Digital cameras abstract
A lens cover for a digital camera provides a diffused light source
for calibration purposes which approximates a flat field light source.
In this way, digital cameras may be calibrated in the field under
user control. The diffuser may be in the form of a light transmissive
hologram. The hologram may have formed thereon a plurality of light
diffusing shapes. The camera may be calibrated either by itself
or advantageously when connected to a host computer. The user may
be prompted through the calibration sequence.
Digital cameras claims
What is claimed is:
1. A digital camera comprising:
an imaging lens contained in the housing;
a diffuser to create the effect of a flat field source, said diffuser
removably positionable in front of the lens; and
a digital imaging device that converts an image into a digital
signal representation of the image, said digital imaging device
having a calibration mode in which said digital imaging device is
calibrated for photo response non-uniformity using said diffuser.
2. The camera of claim 1 wherein said diffuser includes a holographic
plate adapted to diffuse light sufficiently to emulate a flat field
3. The camera of claim 2 wherein said holographic plate is formed
of a plastic film secured to a light transmissive substrate.
4. The camera of claim 3 wherein said device includes an interface
that receives instructions from a host computer.
5. A method of calibrating a camera having an imaging array comprising:
securing a diffuser to the camera;
emulating a flat field source using said diffuser; and
implementing a calibration sequence using said emulated flat field
source to calibrate the imaging array for photo response non-uniformity.
6. The method of claim 5 including prompting the user through the
7. The method of claim 6 including receiving a user input indicating
the amount of time desired for the calibration process.
8. The method of claim 5 including shining a light through said
diffuser and causing the light to be diffused by a plurality of
holographic shapes formed on said diffuser.
Digital cameras description
This invention relates generally to calibrating digital cameras.
Digital cameras may use a solid state sensor as the imaging array.
Typical solid state sensors include charged coupled devices and
active pixel sensors. Cameras using solid state sensors are subject
to photo response nonuniformity (PRNU) and defective pixel maps.
Photo response non-uniformity is systematic and is largely due to
small variations in device processing across a particular imaging
array. The non-uniformity introduces noise to the resulting picture.
However, since the noise is systematic, it can be cancelled out
following an appropriate calibration procedure.
Similarly, the imaging array may have defective pixels or elements
and if the number of defective elements is not too great, the imaging
array may still be useable. For example, calibration techniques
can be utilized to overcome the effect of a relatively small number
of defective elements in the imaging array.
There are a number of problems with calibrating digital cameras
at the factory. For one thing, the quality of the calibration technique
is to some degree a function of how much time is used to accomplish
the calibration. The ideal calibration involves analyzing a large
number of images and extracting the non-uniformity noise from other
noise sources so that the total system noise is reduced. Thus, the
more images that are analyzed the greater the likelihood that camera
noise and photon shot noise may be eliminated so that photo response
non-uniformity may be isolated and calibrated out. Of course, expending
more time during factory calibration, results in increasing expense
to the user.
In addition, it is difficult to convey the PRNU cancellation information
to the user since the calibration data consists of a relatively
large file of noise information. Generally, noise data cannot be
readily compressed and therefore it would be necessary for the factory
to convey a relatively large file in (or with) the camera. This
could result in ineffective use of the camera's memory and awkward
user startup. Particularly in view of the fact that many digital
cameras have relatively little onboard memory, it is not desirable
to provide an extensive file of noise information in cameras with
Of course, it is also possible that the user could attempt to self-calibrate
the camera. One problem the user must face is that calibration techniques
generally require a flat field light source. Generally, inexpensive
flat field sources are not available. The absence of a flat field
light source and the need for some sophistication, would likely
limit the number of users who could effectively self-calibrate digital
Another issue impacting digital camera cost is the necessity for
a relatively high percentage of the elements making up the imaging
array to be properly functional. If a relatively small number of
the individual elements making up the array are defective, this
may not be noticeable. Defective array elements may be compensated
for by the calibration process. The higher the required ratio of
good array elements to the total number of elements in a given array,
the higher the resulting manufacturing cost.
