocurrent at The projection of a glazed and an unglazed slide can be determined empirically, and in a reference current source 11, corresponding reference currents 1Glasdia> 1 paper slide and 1 no slide provided. These reference currents on the one hand and the Photocurrent 1photo, on the other hand, is fed to a comparator 12. The comparator determines the agreement of the photocurrent with one of the reference currents and gives a corresponding voltage signal Upappdia. UGlasdia or Ukein slide on one Encoder 13, which is designed as an OP amplifier.

In accordance with this voltage signal, the OP amplifier in turn inputs Control signal to the control 14 of the electric motor 15, which the lens slide adjusts accordingly.

For explanation it is assumed that the resistor 8 in a cardboard slide is illuminated symmetrically in the film stage and thus the greatest tension Uphoto supplies. 1photo is less with a cardboard slide than with a glass slide, nevertheless the comparator 12 delivers the highest voltage signal UPappdia. "Recognizes" the comparator however, that a glass slide is inserted, it delivers a lower voltage signal UGlasdia The OP amplifier, i.e. the regulator 13 to which this lower voltage signal UGlasdia is fed, in turn, gives a control signal to the motor control 14 until the motor 15 has moved the carriage 4a into a position in which the voltage tapped at the resistor 8 is equal to the voltage signal UGlasdia is. This is the case when the resistor 8 is asymmetrical in the manner shown in Fig.lb is illuminated, in that the slide 4a with lens 4 is in the extended position occupies, in which the lens 4 is at a distance A + ds from the slide film 1.

Hence: by determining the level of the voltage signal, determining one also the amount of displacement of the slide with lens.

But it also follows from this that in the case of the projection of a cardboard slide the photoresistor 8 does not necessarily have to be illuminated symmetrically. It must only to be taken care of that the voltage signal Upappdia exactly is as large} as the voltage UphOto, which the resistor 8 when a Paperboard slide and setting of the slide 4a to the middle of its displacement path D gives away. Whether the resistor 8 is actually illuminated symmetrically or is not secondary, the whole thing is just a question of balancing.

If there is no slide in the beam path, there is no reflection and thus no illumination of the differential photoresistor 8. The latter remains high resistance and thus UphOto corresponds roughly to the supply voltage.

Therefore, if the voltage signal Ukein Dia is given, for example, the supply voltage on the OP amplifier, no control signal for the motor control 14 is output, i.e.

the latter is shut down.

After the presetting of the slide 4a by the OP-amplifier Any change in the position of the slide film is effected 1 or

of the glass slide in a known way a change in the voltage UphOto, which leads to the output of a control signal from the OP amplifier to the motor control 14, i.e. the normal function of an autofocus circuit comes into effect and regulates in a known manner the lens in the preset position for the purpose of automatic Maintaining the sharpness of the image.

In addition, the circuit is in the described embodiment an optical indicator 16 for "glass slide", "unglazed slide" and "no slide" is also provided. The indicator 16 is also controlled by the voltage signal from the comparator 12 and finally causes the display by lighting up different colored LEDs can still use a switch 17 of the electric motor 15 from the autofocus circuit separated and switched to manual focusing.

It should be noted that although the invented Circuit is explained here using an exemplary embodiment in which the light source 5 and the differential photoresistor 8 are connected to the lens 4, the circuit is of course also applicable when the light source 5 and the resistor 8 are both arranged stationary or only one of these components with the lens or are connected to the film stage.

Finally it should be mentioned that the function of the autofocus circuit Mainly only with slides in cardboard frames is necessary to the image sharpness to be maintained even when the slide film is heated in the projection beam path "jumps" in the known way. Since with glass slides the film is between two panes of glass, he has no way to jump, or at most in areas of about 0.1 mm to each side, which is a noticeable blurring of the projected image. The autofocus circuit is used for glass slides therefore mainly to effect the preset discussed above when one disregards the fact that the glass slide in the guide of the film stage also occasionally wobbles, which is then compensated for by the autofocus.

Claims (3)

Claims 1. Autofocus circuit for slide projectors with an electric motor for moving the film stage or the lens along the optical axis, with a differential photoresist emitted by a light source and applied to the slide in the projection position reflected light beam and with a voltage tap on the differential photoresistor due to this Voltage relative to the two poles of the supply voltage of the electric motor Clockwise or counterclockwise rotation is switched or stopped, characterized by the following Features: a the circuit includes a power tap for tapping one of the above the differential photoresistor flowing photocurrent proportional to the partial current (Iphoto), b the circuit contains a reference current source (11) for outputting reference currents (I glass slide; 1 cardboard slide 'I no slide) that correspond to the partial flow (photo) if a Glazed slide, an unglazed slide or if there is no slide in the projection beam path stands, c the circuit contains a comparator (12) for comparing the partial flow ((photo) with the reference currents and to output a corresponding Voltage signal, i.e. a transmitter (13) is planned to which the voltage signal is supplied and which in turn supplies a corresponding control signal to the controller (14), which causes the motor controller to set a preset that is matched to the projected slide to effect the film stage or the lens. 2. autofocus circuit according to claim 1, characterized in that with the comparator (12) optical display means are electrically connected, the corresponding the output control signal take effect and indicate whether a glazed, an unglazed slide or no slide is in the projection beam path. 3. autofocus circuit according to claim 2, characterized in that the optical display means (16) are light-emitting diodes of different colors,