Device for illuminating an auxiliary track of a motion picture film

A laser diode (4) emits a beam which is reflected by a plane mirror (12) and by a spherical focusing mirror (2) in order to form a beam (40) which is focused on a read window which is a segment of a read line (6). Said line extends transversely relative to a film (16) running over a drum (20) which rotates about an axis (22). The window occupies the width of the soundtrack on the film. The light passing through said soundtrack is received by a photo cell (34) which provides the sound signal. The invention is particularly applicable to sound motion pictures.

The present invention relates to projecting motion pictures. 
BACKGROUND OF THE INVENTION 
A film used in such projection comprises a main track which occupies the 
major fraction of the width of the film which has the images to be 
projected following one another thereon. Suitable means are provided for 
causing the main track to run image-by-image between a source of intense 
light and an objective lens. From the recorded image as illuminated in 
this way, the lens projects an image on a screen. To the side of said main 
image-carrying track, the film includes at least one auxiliary recording 
track, which in practice is a soundtrack conveying a sound signal, but 
which could equally well convey some other type of auxiliary signal, for 
example a clock signal or a synchronizing signal, etc. . . . 
Recording on said track is performed optically, i.e. the recorded signal is 
represented by an optical density. It is present practice for a soundtrack 
to comprise a transparent central fraction of its width together with two 
opaque fractions extending symmetrically on either side thereof. The 
signal is constituted by the variable width of the transparent central 
fraction. 
In order to read such a track, it is caused to run at constant speed 
between a light-emitting head and a photoelectric cell which provides an 
electrical signal representative of the sound signal to be reproduced. 
Such a signal may be amplified and applied to loudspeakers in a cinema. 
The light-emitting head illuminates the film by forming a read window whose 
length runs across the width of the auxiliary track and whose width 
extends in the direction of film displacement. 
The auxiliary signal is written, i.e. recorded, in an analogous manner 
using a light-emitting head in conjunction with a read window which is 
illuminated over a fraction only of its length, with said fraction being 
modulated by said signal. 
The passband of such a device, i.e. the band of frequencies which it is 
capable of reproducing, falls off with increasing width of the read 
window. It is therefore desirable to reduce the width of said window. This 
effect is due to the fact that the speed at which the film runs is 
determined by other constraints, and to the fact that the sensitive area 
of the photo-electric cell extends over a greater width than the width of 
the above-defined read window, thus ensuring that the width of the photo 
cell is not a limiting constraint. 
One known technique for reducing the width of the read window is to use a 
light-emitting head including a relatively powerful light source which 
illuminates a diaphragm having a slit therein. An image of the slit 
illuminated in this way is then formed on the film by an objective lens. 
In this prior apparatus, the slit constitutes the above-defined read 
window. 
This prior device suffers from two drawbacks in particular. Firstly, the 
optical quality of the objective lens makes it impossible to obtain a 
window which is as narrow as could be desired, even if the slit through 
the diaphragm is itself sufficiently narrow. This limits the passband of 
the device. Further, given the limited sensitivity of the photo cell, the 
light source must be very powerful indeed since only a small fraction of 
its light passes through the narrow slit through the diaphragm. This gives 
rise to energy consumption and heating which may be objectionable. 
Specific objects of the present invention include: 
increasing the passband of a read and/or write device for use with an 
auxiliary track of a motion picture film (and in particular with a 
soundtrack); and/or 
reducing the electrical power consumed and/or the heat power given off by 
such a device; and/or 
making adjustment of such a device easy; and/or 
making said adjustment independent of the wavelength of the light used; 
and/or 
facilitating the insertion of such a device in an existing motion picture 
apparatus by installing in said apparatus a light-emitting head which is 
compact and/or easy to manufacture. 
SUMMARY OF THE INVENTION 
The present invention provides a read and/or write device for use with an 
auxiliary track (in particular a soundtrack) of a motion picture film, the 
device being intended to provide at least one read signal representative 
of an average optical density over at least one narrow read window which 
is elongate along a read line, and/or to write such a density so as to 
enable such, a read signal to be generated during subsequent reading, the 
line being a line transversal to a film which runs along a run direction, 
the length of the read window covering, at least ,in part, the width of an 
auxiliary recording track written on the film to record an auxiliary 
signal such, that the read signal or the written density represents the 
auxiliary signal, the parasitic width of the window being oriented along 
the direction in which the film runs and limiting the frequency passband 
of the device. 
said device comprising for this purpose: 
the device comprises a source of light; an optical system forming the light 
into a read and/or write beam focused on the film in order to illuminate a 
narrow light spot which forms the read window; and a transducer such as a 
read photo cell receiving the light which is transmitted or reflected by 
the film in response to the read beam, the photo cell providing the read 
signal in response to the received light, and/or a modulator modulating 
the write beam, in response to the auxiliary signal to be written; 
the light source is selected to form an incident light beam which has at 
least a virtual point origin at a finite or an infinite distance and which 
propagates along a mean incident direction, with the point origin being 
disposed on the read line. 
the optical system is constituted by a concave focusing mirror whose 
surface is a portion of a surface of revolution about an axis of 
revolution which coincides with the read line, such that all light rays 
coming from the point origin and encountering the mirror are reflected 
towards the read line, with the normal to the center point of the mirror 
forming a mean angle of inclination with the mean incident direction, the 
angle of inclination and the angular extent of the mirror on either side 
of the central point in a plane passing through the read line being 
selected so that the beam reflected by the mirror illuminates the entire 
length of the read window. 
