GB2101303A - Automatic focussing for slide projectors - Google Patents

Abstract

Method and apparatus for automatic focussing of slide projectors for glazed and unglazed slides (4). A secondary light beam (25) is used additional to a main projection beam (10) which acts independently of glazed or unglazed slides. The secondary beam is reflected to a double-cell photo receiver (14, 15). The total available intensity of incident light is applied by the photo receiver and is converted in a control connection (16-22) into electrical signals that act on a displacing means (23) for moving the support (9) for the lens (6) which also carries the secondary light source (11, 12). <IMAGE>

Description

SPECIFICATION Process and device for automatic focusing for projectors, for example slide projectors The invention relates to automatic focusing for projectors, for example, slide projectors.

It is known to use light projection via the slide, and a double photo receiver of which its component cells or zones have differential characteristics to incident light, and according to light absorption by which lens displacing means is controlled.

In such known focusing systems as for example according to DE-PS 1251975, one component of the photo receiver is made effectively weaker in relation to incident light, which can be done by diaphragms, masks or other light-weakening measures. This relative weighting permits relatively strong reflections from the front of a forward cover glass to be suitably subdued so as not be overdrive lens displacing means. This weighting does however produce problems, particularly for focusing in response to a secondary light beam for which slide reflections will be of low intensity relative to glare from the main light projection beam. Satisfactory operation is not attainable, at least without expensive glare compensation provisions to avoid misadjustments whether due to main beam glare or to glare from the slide itself.

The invention seeks a simpler solution to the problem of focusing projectors, for example slide projectors where glazed and unglazed slides are readily distinguished and appropriate focal plane adjustments made for both.

The invention solves this problem in such a way that for distinguishing a glazed or unglazed slide the variable total intensity of the light beam coming from the slide is applied to a photo receiver and variable light strength at the receiver surface is used for distinguishing unglazed and glazed slides.

Thus, for automatic focusing of projectors, especially slide projectors with a secondary light beam reflected by the slides to a photo receiver, a secondary beam source is arranged laterally of a displaceable lens carrier on a side thereof, and the photo-receiver has two balanced photoconductive cells also arranged laterally of the main lens support to be positioned according to glazed or unglazed slides by a signal from the photo receiver.

In preferred embodiments, two bias voltage setters connected in series with and one to each side of the photo conductive cells with a detuning potentiometer associated with one of the bias voltage setters, say directly in series. Across the potentiometer, there can advantageously be a transistor controlled by a voltage comparator for a reference voltage transmitter and signal from between the photo-conductive cells.

Advantageously the detuning potentiometer can be set at a constant potential so as to reduce the sensitivity of the control device in relation to fluctuations in the luminosity of the secondary light source.

Position control of a glazed slide there can be via a further control circuit and a throw-over switch, controlled by the first control circuit and operative to switch between outputs of the those control circuit in supplying a motor for adjustment of the main lens carrier. Then, the further control circuit has a reference value and an actual position value inputs and operates to null the voltage difference.

The light strength of a reflected secondary light beam is, with a glazed slide and due to its front and rear cover glasses much higher on the average, typically twice as much as for an unglazed slide as relatively light intensive reflections arise from air-glass-surfaces. Known arrangements are thus balanced if one half of such reflected beam strength is applied to two photo receivers, i.e. with one photo receiver receiving only a variable predetermined fixed portion of the beam strength based on an aprropriate weakening of the secondary beam. The total intensity of the light beam is thus present but not significant, whereas, according to the invention simple use is made of that total light intensity with obvious consequent advantage.

Embodiments of the invention will now be described in more detail, by way of example, with reference to the schematic accompanying drawings.

Figure 1 shows a slide with a first embodiment of device for automatic focusing; Figure 2 shows an idealised intensity diagram for the reflections from an unglazed slide; Figure 3a is an idealised intensity diagram for reflections from a glazed slide with relatively weak reflectivity; Figure 3b shows a similar diagram for a glazed slide with greater reflectivity; Figure 4 shows a second embodiment of automatic focusing control device; Figure 5 shows a third embodiment of automatic focusing control device; Figure 6 shows a fourth embodiment using two control devices.

