Linear integrating cavity light source used for generating an intense beam of light

A linear light source for a film scanner includes means for generating an intense beam of light and an elongated cylindrical integrating cavity having diffusely reflective walls, and defining an input port through which the intense beam is introduced into the cavity and an output slit parallel to the long axis of the cylindrical integrating cavity to emit a uniform line of light.

FIELD OF THE INVENTION 
The present invention relates to a linear light source, and more 
particularly to a linear light source for use in a film scanner employing 
a linear image sensor. 
BACKGROUND OF THE INVENTION 
Apparatus such as document and film scanners having linear image sensing 
arrays to produce an electronic image signal by sensing the original a 
line at a time, employ linear light sources to illuminate the original one 
line at a time. U.S. Pat. No. 4,186,431 issued to Engel et al., Jan. 29, 
1980, shows a linear light source for use in a document scanner. The light 
source comprises an elongated incandescent lamp inside a specularly 
reflective tube. The reflective tube is provided with a slit to emit a 
line of light, and a light diffusing strip over the slit to produce a 
pattern of uniform intensity along the slit. 
For a film scanner, operating at normal film projection rates, e.g. 24 
frames per second, an intense uniform source of light is required. For 
optimum scratch suppression, it is also desirable for the light to be 
diffuse and nearly uniform in angular distribution (i.e. Lambertian). The 
straight line light source described in U.S. Pat. No. 4,186,431 suffers 
from the drawback that the incandescent light source is not as bright as 
desired for a high resolution film scanner and does not have the best 
color temperature for scanning color film. Intense light sources such as 
Xenon arc lamps and lasers are not produced in a linear configuration like 
the elongated incandescent lamp. Furthermore, the intensity distribution 
produced by the light source can be improved. 
It is therefore the object of the present invention to provide apparatus 
for producing a line of illumination that is free from the shortcomings 
noted above. 
SUMMARY OF THE INVENTION 
According to the present invention, the object is achieved by providing 
apparatus for producing a line of illumination comprising a light source 
for producing an intense beam of light, and an elongated cylindrical light 
integrating cavity having diffusely reflective walls and defining an input 
port through which the intense beam of light is introduced to the cavity, 
and an output slit parallel to the long axis of the cylindrical 
integrating cavity through which a line of diffuse illumination exits the 
cavity. In a preferred embodiment, the input port is arranged off axis of 
the output slit so that the intense beam of light cannot exit the slit 
directly without at least one diffuse reflection inside the cavity. With 
the apparatus of the present invention, the light source may be larger 
than the dimensions of the integrating cavity, thereby permitting the use 
of powerful light sources such as Xenon arc lamps or lasers that do not 
have an elongated geometry. 
According to another aspect of the invention, the temporal uniformity of 
the line light source is improved by further providing a feedback port in 
the integrating cylinder, sampling the light emitted from the feedback 
port with a feedback detector, and controlling the power to the light 
source with a feedback circuit to remove temporal variations. In a 
preferred mode of practicing the invention, the detector is coupled to the 
output port via a fibre optic coupling. 
The angular distribution of the light exiting the output slit is optimized 
for film scanning by shaping the edges of the output slit such that the 
edges of the slit lie in a plane tangent to the surface of the cylindrical 
cavity. In a preferred embodiment of the apparatus, the cavity is formed 
from a reflective polytetrafluoro ethylene material.

MODES OF PRACTICING THE INVENTION 
Referring to FIG. 1, apparatus for producing a line of illumination in a 
film scanner according to the present invention is shown schematically. 
The apparatus includes a light source 10, such as a Xenon arc lamp for 
producing an intense beam 12 of light. The light beam 12 is spectrally 
filtered by a filters 14 to remove infrared and ultraviolet radiation, and 
is focused by a lens 16 onto an input port 18 of a cylindrical integrating 
cavity 20. Although a right circular cylindrical cavity is shown, other 
cross sections, such as rectangular, elliptical, etc. can be employed. 
