Imaging apparatus offering dynamic range that is expandable by weighting two image signals produced during different exposure times with two coefficients whose sum is 1 and adding them up

• US 6,677,992 B1
• Filed: 10/22/1998
• Issued: 01/13/2004
• Est. Priority Date: 10/23/1997
• Status: Expired due to Fees

• Current Assignee: Olympus Corporation
• Original Assignee: Olympus Corporation
• Inventors: Ohno, Wataru, Yamashita, Shinji, Tsunakawa, Makoto, Mochida, Akihiko, Ogasawara, Kotaro, Saito, Katsuyuki, Matsumoto, Kanichi, Kusamura, Noboru, Tashiro, Hideki, Kami, Kuniaki, Yajima, Manabu
• Primary Examiner(s): Christensen, Andrew
• Assistant Examiner(s): Harris, Tia M.
• Application Number:
  US09/176,966
• Time in Patent Office: 1,909 Days
• Field of Search: 348/299, 348/298, 348/230.1
• US Class Current: 348/229.1
• CPC Class Codes: H04N 23/45 for generating image signals from two or more image sensors being of different type or operating in different modes, e.g. with a CMOS sensor for moving images in combination with a charge-coupled device [CCD] for still images
• CPC Class Codes: H04N 23/555 for picking-up images in sites, inaccessible due to their dimensions or hazardous conditions, e.g. endoscopes or borescopes
• CPC Class Codes: H04N 23/74 by influencing the scene brightness using illuminating means
• CPC Class Codes: H04N 25/42 by switching between different modes of operation using different resolutions or aspect ratios, e.g. switching between interlaced and non-interlaced mode
• CPC Class Codes: H04N 25/589 with different integration times, e.g. short and long exposures
• CPC Class Codes: H04N 25/71 Charge-coupled device [CCD] sensors; Charge-transfer registers specially adapted for CCD sensors

Abstract

An object is imaged continuously during a first exposure time and a second exposure time shorter than the first exposure time. Weights, one of which decreases monotonously and the other of which increases monotonously, are applied to first and second resultant image signals under the condition that the sum of the weights is 1. The first and second image signals that have been weighted are added up, thus producing a synthetic picture signal. When a luminance level is low, the ratio of the first image signal, which has been produced during the longer exposure time, to the second image signal is increased. This results in an image demonstrating a high signal-to-noise ratio. When the luminance level is high, the ratio of the second image signal, which has been produced during the shorter exposure time, to the first image signal is increased. This results in a synthetic image that proves a wide dynamic range, depicts a smoothly varying brightness level, and exhibits a characteristic of being seen as almost natural. Moreover, the first and second image signals are produced to resemble those produced during mutually different exposure times by controlling an amount of light incident on an imaging device or by varying an amount of illumination light.

Claims

1. An imaging apparatus comprising:

an imaging device for successively outputting a first image signal produced by imaging an object during a first exposure time and a second image signal produced by imaging said object during a time shorted than said first exposure time;

an image signal producing circuit composed of;

first and second weighting circuits for applying a first weight, which decreases monotonously with an increase in light level from a light level of substantially no luminance up to a light level causing said first and second image signals to have a saturation value, and a second weight, which increases monotonously from said light level of substantially no luminance up to said light level causing said first and second image signals to have said saturation value, to said first and second image signals under the condition that the sum of said first and second weights is 1; and

an adding circuit for adding together first and second picture signals produced by applying said first and second weights to said first and second image signals by means of said first and second weighting circuits; and

a signal processing circuit for processing a picture signal output from said adding circuit and producing a video signal according to which an image can be displayed on a display device.

2. An imaging apparatus according to claim 1, further comprising an exposure time setting facility for varying at least one of said first and second exposure times.

3. An imaging apparatus according to claim 2, wherein said exposure time setting facility uses a waveform-detected signal produced by detecting the waveform of said first or second image signal.

4. An imaging apparatus comprising:

an imaging device for successively outputting a first image signal produced by imaging an object during a first exposure time, and a second image signal that has a saturation value relative to a higher light level than said first image signal;

a picture signal producing device composed of;

first and second weighting circuits for applying a first weight, which decreases monotonously with an increase in light level from a light level of substantially no luminance up to a light level causing said first and second image signals to have the saturation value, and a second weight, which increases monotonously from said light level of substantially no luminance up to said light level causing said first and second image signals to have said saturation value, to said first and second image signals under the condition that the sum of said first and second weights is 1; and

an adding circuit for adding together first and second picture signals produced by applying said first and second weights to said first and second image signals by means of said first and second weighting circuits; and

a signal processing circuit for processing a picture signal output from said adding circuit and producing a video signal according to which an image can be displayed on a display.

5. An imaging apparatus according to claim 4, wherein said imaging device images an object during a second exposure time that is shorter than said first exposure time, and thus outputs said second image signal that has the saturation value relative to a higher light level than said first image signal.