Thus, there is a continuing need to enable cost effective calibration
of digital cameras. There is also a need for flat field calibration
techniques that are applicable to use in the field. Similarly, there
is a continuing need to enable manufacturers to decrease the required
ratio of good elements to total elements in an imaging array while
still producing fully adequate image quality in the resulting stored
In accordance with one aspect of the present invention, a lens
cover for covering the lens of a digital camera may include a diffuser
which produces a diffused light source. The diffuser is connectable
to the camera.
In accordance with another aspect of the present invention, a digital
camera includes a housing and an imaging lens contained in the housing.
A diffuser is removably positionable in front of the lens. A digital
imaging device converts the image into a digital signal representation
of the image which can be calibrated when the diffuser is connected
to the housing over the lens.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic depiction of a digital camera in accordance
with one embodiment of the present invention;
FIG. 2 is an enlarged view of a portion of one embodiment of the
diffuser shown in FIG. 1;
FIG. 3 is a block diagram of a host computer configured to implement
one embodiment of the present invention; and
FIG. 4 is a flow diagram of a procedure implemented by the system
shown in FIG. 1 for field calibration of digital cameras.
Referring to FIG. 1, a digital camera 10 includes a lens cover
12, a lens 14, a housing 7, and an image processor 18. The camera
may be connected by bus 20 to a host computer. The camera 10 may
be adapted to take still pictures, movies or both still pictures
The lens cover 12 may be implemented as a removable cover which
is snapped onto the lens in a conventional fashion. Alternatively,
any one of the variety of conventional mechanisms for displacing
the cover 12 into position in front of the lens 14 may be used.
The lens cover 12 protects the lens 14, prevents adverse UV effects
on downstream components such as the imaging array 15 and the lens
14 and assists in calibration, in a fashion described hereinafter.
The imaging array 15 may be any conventional solid state imaging
array including a charge coupled device (CCD) image array or a CMOS
(Complementary Metal Oxide Semiconductor) imaging array which is
sometimes also called an active pixel sensor (APS). The image processor
18 may include the necessary electronics to convert the information
stored on the imaging array 15 into a digital signal, to process
the signal, and to store the signal. For example, the processor
18 may include an LCD screen if desired, and it may interpolate
missing color components, convert and compress signals, adjust the
image based on calibration information and the like. The digital
camera may work in a stand alone mode or it may be connectable to
a host computer. It may also interact with external memory through
a port 22. In this way, images may be captured, processed and stored.
The lens cover 12 includes a light transmissive support 26 and
a diffuser 16. The diffuser 16 may be formed of any material capable
of forming a sufficiently diffused light source. By "sufficiently
diffused", it is intended to refer to the ability of the cover
12 to diffuse light sufficiently to create a sufficiently close
approximation to a flat field to enable calibration of a camera.
One suitable implementation of the diffuser 16 is a light transmissive
plastic film hologram formed with a pattern of light diffusing elements
thereon. As shown in FIG. 2, one embodiment of the hologram includes
a plurality of closely spaced conical shapes, such as the pyramid
shapes 24. These pyramid shapes receive the incoming light, indicated
as A, and diffuse the light in a variety of directions. This reflection
may occur by reflecting the light off of the sides 25 of the shapes
24 so that a relatively uniform light source is created passing
outwardly from the diffuser 16. The hologram may be formed on a
thin plastic film using commercially available embossing equipment.
It is desirable that the density of the diffusing shapes 24 be sufficient
to create a relatively diffused light source which sufficiently
emulates a flat field light source for camera calibration purposes.
The support 26 may be formed of any suitable light transmissive
glass or plastic material. It functions to protect the lens 14 and
the diffuser 16. The support 26 also supports the diffuser 16 when
the diffuser 16 is formed from a plastic film hologram. In many
embodiments it may be desirable to form the support 26 of a material
that is UV absorbing to prevent UV damage to downstream components
such the lens 14 and the imaging array 15.