Preferably, the light is brought to a focus just inside the cavity as 
shown in FIG. 1, and diverges before striking the opposite wall of the 
cavity. The internal surface 22 of the integrating cavity is diffusely 
reflecting. The integrating cavity 20 defines an output slit 24 that emits 
a line of light to illuminate a film 26. The image on the film is sensed 
one line at a time by a linear image sensor 28, such as a CCD linear image 
sensor. The film is imaged on the linear image sensor 28 by a lens 29 and 
is advanced in the direction of arrow A to effect an area scan of the film 
image. 
Since bright light sources such as Xenon arc lamps vary in intensity due to 
wandering of the plasma in the arc, means are provided for stabilizing the 
output of the light source in time. In the prior art, light from the 
source was sampled by placing a partially silvered mirror or beam splitter 
in the light beam, and extracting a sample of the light beam for a 
feedback signal. The present inventors have discovered through 
experimentation that much better temporal control of the illumination 
intensity is achieved by sampling the diffuse light from the integrating 
cavity. Accordingly, a feedback port 30 is provided in the integrating 
cavity 20 to remove a sample of the diffuse light. The light exiting the 
feedback port 30 is directed to a photosensor such as a silicon photo 
diode 32. A neutral density filter 36 is optionally placed over the 
photocell 32 to control the intensity of the light received. The signal 
generated by the photocell 32 is detected in a feedback circuit 34, which 
generates a control signal for lamp power supply 38, to remove intensity 
fluctuations from the light source 10. It has been discovered through 
experimentation that the noise in the light intensity from the output slit 
is further reduced by between 3 to 8 db when a fiber optic bundle 40 is 
employed to direct the light from the integrating cavity 20 to detector 
32. 
FIG. 2 shows a presently preferred embodiment of integrating cavity 20 for 
scanning 35 mm film in a telecine. The integrating cavity is preferably 
machined from a block of diffusely reflecting polytetrafluoro ethylene 
plastic, known as Spectralon.TM. available from the Labsphere Corporation, 
North Sutton, N.H. For the 35 mm scanning application, the integrating 
cavity is a circular cylinder 38 mm long and 20 mm in diameter. The input 
port 18 is a round hole 6-8 mm in diameter, the feedback port 30 is a 
round hole to the end of the cylindrical cavity 4 mm in diameter. The exit 
slot 24 is 2 mm wide by 30 mm long. 
Alternatively, the cavity 20 may be constructed from a material such as 
aluminum, with a diffusely reflective coating on the internal surfaces, 
such as barium sulfate based paint (available as Eastman White.TM. from 
the Eastman Kodak Company, Rochester, N.Y.). 
To provide optimum illumination in a film scanner, it is important to 
tailor the angular distribution of light from the slit so that optimum 
scratch suppression is provided. Experiments were performed on the 
geometry of the sides of the output slit 24 to determine the effect on 
angular distribution of the light, and hence on scratch suppression. FIGS. 
3A-C show different slit geometry configurations for the sides of output 
slit 24 that were tried. In FIG. 3A, the sides slope such that the slit 
widens away from the cavity. In FIG. 3B, the slit narrows away from the 
cavity, and in FIG. 3C, the edges of the slit are formed by a cutting 
plane that is substantially tangent to the circular cavity. FIG. 4 shows 
the angular distributions of light that were measured in a direction 
perpendicular to the slit length for each of the slit configurations 3A, 
B, and C. Tests showed that the configuration shown in FIG. 3C provides 
the best scratch suppression, and is therefore presently preferred. 
FIG. 5 is a graph illustrating the light intensity measured along the slit 
24, showing the good uniformity achieved by the linear light source of the 
present invention. The measured uniformity is a "Frown" of -6% over the 
central 21 mm, with a sharp roll off at the edges. It was found through 
experimentation that uniformity of the light intensity along the slit can 
be further improved by lengthening the cavity, however, the light emitting 
efficiency of the system is thereby reduced. In the preferred mode for use 
in a 35 mm telecine film scanner, the light source was a Cermax.TM. LX 
300F Xenon Arc Lamp in a R300 lamp holder. The power supply is a Cermax 
PS300-1 power supply available from ILC Technology, Sunnyvale, CA. 