Preferably, the mean angle of inclination lies between 5.degree. and 
85.degree.. 
Also, preferably, the surface of revolution is a spherical surface so as to 
be easily formed, and so that coincidence between one of the axes of 
revolution and the read line is intrinsically achieved by situating the 
point origin and the center of the read spherical surface on the line. 
The present invention also provides a head which is intended for mounting 
in a motion picture apparatus in order to constitute a read and/or write 
device, the head comprising: a frame; the light source carried by the 
frame; the focusing mirror carried by the frame; and frame fixing means 
for fixing the frame to the motion picture apparatus in such a manner that 
the position of the source relative to the mirror is adjusted prior to 
fixing the head to the motion picture apparatus. 
The head has a longitudinal direction along a mean direction of the read 
beam, with a front end and a rear end such that the beam propagates from 
the rear end towards the front end, with two directions substantially 
perpendicular to each other and to the longitudinal direction being a 
running direction which is intended to be oriented parallel to the running 
direction of the film, and a transverse direction oriented parallel to the 
read line. 
The invention preferably includes a front assembly and a rear assembly 
interconnected by the frame, the rear assembly comprising the focusing 
mirror and the source of light, with the source being offset relative to 
the mirror along said transverse direction towards a side which is a light 
arrival side, the source emitting a primary light beam which is forwardly 
directed. The front assembly comprises a mirror receiving the primary beam 
and reflecting it rearwardly towards the focusing mirror in order to form 
the incident beam. 
The frame fixing means are in the form of a hollow cylindrical or prismatic 
fixing structure having generator lines extending along the longitudinal 
direction, the fixing means belonging to the front assembly and locally 
surrounding the read and/or write beam, thereby enabling the frame to be 
fixed by the fixing structure sliding in an illumination opening formed in 
a support wall belonging to the motion picture apparatus, with the opening 
being provided to pass said read beam. 
The mirror is offset relative to the structure along the transverse 
direction towards the light arrival side. 
The head further includes means for adjusting and fixing its position, the 
means bearing against said wall of the support in order to enable the 
position of the read beam to be adjusted by sliding the fixing structure 
in the illumination opening. 
The light source is a semiconductor laser diode emitting a primary beam 
having a point origin at finite distance, the beam having relatively low 
divergence (about 20.degree. ), thereby avoiding objectionable energy 
losses and/or heating. 
The present invention is naturally equally applicable to cases where the 
above-mentioned auxiliary signal is in analog form or is in digital form.

DETAILED DESCRIPTION OF THE INVENTION 
As shown in FIG. 1, a moving picture projector serves to project a film 16 
which is paid out from a supply spool (not shown) and which is taken up by 
a take-up spool (also not shown). The projector performs two functions 
simultaneously. It projects images on a screen and it provides amplifiers 
and loudspeakers with two electrical signals representative of 
stereophonic sound. The film is provided, for this purpose, with a main 
track for the images and with two auxiliary tracks disposed on the same 
side of the main track for conveying the sound. In order to simplify the 
explanation, it is assumed below that there is only one such track and it 
is referred to as the soundtrack. 
The film passes initially in front of a powerful projection lamp 28 which 
illuminates an image via a condenser lens 30. The light transmitted 
through said recorded image is projected by an optical lens 32 on a 
projection screen (not shown). The film is driven discontinuously, 
image-by-image, by a drive device (not shown). Thereafter, it forms a loop 
17 of cyclically variable length enabling it to be subsequently driven 
continuously over a read drum 20 by a take-up wheel 36 (see FIG. 4), 
thereby enabling the soundtrack to be read. To this end, the drum 20 which 
rotates about an axis 22 parallel to said transverse direction is narrower 
in said transverse direction than the film, thereby allowing light to pass 
freely through the soundtrack. 
A photo cell 34 having a sensitive area 35 (FIGS. 2 and 4) is located on a 
longitudinally forward side of the film in the immediate proximity of the 
film and the drum 20 and facing the soundtrack, thereby enabling it to 
receive that fraction of illuminating light which has passed through the 
soundtrack and thus serving to supply the read signal. This signal is 
representative of the sound which corresponds to the images being 
projected at the same instant. 