6. An imaging apparatus according to claim 4, wherein during a second exposure time during which said second image signal is produced, an amount of light coming from said object and falling on said imaging device through an imaging window is restricted by an amount-of-light restricting device, and said imaging device outputs said second image signal that has the saturation value relative to a higher light level than said first image signal.

7. An imaging apparatus according to claim 4, wherein said weights applied by said picture signal producing device are such that:

when a light level is much lower than the light level causing said first image signal to have the saturation value, said first weight is larger than said second weight; and

when the light level is higher than the light level causing said first image signal to have the saturation value, said second weight is larger than said first weight.

10. An imaging apparatus according to claim 4, wherein said imaging device includes a progressive imaging device having two systems of horizontal transfer output circuits.

11. An imaging apparatus according to claim 10, wherein said signal processing means produces a video signal conformable to the interlacing.

12. An imaging apparatus according to claim 4, wherein said image signal producing circuit includes a weight data storage that stores data of said first weight to be applied to said first image signal and data of said second weight to be applied to said second image signal.

13. An imaging apparatus according to claim 12, wherein said weight data storage stores data of a weight to be applied to said first image signal and data of a weight to be applied to said second image signal, and the weights exhibit different characteristics.

14. An imaging apparatus according to claim 13, further comprising a brightness detector for detecting brightness information of an object in an output signal of said imaging device, wherein a weight to be applied to said first image signal and a weight to be applied to said second image signal are selected based on the brightness information of an object, and thus read from said weight data storage.

15. An imaging apparatus according to claim 5, wherein said imaging device images said object during said first exposure time, and produces said first image signal and second image signal.

16. An imaging apparatus according to claim 4, wherein said picture signal producing device produces a synthetic picture signal exhibiting such a characteristic that a differential coefficient thereof relative to a light level varies continuously with an increase in light level up to a light level causing said second image signal to have the saturation value.

17. An imaging apparatus according to claim 16, wherein the differential coefficient decreases monotonously relative to the light level.

8. An imaging apparatus comprising:

an imaging device for successively outputting a first image signal produced by imaging an object during a first exposure time, and a second image signal that has a saturation value relative to a higher light level than said first image signal;

a picture signal producing device composed of;

first and second weighting circuits for applying a first weight, which decreases monotonously with an increase in light level up to a light level causing said first and second image signals to have a saturation value, and a second weight, which increases monotonously therewith, to said first and second image signals under the condition that the sum of said first and second weights is 1; and

an adding circuit for adding together first and second picture signals produced by applying said first and second weights to said first and second image signals by means of said first and second weighting circuits; and

a signal processing circuit for processing a picture signal output from said adding circuit and producing a video signal according to which an image can be displayed on a display;

wherein

said first weight function F1(x) is cos²(px) where x is a light level and p is a coefficient, and said second weight function F2(x) is sin²(px).

9. An imaging apparatus according to claim 8, wherein said coefficient p satisfies p≈

π

/(2s) where s is a light level of light incident on said imaging device.

18. An endoscopic imaging apparatus, comprising:

an endoscope having an elongated insertion unit, irradiating illumination light to an object through an illumination window located at the distal end of said insertion unit, and including an imaging device for successively outputting a first image signal produced by imaging said object illuminated with said illumination light during a first exposure time, and a second image signal that has a saturation value relative to a higher light level than said first image signal;

an image signal producing circuit composed of;

first and second weighting circuits for applying a first weight, which decreases monotonously with an increase in light level from a light level of substantially no luminance up to a light level causing said first and second image signals to have the saturation value, and a second weight, which increases monotonously from said light level of substantially no luminance up to said light level causing said first and second image signals to have said saturation value, to said first and second image signals under the condition that the sum of said first and second weights is 1; and

an adding circuit for adding together first and second picture signals produced by applying said first and second weights to said first and second image signals by means of said first and second weighting circuits;

a signal processing circuit for processing a picture signal output from said adding circuit and producing a video signal which can be displayed; and

a display for displaying a synthetic image represented by said picture signal when inputting said video signal.

19. An endoscopic imaging apparatus according to claim 18, wherein said imaging device images said object during a second exposure time that is shorter than said first exposure time, and thus outputs said second image signal that has the saturation level relative to a higher light value than said first image signal.

20. An endoscopic apparatus according to claim 18, wherein during a second exposure time during which said second image signal is produced, an amount of light coming from said object and falling on said imaging device through an imaging window is restricted by a light restricting device, and said imaging device outputs said second image signal that has the saturation value relative to a higher light level than said first image signal.

21. An endoscopic imaging apparatus according to claim 18, further comprising a light source unit for supplying illumination light to a light guide that runs through said insertion unit of said endoscope, propagates illumination light, and emits the illumination light through said illumination window.

