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An electromechanical optical shutter
This content has been downloaded from IOPscience. Please scroll down to see the full text. 1979 J. Phys. E: Sci. Instrum. 12 678 (http://iopscience.iop.org/0022-3735/12/8/003) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 131.91.169.193 This content was downloaded on 28/08/2015 at 09:19
Please note that terms and conditions apply.
Apparatus and techniques J. Phys. E: Sci. Instrum., Vol. 12, 1979. Printed in Great Britain
An elect ro mec han ical optical shutter P Naish University of Oxford, Department of Experimental Psychology, South Parks Road, Oxford OX1 3UD, UK Receiced 8 August 1978, in final form 13 March 1979
Abstract Constructional details are given of an inexpensive and easily made electromechanical shutter. The device may be placed at any convenient point in an optical system, without modification to that system, and causes no scanning of the final image. There are many situations in which it is required to establish a period of illumination, with well defined moments of onset and offset. Two basic methods are available for producing this effect: the light source may be turned on and off, or the light path may be interrupted with a shutter. Being nonmechanical, the former technique obviously has advantages over the latter, but in practice it is not without its difficulties. Clearly the slow response of a filament lamp rules out its use and the variation in rise time with wavelength in fluorescent tubes renders them unsuitable for some situations. Thus, Mollon and Polden (1978) have shown that many tachistoscopes using such lamps, whilst exhibiting very fast broadband responses, are in the important visible region producing rise and fall times approaching 20 ms. For many applications an electronically controlled shutter is ideal and the device described here was constructed for use in front of a standard 35 mm projector, set up for making psychophysical measurements. Commercially made electromechanical shutters exist, but many suffer from a number of disadvantages, chief of which is often the extremely high cost. It was decided, therefore, to produce a shutter of simple construction, using inexpensive, readily available materials and possessing the following features : (i) the instrument should not require modification to the optical system into which it was inserted; (ii) even when not placed at a point of convergence in the light path, the shutter should cause no perceptible 'scanning' across the image; (iii) simple changing from normally closed to normally open operation would be desirable. To achieve a high operating speed with the shutter it was necessary to devise a low-inertia system, with a low level of air damping; obviously lightness of moving parts is indicated. To reduce inertia further, it was decided to use a system based on rotation, with a small radius of gyration and small angular displacement; a decrease in these quantities also reduces air resistance. To cover a lens of reasonable aperture requires a fairly large shutter area, so, if the radius of gyration were not to be too large, it would be necessary to divide the area into a number of independently pivoted sections, giving the appearance of a Venetian blind. The total inertia of such a system is the sum of its parts, whereas the moment of inertia of a single rotating body is proportional to the square of the radius. Hence, within reason, the greater the number of sections into which the shutter is divided the better. Moreover, with 0022-3735/79/080678+ 02 $01.00 @J 1979 The Institute of Physics
narrower individual sections the shadowing caused by obstruction of non-paraxial rays will be reduced. However, as well as practicality of construction, the increase in friction at the pivots becomes a controlling factor. The prototype shutter was to be used in front of a 50 mm diameter projector lens and it was found convenient to divide the area into six independent vanes; it will be appreciated that the simultaneous opening of six regions, right across the lens aperture, virtually eliminates scanning effects in the projected image. The broad concept of this design is clearly not entirely novel; for example, the Focal Encyclopedia of Photography (1965) sketches the principle in a list of early forms of camera shutter, although no details of the drive mechanism are included. Figures 1 and 2 and the following details will be adequate for one to be able to design a shutter to suit any system; not all dimensions or details are given, since size requirements and application will dictate the number of vanes, type of support stand and so on.
Foil vane Solenoid
Nylon filament PTFE
bearing
Figure 1 An isometric view of the shutter.
U
10 mm
Travel and tension adjustment
Locking sci-ew
Figure 2 A side elevation of the shutter.
The frame of the shutter consists of two aluminium plates, drilled to take four silver steel spacing dowels. The long axis of each plate is also drilled, to take a series of nylon, or better, PTFE, plugs cut from a rod of the material. These form the bearing surfaces for the vanes, and each plug is drilled to take a 1.5 mm diameter vane support. In the prototype these have been formed from silver steel but, to avoid the possibility of corrosion in the bearings, although none has been found to date, it may be preferred to use another material, such as stainless steel, for the supports. These shafts are cut to length and then placed in an electric drill to file, emery and finally metal-polish the ends into smooth hemispheres, which run
Apparatus and techniques inside the PTFE.Near one end of each of these rods fine emery cloth is used to produce a slightly matt surface, which prevents slip of the operating cord. The centre sections of the rods are filed away, to leave a thin ‘knife-blade’ support region for the vanes. These are cut from aluminium foil and glued in place: they may be painted matt black. The drive cord for the vanes is a fine nylon monofilament, ‘invisible’ sewing thread being found most suitable. The cord is supported at one end by a spring, is passed once round each vane support, in the roughened region, and then attached to a solenoid. The plunger of the latter is adjusted to give sufficient motion to rotate the vanes through 90”. Clearly the vanes may be set up in the normally open or normally closed configuration and angular adjustment is most easily effected by means of a thin metal rod, covered with rubber sleeving, to prevent slip. This tool, rubbed against the support rods, rotates them against the restraint of the drive cord.