The projection apparatus has a main light source 1, a heat protection filter 2 and a condenser lens 3. A slide 4 to be displayed is shown fitted in a prism shaped stage 5. This is projected through projection lens 6 to a display plane, not shown but, as will be clear, is dependent on the position of focal plane 7 for any given lens 6. For a particular desired or fixed display plane, the focal plane of the lens 6 will be shifted automatically so that the slide 4 is reproduced sharply at the display plane.

For automatic focusing, there is a secondary light source 11 and a lens 12, arranged displaceably on main lens carrier or support 9 and on one side thereof adjacent to the slide support, and a lens 1 3 and a light receiver including two photo-conductive cells 1 4 and 1 5. These photoconductive cells 14 and 1 5 are parts of the focus control system. The secondary light source 11 with lens 12 is inclined at an angle to the optical axis of the main projection apparatus and, via the lens 13, produces a convergent light beam 25 incident upon both of the photo-conductive cells 14 and 15.

Two bias voltage setters 1 6 and 1 7 are connected in series with the two photoconductive cells 14 and 1 5. A detuning circuit is connected to the bias voltage setter 17, which detuning circuit consists of a detuning potentiometer 18, a transistor 1 9 connected in parallel to this and a voltage comparator 20 with a reference voltage source 21. A motor 23 controlled from an amplifier 22 serves to move the lens support 9 until the series resistance connections of the photo-conductive cell 14 and the bias voltage setter 16, on the one hand, and the photo-conductive cell 1 5, the bias voltage distributor 17 and the detuning potentiometer 1 8, on the other hand, balance.

Conditions for an unglazed slide are indicated in Figure 2. Reflection 30a from the slide 4 impinges upon the double photo-receiver 14, 15 with an idealised intensity distribution 30b. Reflection due to glazing of the main projection apparatus is ignored as it will always contribute regardless of the nature of the slide, and be a substantially constant component. The photo-conductive cells 14 and 1 5 respond to the incident light strength E-1 by presenting a corresponding electrical resistance R-1 (unglazed), which is small in relation to dark or unlit resistance R-O.The corresponding current 1-1 flowing through the photo-conductive cells 1 4, 1 5 produces at its junction with the bias voltage setter 1 7 a voltage V-1 which is compared with the reference voltage V-2 1 of the reference voltage source 21, by the comparator 20. The value of the reference voltage is decided empirically and will be set or adjusted suitably for operation as required herein. The voltage V-l (unglazed) is smaller than the reference voltage V-2 1 so that the control transistor 1 9 short circuits the detuning potentiometer 1 8. The series resistance connections are then so balanced that the resistance values 16 and 14 on the one hand and 15 and 17 on the other hand are equal.

Figures 3a and 3b show conditions for glazed slides 4 with two cover glasses 4a and 4b. The reflection 30c has an idealised intensity distribution 30dfor one type of slide 4 that is sufficiently light transmissive for there to be significant reflection from the backing glass 4b.

The idealised distribution 30e corresponds to a more reflective or unbacked slide. Such reflected light beams are also applied for total absorption by the double cell photo receiver system 14, 15.

The photo-conductive cells 14, 1 5 now have a resistance value R-2 (glazed) corresponding to light strengths E2 or E3, greater than that of E- 1.

The resistance R2 (glazed) is thus smaller than the resistance value R-1 (unglazed) by a factor of 1.5 to 2. The current 1-2 flowing through the photo conductive cells 14, 1 5 is greater than 1-1 and causes a voltage V-2 at the bias voltage distributor 17, which voltage is applied to the voltage comparator 20. The voltage V-2 (glazed) is greater than the reference voltage V-21 causing the transistors 1 9 to be cut off.The detuning potentiometer 1 8 is thus connected to the series connections 1 6, 1 4 and 1 5, 1 7. The result for glazed slides is a requirement to balance the resistance values 1 6, 14 with the resistance values 1 5, 1 7, 18. Accordingly the motor 23 operates in order to correct the reflective focal plane for the glazed slide.