The illuminating light in question is directed onto the film in the form of 
a read beam 40 by means of a removable light-emitting head comprising a 
frame 14 which is generally in the form of a truncated hollow cone about a 
longitudinal axis 42 coinciding with the axis of the beam 40. This frame 
fits more or less around the rear portion of the read beam 40 where it 
exists inside the frame. It is forwardly extended from its small base as 
shown in more detail in FIG. 3, by a short tubular fixing structure having 
longitudinally extending generator lines 25 and which is a push fit in a 
circular illumination opening. This opening is formed in a thick support 
wall 27 forming a part of the main frame of the projector. It faces the 
photo cell 34. Fixing and position-adjusting means are provided for the 
frame. They are represented symbolically in FIG. 3 in the form of a screw 
24 whose head bears against the front face of the wall 27 and whose shank 
passes through the wall 27 and screws into a tapped hole fixed to the 
frame 14. Slack is taken up by a return spring 26 surrounding the shank of 
screw 24 and pressing against the rear face of the wall 27 of the 
projector frame. 
The emitted light is monochromatic and coherent. It is emitted in the form 
of a primary beam 3 having a wavelength of 0.78 micrometers, for example, 
by a laser diode 4 which is carried at the rear of the frame 14 and which 
forms a portion of the above-specified rear assembly 2, 4. This diode is 
powered and regulated from a housing 5. In a write device, it would be 
modulated from said housing. It is mounted in such a manner as to enable 
its position to be adjusted. It provides a low divergence beam having a 
substantially point origin. This beam is directed forwardly and is 
reflected by a plane mirror 12 carried at the front of the frame 14 in 
order to form the above-specified incident beam 9. The mirror is disposed 
in such a manner as to cause the virtual point origin 8 of the incident 
beam (possibly after adjusting the position of the diode 4) to be situated 
on a read line 6 which is in alignment with a generator line of the drum 
20 facing the sensitive area 35 of the photo cell 34. The desired result, 
which is indeed obtained, is that the read line 6 is in the form of an 
extension of a line transverse to the length of film whose main track is 
pressing against the drum 20. The portion of the read line 6 which extends 
over the width of the soundtrack constitutes the above-mentioned elongate 
read window. 
The plane shape of the mirror has the advantage of forming an 
aberration-free point virtual image, while also being simple to make and 
cheap. 
The position of the read image is naturally adjusted so as not to be 
exactly on a generator line of the cylindrical surface of the read drum 
20, but so as to lie on the printed face of the film 16. 
Although the incident beam constituted in this way has a point origin at 
finite distance, it should be understood that it would also be possible, 
within the scope of the present invention, to make use of an incident beam 
whose point origin would still lie on the same read line, but at an 
infinite distance, i.e. for the incident beam to be a parallel beam whose 
direction corresponds with the direction of the read line. 
The read beam 40 is formed from the incident beam 9 by reflection on a 
focusing mirror 2 which is carried at the rear of the frame 14. The 
incident beam is allowed to pass through a large opening 15 formed for 
this purpose in the frustoconical wall of the frame 14. The mirror 2, or 
more precisely its reflecting surface, constitutes a portion of a 
spherical surface whose center 10 is situated on the read line 6 
substantially half-way between the virtual source 8 and the middle of the 
width of the soundtrack on the film 16. This center 10 constitutes the 
center of curvature of the mirror 2. 
Although the use of a spherical surface appears to be advantageous in the 
context of the present invention for reasons of ease of manufacture, it 
must be understood that the essential point is merely that the focusing 
mirror constitutes a surface of revolution about the read line. This 
ensures that any incident light ray considered as being a straight line 
passing through the point source 8 is reflected by the focusing mirror 
along a ray direction which passes exactly through the read line. This 
ensures that the width of the read window formed on the film by the rays 
is theoretically nil, the width being measured in the direction of film 
displacement. 
This is true for geometrical objects, but it should be understood that in 
fact phenomena related to diffraction and to the size of the source ensure 
that the width may be greater than the wavelength of the light being used 
but is nevertheless much smaller than widths obtainable in prior art 
devices for the same amount of illumination. 
Nevertheless, the entire width of the soundtrack must be illuminated. This 
is obtained, in particular, by suitably choosing the above-mentioned mean 
angle of inclination. This angle 7 may be measured, for example, from the 
center 11 of the mirror 2. It extends between directions going from the 
center towards said point source 8 and towards said center of curvature 
10. It is equal to 10.degree., for example, so that the angular extent of 
the incident beam is equal to 20.degree., for example. 
The advantages of the device described stem, in particular, from the fact 
that practically all of the light energy emitted by the diode 4 passes 
through an extremely narrow read window. However, it may be advantageous 
to loose a small fraction of this energy by causing the reflecting surface 
of the mirror 2 or of the mirror 12 to be specially shaped to make the 
illumination of the window as uniform as possible, e.g. by having a 
surface which is generally rectangular in shape with concave sides. 
Alternatively specially shaped could be used for the same purpose.