References Mollon J D and Polden P G 1978 On the time constants of tachistoscopes Q. J . Exp. Psycho/. 30 555-68 F ~Encyc/opedia ~ ~ /ofPho~og,.aphy1965 vo1 2 (London and N~~ York: ~~~~l press) p 1368
ligorc 3 An oscillograph of the shutter response to a 20 ms pulse. The behaviour of the prototype shutter hzs been examined, using an oscilloscope and photocell, and figure 3 shows a typical response to a 20 ms pulse. The horizontal scale is 5 ms/division. The rise and fall times are each of the order of 2-3 ms; to some extent a trade-off can be achieved between these, by adjusting the spring tension to make one faster at the expense of the other. In the short term the shutter exhibits very good reproducibility: figure 3 was taken from a storage oscilloscope and consists of three superimposed traces obtained from successive operations of the shutter. There is no visible difference between the exposures. Long-term reliability is also good and two further shutters have been constructed which, together with the first shutter and three projectors, form a three-field projection tachistoscope, controlled by TTL circuitry. Also, a very compact two-vaned version has been produced, for use on an Aimark projection perimeter. Apart from being too slow for certain applications, the only major disadvantage of this form of shutter is that in the open position there is still a degree of obstruction of the light. It was found that placing an open shutter over a projector lens focused on a screen 5 m away reduced the incident light by only 7%. For most purposes this reduction is acceptable and it may be concluded that in many circumstances this form of shutter would make a good low-cost alternative to other forms of rapid light control.
Acknowledgments I am grateful to Peter Coughlin for very helpful discussions during the development of this apparatus. The design and construction were carried out whilst the author was in receipt of an SSRC Studentship. 679
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My IOPscience
An electromechanical optical shutter
This content has been downloaded from IOPscience. Please scroll down to see the full text. 1979 J. Phys. E: Sci. Instrum. 12 678 (http://iopscience.iop.org/0022-3735/12/8/003) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 131.91.169.193 This content was downloaded on 28/08/2015 at 09:19
Please note that terms and conditions apply.
Apparatus and techniques J. Phys. E: Sci. Instrum., Vol. 12, 1979. Printed in Great Britain
An elect ro mec han ical optical shutter P Naish University of Oxford, Department of Experimental Psychology, South Parks Road, Oxford OX1 3UD, UK Receiced 8 August 1978, in final form 13 March 1979
Abstract Constructional details are given of an inexpensive and easily made electromechanical shutter. The device may be placed at any convenient point in an optical system, without modification to that system, and causes no scanning of the final image. There are many situations in which it is required to establish a period of illumination, with well defined moments of onset and offset. Two basic methods are available for producing this effect: the light source may be turned on and off, or the light path may be interrupted with a shutter. Being nonmechanical, the former technique obviously has advantages over the latter, but in practice it is not without its difficulties. Clearly the slow response of a filament lamp rules out its use and the variation in rise time with wavelength in fluorescent tubes renders them unsuitable for some situations. Thus, Mollon and Polden (1978) have shown that many tachistoscopes using such lamps, whilst exhibiting very fast broadband responses, are in the important visible region producing rise and fall times approaching 20 ms. For many applications an electronically controlled shutter is ideal and the device described here was constructed for use in front of a standard 35 mm projector, set up for making psychophysical measurements. Commercially made electromechanical shutters exist, but many suffer from a number of disadvantages, chief of which is often the extremely high cost. It was decided, therefore, to produce a shutter of simple construction, using inexpensive, readily available materials and possessing the following features : (i) the instrument should not require modification to the optical system into which it was inserted; (ii) even when not placed at a point of convergence in the light path, the shutter should cause no perceptible 'scanning' across the image; (iii) simple changing from normally closed to normally open operation would be desirable. To achieve a high operating speed with the shutter it was necessary to devise a low-inertia system, with a low level of air damping; obviously lightness of moving parts is indicated. To reduce inertia further, it was decided to use a system based on rotation, with a small radius of gyration and small angular displacement; a decrease in these quantities also reduces air resistance. To cover a lens of reasonable aperture requires a fairly large shutter area, so, if the radius of gyration were not to be too large, it would be necessary to divide the area into a number of independently pivoted sections, giving the appearance of a Venetian blind. The total inertia of such a system is the sum of its parts, whereas the moment of inertia of a single rotating body is proportional to the square of the radius. Hence, within reason, the greater the number of sections into which the shutter is divided the better. Moreover, with 0022-3735/79/080678+ 02 $01.00 @J 1979 The Institute of Physics
narrower individual sections the shadowing caused by obstruction of non-paraxial rays will be reduced. However, as well as practicality of construction, the increase in friction at the pivots becomes a controlling factor. The prototype shutter was to be used in front of a 50 mm diameter projector lens and it was found convenient to divide the area into six independent vanes; it will be appreciated that the simultaneous opening of six regions, right across the lens aperture, virtually eliminates scanning effects in the projected image. The broad concept of this design is clearly not entirely novel; for example, the Focal Encyclopedia of Photography (1965) sketches the principle in a list of early forms of camera shutter, although no details of the drive mechanism are included. Figures 1 and 2 and the following details will be adequate for one to be able to design a shutter to suit any system; not all dimensions or details are given, since size requirements and application will dictate the number of vanes, type of support stand and so on.