Such movement, i.e. difference between lens carrier positions for glazed and unglazed slides and set by the detuning potentiometer 18, is determined to achieve automatic focusing effective for as many as possible types of glazed slides to give a substantial improvement of display definition, say where glazed and unglazed slides are projected alternately.

Figure 4 shows a simplified system'where me detuning potentiometer 1 8 is always in series circuit with the photo-conductive cells 14 and 1 5.

Resistance conditions will be different for glazed or unglazed slides, so that the motor 23 will be operated via the amplifier 22. The two balance conditions will be empirically determined to give, for both glazed and unglazed slides, suitable positions of the lens support 9 and thus the focal plane 7.

Another version of focusing control is shown in Figure 5. The systems of Figures 1 and 4 can be particularly sensitive to variations of the luminosity of the light source 11. A system according to Figure 5 is not so sensitive as the detuning potentiometer 1 8 is at constant potential. Otherwise, operation is the same as previously described.

A further version of focusing control is shown in Figure 6. Distinguishing glazed and unglazed slides again requires intensity balancing from the light beam 25 within block 27, not shown in detail here. As before, for an unglazed slide, balancing is of the series connections 1 6, 14 and 1 5, 1 7 to move the lens support 9 via the regulator motor 23 into the focal plane 7. A switch 28 is provided to adopt position 28a for connection to lead 29 from the control block 27.

If a glazed slide is detected, then the control block 27 switches the switch 28 via lead 30 to its position 28b for connection to a lead 31 from a further control block 32. This control block 32 then takes over driving into a predetermined constant focal plane 33, in which each glazed slide, independently of the particular type of slide, is reproduced sharply by the lens 6 at a fixed projection plane, again not illustrated.

Accordingly, the slide 4 is so held in the prismshaped stage 5 that each glazed slide independently of the thickness of the symmetric halves of the glass frame always takes up a uniform position in the stage 5.

The control device 32 shifts the lens support 9, via the motor 23, in such a way that the voltage difference between a preset 34 and a potentiometer 35 is regulated to null.

Claims (9)

1. A method of automatically focusing a projector for glazed and unglazed slides, wherein a secondary light beam is reflected from the slide to a photo receiver which produces a control signal for shifting the lens of the projector characterised in that the total intensity of the secondary light beam as reflected from the slide is applied to the photo receiver to produce said control signals differing according to whether the slide is glazed or unglazed.

2. Apparatus for carrying out the process according to claim 1, characterised by a source for the secondary beam arranged laterally of a displaceable lens carrier on a side thereof, and, for reception of secondary beam reflection from the slide, and a double photo receiver with two balanced photo-conductive cells also arranged laterally of the displaceable lens carrier.

3. Apparatus according to claim 2, characterised by two bias voltage setters connected in series with and one to each side of the photoconductive cells, a detuning potentiometer associated with one of said bias voltage setters, and means for deriving said control signal from between the photoconductive cells.

4. Apparatus according to claim 3, characterised in that the detuning potentiometer is in series with said one bias voltage setter.

5. Apparatus according to claim 3, characterised in that the detuning potentiometer is separately supplied with voltage.

6. Apparatus according to claim 3, 4 or 5, characterised by a transistor is connected in parallel to the detuning potentiometer selectively to short circuit the latter according to output from a comparator for a reference voltage and a voltage from the junction of the photo receiver and said one bias voltage setter.

7. Apparatus according to any one of claims 2 to 6, characterised by a further control circuit and a throw-over switch controlled by output from said photo receiver to connect alternatively said control signal or a signal from said further control circuit to a motorfordisplacementofthe lens carrier, said motor being operable so as to null reference and factual position representative signals.

8. Method of automatically focusing a slide projector substantially as herein described with reference to the accompanying drawings.

9. Apparatus for automatically focusing a slide projector arranged and adapted to operate substantially as herein described with reference to, and as shown in, the accompanying drawings.