The feedback circuit 34 is shown schematically in FIG. 6. The feedback 
circuit includes a first operational amplifier 40 for converting the 
current signal from the photocell to a voltage. A second operational 
amplifier 42 generates a dc reference voltage for controlling the 
brightness of the linear light source. The operational amplifier 42 
receives a reference voltage through a switch S1 from a voltage reference 
V.sub.ref through a first resistor R1 to control the intensity of the 
light for operation of the film scanner at 30 frames per second and a 
second reference voltage through a resistor R2 for controlling operation 
of the film scanner at 24 frames per second. The reference voltage 
supplied by operational amplifier 42 and the voltage from the photocell 
supplied by operational amplifier 40 are combined at a summing node 44 and 
applied to the input of an operational amplifier 46 configured as an 
integrator. Operational amplifier 46 maintains a constant dc level output 
voltage and compensates for instantaneous variation in the signal from the 
photocell. The output voltage from operational amplifier 46 is supplied to 
the base of the transistor 48 operating as a buffer amplifier. The output 
from the transistor 48 is applied to the control input of the lamp power 
supply. 
For larger film formats, e.g. greater than 50 mm wide, it has been found 
that the uniformity of intensity along the slit varies more than is 
desired if the light input is through a single centrally located input 
port as in the examples described above. To improve the uniformity in a 
light source of intermediate length, e.g. 50-75 mm, a cylindrical lens 50 
and a slot shaped input port 18 may be employed as shown in FIG. 7. For 
longer sources, e.g. 75 mm or more, more than one light input source 10 
and 10' may be employed as shown in FIG. 8 introducing the light beams 
through a plurality of input ports 18, 18' as shown in FIG. 8. The 
multiple light beams may also be introduced into the integrating cavity 
through slits as shown in FIG. 7, or through the ends of the cavity as 
shown in FIG. 9. 
An alternative means of introducing the light into the integrating cavity 
for improved uniformity at the output slit is to employ an area-to-line 
fibre optic converter 52 as shown in FIG. 10. The area end of the 
area-to-line fibre optic converter 52 is illuminated by light source 10 
and the linear end of the area-to-line fibre optic converter 52 is located 
adjacent at input slit 18 in the light integrating cavity 20. 
In one application for scanning wide film a light source of the type shown 
in FIG. 10 was constructed having an integrating cavity 215 mm long 
.times.35 mm in diameter. The input slit was 2.0 mm wide by 200 mm long; 
and the output slit was 1.5 mm wide by 200 mm in long. The light source 10 
was a tungsten halogen projector lamp. 
The light source for producing the intense beam of light may also comprise 
a laser where an intense monochromatic line of light is desired. As is 
known in the art, to suppress laser speckle, a suitable dithering optical 
element such as a vibrating mirror in the laser optical path can be used 
to "blur" the speckle. 
Where less intense light source is required, a tungsten lamp with a coiled 
filament can be employed as a light source. FIG. 11 shows an embodiment, 
where the light source 10 is a tungsten lamp 54 with a coiled filament 56. 
In the above examples, the input port was arranged off axis of the output 
slit so that direct illumination from the light source could not exit the 
output slit without at least one diffuse reflection in the integrating 
cavity. Further protection from light directly exiting the output slit may 
be provided by arranging a baffle in the interior of the cavity between 
the input port and the output port. Alternatively, the input port may be 
arranged opposite the output slit and means for baffling or diffusing the 
input light provided within the integrating cavity. FIG. 12 is a schematic 
cross sectional view of the integrating cavity 20 with the input port 18 
directly across from the output slit 24. A diffuser such as a glass ball 
or a rod 58 having a diffusely reflective surface is placed between the 
input port 18 and the output slit 24 to prevent light from passing 
directly from the input port 18 to the output slit 24. 
Industrial Applicability and Advantages 
The linear light source of the present invention is useful in a film 
scanner of the type employing a linear image sensor. The light source is 
advantageous in that an intense line of light is produced having a highly 
uniform intensity along the line and a nearly uniform angular distribution 
for optimum scratch suppression in a film scanner. The linear light source 
further has the advantage that temporal fluctuations are minimized by 
employing the light from the integrating cavity as a feedback signal to 
power the light source.