22. An endoscopic imaging apparatus according to claim 21, wherein during a second exposure time during which said imaging device produces a second image signal, said light source unit allows a light restricting device to restrict illumination light to be supplied to said light guide, and said imaging device outputs said second image signal that has the saturation value relative to a higher light level than said first image signal.

23. An endoscopic imaging apparatus according to claim 22, wherein said light restricting device is a light restriction filter for restricting illumination light generated by a lamp and supplying resultant light to said light guide.

24. An endoscopic imaging apparatus according to claim 22, wherein said light restricting device restricts an amount of illumination light generated by a lamp.

25. An endoscopic imaging apparatus according to claim 18, wherein said endoscope is an electronic endoscope and said imaging device being provided with the ability to photoelectrically convert a signal, placed on the image plane of an objective optical system located at the distal end of said elongated insertion unit.

26. An endoscopic imaging apparatus according to claim 18, wherein said endoscope is a TV camera-mounted endoscope composed of an optical endoscope having an image propagating device for propagating an object image formed by an objective optical system located at the distal end of said elongated insertion unit to an optical system, and a TV camera that is mounted on an eyepiece of said optical endoscope and has an imaging device, which is included in said imaging device and provided with the ability to photoelectrically convert a signal, incorporated therein.

27. An endoscopic imaging apparatus according to claim 18, wherein said endoscope includes a common image formation optical system, and said imaging device is composed of a plurality of imaging devices located at positions respectively at which a plurality of images are formed through a plurality of apertures by said image formation optical system.

28. An endoscopic imaging apparatus according to claim 19, wherein said imaging device images said object during said first exposure time so as to produce said first image signal and second image signal.

29. An endoscopic imaging apparatus according to claim 18, wherein said imaging device has a color separation filter used to achieve color imaging under white illuminating light.

30. An endoscopic imaging apparatus according to claim 18, wherein said imaging device is a field-sequential imaging device that includes a monochrome imager not having a color separation filter, and that carries out color imaging under illumination of field-sequential light having a plurality of wavelengths.

31. An imaging apparatus, comprising:

an imaging device for successively outputting a first image signal produced by imaging an object during a first exposure time, and a second image signal that is produced by imaging said object during a second exposure time shorter than said first exposure time and that has a saturation value relative to a higher light level than said first image signal;

a picture signal producing circuit composed of;

first and second weighting circuits for applying a first weight, which decreases monotonously with an increase in light level from a light level of substantially no luminance up to a light level causing said first and second image signals to have the saturation value, and a second weight, which increases monotonously from said light level of substantially no luminance up to said light level causing said first and second image signals to have said saturation value, to said first and second image signals under the condition that the sum of said first and second weights is 1; and

an adding circuit for adding together first and second picture signals produced by applying said first and second weights to said first and second image signals by means of said first and second weighting circuits; and

a signal processing circuit for processing a picture signal output from said adding circuit and producing a video signal according to which an image can be displayed on a display.

32. An imaging apparatus, comprising:

an imaging device for successively outputting a first image signal produced by imaging an object during a first exposure time, and a second image signal that has a saturation value relative to a higher light level than said first image signal;

a picture signal producing circuit composed of;

first and second weighting circuits for applying a first weight, which decreases monotonously with an increase in light level from a light level of substantially no luminance up to a light level causing said first and second image signals to have the saturation value, and a second weight, which increases monotonously from said light level of substantially no luminance up to said light level causing said first and second image signals to have said saturation value, to said first and second image signals under the condition that the sum of said first and second weights is 1; and

an adding circuit for adding together first and second picture signals produced by applying said first and second weights to said first and second image signals by means of said first and second weighting circuits; and

a signal processing circuit for processing a picture signal output from said adding circuit and producing a video signal according to which an image can be displayed on a display.

33. An imaging apparatus, comprising:

an imaging device for successively outputting a first image signal produced by imaging an object during a first exposure time, and a second image signal that has a saturation value relative to a higher light level than said first image signal;

a picture signal producing circuit composed of;

first and second weighting circuits for applying a first weight, which decreases monotonously with an increase in light level from a light level of substantially no luminance up to a light level causing said first and second image signals to have the saturation value, and a second weight, which increases monotonously from said light level of substantially no luminance up to said light level causing said first and second image signals to have said saturation value, to said first and second image signals under the condition that the sum of said first and second weights is 1; and

an adding circuit for adding together picture signals produced by applying said first and second weights to said first and second image signals by means of said first and second weighting circuits; and

a signal processing circuit for processing a picture signal output from said adding circuit and producing a video signal according to which an image can be displayed on a display.

34. An imaging apparatus according to claim 33, wherein when a light level is higher than the light level causing said first image signal to have the saturation value, said first weighting circuit outputs a weight coefficient of 0.

35. An imaging apparatus according to claim 33, wherein when a light level is higher than the light level causing said first image signal to have the saturation value, said picture signal is determined substantially with a weight coefficient that is applied to said second image signal by said second weighting circuit.