While the diffuser 16 is illustrated as being a plastic film hologram,
other forms of holograms may be utilized as well. In addition, diffusers,
such as frosted glass diffusers, may be suitable in particular applications.
While the diffuser 16 may not form a true flat field light source
in the sense of a plurality of collinear light rays, through the
creation of a relatively diffused pattern of light, an effect comparable
to that of a flat field can be developed. This diffused light, which
would impact the array 15 at a variety of random angles, sufficiently
emulates a flat field to enable calibration.
Because the calibration is done in the field, for example by the
user of the camera, it is not necessary to load the camera in the
factory with a large amount of information related to calibration.
In addition, the user can select the degree of calibration that
the user desires to implement. In this way, an economical calibration
system can be implemented. The manufacturer may also benefit by
compensating for dead pixels through better calibration. This could
mean cost savings in terms of lower imaging array discard rates.
Referring now to FIG. 3, an illustrative host computer 30 includes
a processor 32 and a system memory 34. The processor 32 and system
memory 34 are connected to a bus 36 by a bridge 38. An interface
40 for connection to a digital camera may be connected to the bus
36. For example, the interface 40 could be a host for a Universal
Serial Bus (USB) 20 which connects to the bus 20 of the digital
camera 10 (FIG. 1). A monitor 59 may also be connected to the bus
36. The bus 36 may connect through a bridge 44 to a bus 46. The
bus 46 supports an input/output interface 48 which operates a keyboard
50 and a mouse 52. An interface 54 may be connected to the bus 36
which connects to a memory 56. The host computer memory 56 may include
a number of application programs 58 including a digital camera installation
and calibration program.
When it is desired to calibrate the digital camera 10, the camera
may be connected to the host computer 30 as described above. The
installation and calibration programs 58 may be installed into the
memory 56 of the host computer and the application program 58 may
be run to calibrate the camera 10. However, some cameras may calibrate
themselves, without requiring a host computer.
Referring to FIG. 4, the calibration program initially checks to
determine whether or not a calibration request has occurred (diamond
60). If not, the program awaits a calibration request. Otherwise,
the host prompts the user to determine whether the cover 12 is in
place. This could be done by a message on the monitor 59. If the
cover is not in place, the program awaits the user's positioning
of the calibrating cover 12. Once the user indicates that the calibration
cover is in place, the user may be prompted to input a calibration
time as indicated in block 64. The user can indicate that the cover
is on or provide a calibration time through the keyboard 50, for
example. A default calibration time may be preloaded into the program.
The calibration time to some degree determines how many images
are used for calibration and therefore the extent to which the photo
response non-uniformity may be effectively eliminated.
The host computer 30 (or the camera itself) then determines whether
the desired calibration time has been exceeded (diamond 66) and
implements the calibration function using the image processor 18
in the camera 10. The user is signaled that the calibration is completed,
as indicated in block 68, and the calibration results are stored
in the camera and/or the host computer.
A number of images produced with the diffuser 16 may be analyzed
and/or compared to other images. Assuming all elements in the imaging
array 15 would be exposed to light g through the action of the diffuser
16, one can use an iterative process to eliminate other noise components
to isolate the PRNU components. Thereafter, systematic noise from
PRNU or dead pixels may be removed through the calibration process.
A digital camera can be calibrated to overcome the effects of photo
response non-uniformity and defective pixels in a cost effective
fashion. Moreover, the user can control the calibration process
and can achieve a desired degree of calibration effectiveness based
on the amount of time which the user is willing to invest. The manufacturer
may benefit by providing higher quality cameras without incurring
the expense of calibration and possibly through the use of imaging
arrays with a higher percentage of defects (whose effect may be
neutralized by the calibration process).
While the present invention has been described with respect to
a limited number of embodiments, those skilled in the art will appreciate
numerous modifications and variations therefrom. It is intended
that the appended claims cover all such modifications and variations
as fall within the true spirit and scope of the present invention.