Foil vane Solenoid
Nylon filament PTFE
bearing
Figure 1 An isometric view of the shutter.
U
10 mm
Travel and tension adjustment
Locking sci-ew
Figure 2 A side elevation of the shutter.
The frame of the shutter consists of two aluminium plates, drilled to take four silver steel spacing dowels. The long axis of each plate is also drilled, to take a series of nylon, or better, PTFE, plugs cut from a rod of the material. These form the bearing surfaces for the vanes, and each plug is drilled to take a 1.5 mm diameter vane support. In the prototype these have been formed from silver steel but, to avoid the possibility of corrosion in the bearings, although none has been found to date, it may be preferred to use another material, such as stainless steel, for the supports. These shafts are cut to length and then placed in an electric drill to file, emery and finally metal-polish the ends into smooth hemispheres, which run
Apparatus and techniques inside the PTFE.Near one end of each of these rods fine emery cloth is used to produce a slightly matt surface, which prevents slip of the operating cord. The centre sections of the rods are filed away, to leave a thin ‘knife-blade’ support region for the vanes. These are cut from aluminium foil and glued in place: they may be painted matt black. The drive cord for the vanes is a fine nylon monofilament, ‘invisible’ sewing thread being found most suitable. The cord is supported at one end by a spring, is passed once round each vane support, in the roughened region, and then attached to a solenoid. The plunger of the latter is adjusted to give sufficient motion to rotate the vanes through 90”. Clearly the vanes may be set up in the normally open or normally closed configuration and angular adjustment is most easily effected by means of a thin metal rod, covered with rubber sleeving, to prevent slip. This tool, rubbed against the support rods, rotates them against the restraint of the drive cord.
References Mollon J D and Polden P G 1978 On the time constants of tachistoscopes Q. J . Exp. Psycho/. 30 555-68 F ~Encyc/opedia ~ ~ /ofPho~og,.aphy1965 vo1 2 (London and N~~ York: ~~~~l press) p 1368
ligorc 3 An oscillograph of the shutter response to a 20 ms pulse. The behaviour of the prototype shutter hzs been examined, using an oscilloscope and photocell, and figure 3 shows a typical response to a 20 ms pulse. The horizontal scale is 5 ms/division. The rise and fall times are each of the order of 2-3 ms; to some extent a trade-off can be achieved between these, by adjusting the spring tension to make one faster at the expense of the other. In the short term the shutter exhibits very good reproducibility: figure 3 was taken from a storage oscilloscope and consists of three superimposed traces obtained from successive operations of the shutter. There is no visible difference between the exposures. Long-term reliability is also good and two further shutters have been constructed which, together with the first shutter and three projectors, form a three-field projection tachistoscope, controlled by TTL circuitry. Also, a very compact two-vaned version has been produced, for use on an Aimark projection perimeter. Apart from being too slow for certain applications, the only major disadvantage of this form of shutter is that in the open position there is still a degree of obstruction of the light. It was found that placing an open shutter over a projector lens focused on a screen 5 m away reduced the incident light by only 7%. For most purposes this reduction is acceptable and it may be concluded that in many circumstances this form of shutter would make a good low-cost alternative to other forms of rapid light control.
Acknowledgments I am grateful to Peter Coughlin for very helpful discussions during the development of this apparatus. The design and construction were carried out whilst the author was in receipt of an SSRC Studentship. 679
