Camera flash module and method for controlling same

• US 20090160944A1
• Filed: 12/17/2008
• Published: 06/25/2009
• Est. Priority Date: 12/21/2007
• Status: Active Grant

• Current Assignee: Nokia Technologies Oy (Nokia Corporation)
• Original Assignee: Nokia Corporation
• Inventors: Trevelyan, Philip, Ivanov, Angel, Kirilov, Emil, Vasilev, Ivan
• Granted Patent: US 8,018,525 B2
• Time in Patent Office: Days
• US Class Current: 348/187
• CPC Class Codes: H04N 23/74 by influencing the scene brightness using illuminating means

Abstract

An auto-exposure algorithm for controlling a camera flash uses image processing to identify important areas of the image affected by the flash, while disregarding highly reflective/illuminated areas and uses a ND filter to linearize the flash triggering with highly reflective scenes. The camera flash is controlled by the auto-exposure algorithm in two stages:

a pre-flash stage followed by a main-flash stage. In the pre-flash stage, two images are captured under the same camera settings regarding the exposure time, gain, iris and resolution. One image is captured with flash and one without. From the difference between the two images, a reference pixel is used to determine the flash intensity in the main-flash stage.

Claims

1. A method, comprising:

capturing a first test image without using a flash;

capturing a second test image using the flash with a predetermined intensity;

obtaining a difference image indicative of a difference between the second test image and the first test image, wherein the difference image comprises a plurality of difference image pixels, each difference image pixel having a pixel value;

identifying a reference pixel among the difference image pixels, wherein the first test image comprises a first test pixel corresponding to the reference pixel, and the second test image comprises a second test pixel corresponding to the reference pixel; and

computing illumination amount by the flash for subsequent image capturing based at least partly on pixel value of the first test pixel and pixel value of second test pixel.

2. The method of claim 1, wherein the reference pixel is identified based at least partly on a highest pixel value among the difference image pixels.

3. The method of claim 1, further comprising:

removing one more of the difference image pixels belonging to objects smaller than a predetermined size in the difference image before said identifying.

4. The method of claim 3, wherein each of the objects smaller than the predetermined size is identified based at least partly on number of contiguous pixels in a pixel array having a highest pixel value among the difference image pixels.

5. The method of claim 4, wherein the pixel array comprises a plurality of edge pixels, said method comprising:

applying a pixel filter to remove the edge pixels for obtaining remaining pixels in the pixel array, wherein the number of remaining pixels is used to compare with the predetermined size.

6. The method of claim 1, further comprising:

removing some of the difference image pixels belonging to reflective objects in the difference image before said identifying.

7. The method of claim 6, wherein the difference image pixels comprise color image pixels, said method further comprising:

performing a histogram analysis on the color image pixels for determining the reflective objects.

8. The method of claim 1, further comprising:

determining camera settings for the capturing of the first test image and the second test image, wherein the camera settings comprise an exposure time and the predetermined intensity of the flash, wherein average luminance of one or more preview images are used for setting the predetermined intensity of flash such that average luminance of the second test image is within a predetermined range.

9. The method of claim 1, wherein the illumination amount by the flash for the subsequent image capturing is achievable by causing the flash to provide flash intensity by a trigger pulse having a pulse width, wherein when the pulse width is smaller than a predetermined value, the pulse width is increased and the flash intensity is reduced accordingly so as to maintain the illumination amount by the flash.

10. The method of claim 9, wherein a neutral density filter is used to reduce the flash intensity.

11. The method of claim 9, wherein an iris is used to reduce the flash intensity.

12. The method of claim 1, wherein the first test image comprises a plurality of first image pixels, each of the first pixels has a pixel value, said method further comprising:

determining in the first test image areas having first image pixels with pixel values within a predetermined range;

removing corresponding areas in the difference image before said computing.

17. A computer readable storage medium embedded therein a software program for use in a camera, said software program comprising programming codes for carrying out the method of claim 1.

18. A camera, comprising:

a flash module as defined in claim 11;

an imaging sensor; and

an optical module for forming an image on the imaging sensor, wherein the imaging sensor is configured to provide the image data.

13. A flash module for use in a camera, the module adapted to control a camera flash, said module comprising:

a mechanism for triggering the camera flash; and

a processor arranged to receive image data from the camera, wherein the processor is adapted to carry out;

capturing a first test image without using the camera flash;

capturing a second test image using the camera flash with a predetermined intensity;

obtaining a difference image indicative of a difference between the second test image and the first test image, wherein the difference image comprises a plurality of difference image pixels, each difference image pixel having a pixel value;

identifying a reference pixel among the difference image pixels, wherein the first test image comprises a first test pixel corresponding to the reference pixel, and the second test image comprises a second test pixel corresponding to the reference pixel; and

computing illumination amount by the flash for subsequent image capturing based at least partly on pixel value of the first test pixel and pixel value of the second test pixel.

14. The flash module of claim 13, wherein the processor comprises a software program having programming codes for performing said obtaining, identifying and said computing.

15. The flash module of claim 13, further comprising a neutral density filter, wherein the illumination amount by the flash for the subsequent image capturing is achievable by causing the flash to provide flash intensity by a trigger pulse having a pulse width, wherein when the pulse width is smaller than a threshold value, apply the neutral density filter to reduce the flash intensity while increasing the pulse width so as to maintain the illumination amount by the flash.

16. The flash module of claim 13, further comprising an iris, wherein the illumination amount by the flash for the subsequent image capturing is achievable by causing the flash to provide flash intensity by a trigger pulse having a pulse width, wherein when the pulse width is smaller than a threshold value, apply the iris to reduce the flash intensity while increasing the pulse width so as to maintain the illumination amount by the flash.

19. An apparatus, comprising:

means for capturing a first test image without using a flash and a second test image using the flash with a predetermined intensity;

means for obtaining a difference image indicative of a difference between the second test image and the first test image, wherein the difference image comprises a plurality of difference image pixels, each difference image pixel having a pixel value;

means for identifying a reference pixel among the difference image pixels, wherein the first test image comprises a first test pixel corresponding to the reference pixel, and the second test image comprises a second test pixel corresponding to the reference pixel; and

means computing illumination amount by the flash for subsequent image capturing based at least partly on pixel value of the first test pixel and pixel value of second test pixel, wherein the reference pixel is identified based at least partly on a highest pixel value among the difference image pixels

20. The apparatus of claim 19, further comprising:

means for removing one more of the difference image pixels belonging to objects smaller than a predetermined size in the difference image, and some of the difference image pixels belonging to reflective objects in the difference image before identifying the reference pixel.