Patent Application
20100141571
1. Field of the Invention
The present invention generally relates to image sensors, and more particularly, to an image sensor with an integrated ambient light meter for controlling electronic display brightness.
2. Description of the Prior Art
Electronic devices utilize display devices to effectively communicate information. Different display technologies, such as Liquid Crystal Displays (LCD), Organic Light Emitting Diodes (OLED), Field Emission Displays, and Plasma Displays, are selected for use depending on the intended application.
The LCD and OLED are flat display devices that have the advantage of being thinner and lighter over conventional displays, such as cathode ray tube displays, and thus receive widespread popularity in a wide range of modern electronic device applications.
The LCD is constructed as transmissive LCD, reflective LCD or transreflective LCD, differing by the relative alignment between light source and liquid crystal (LC) molecules. The transmissive LCD is made of the LC molecules arrayed in front of, and illuminated by, a backlight module (“backlight”). The reflective LCD is made of the LC molecules and a reflector that reflects ambient light to illuminate the LC molecules. The transreflective LCD uses both backlighting and reflecting methods to illuminate the LC molecules.
The backlight mentioned above may be built from a variety of lighting elements, such as light emitting diode (LED), cold cathode fluorescent lamp (CCFL), or electroluminescence panel (ELP). Among these lighting elements, the LED is currently emerging as the preferred technology due to its long lifetime, low cost, resilience to vibration, low voltage and precise control of its intensity.
The OLEDs are composed of light-emitting organic materials. The material emits light when it is excited by an electric current, and as a result requires no backlighting. The amount of electric current can be actively controlled within the OLED structure, thereby controlling the brightness of the display.
In order to conserve power, such as in a handheld or portable electronic device, the display brightness may be adaptively adjusted based on the available light (or ambient light) in the environment surrounding the display. For example, the display is dimmed in a dark room, whereas the display intensity is increased in a bright lighting condition. Furthermore, controllability of the LCD display has the added benefit of reducing eye strain on the user and increasing visibility of the display. A conventional method of controlling the display brightness mentioned above involves employing a small discrete photo sensor to measure the ambient light. A control loop is employed to adjust the brightness based on measured ambient light.
An image sensor is also a crucial electronic component that is widely used in modern electronic devices. Particularly, semiconductor-based image sensors, such as charge-coupled devices (CCDs) or complementary metal-oxide-semiconductor (CMOS) image sensors (commonly referred to as CISs), are popular in, for example, cameras and camcorders for converting images of visible light into electronic signals that can then be stored, transmitted and/or displayed. As image sensors consume substantially more power than discrete photo sensors, image sensors are seldomly used, if at all, for controlling the backlight particularly in portable electronic devices or battery-operated devices.
For devices that contain an image sensor, a separate ambient light sensor is still needed for brightness adjustment. This solution increases the cost, space and complexity of the device design. For the above reasons, a need has arisen to propose a novel method and apparatus for an image sensor that can be used to automatically adjust the display brightness based on the ambient light in the environment.
In view of the foregoing, it is an object of the present invention to provide a method and apparatus for an image sensor to automatically adjust the display brightness based on the ambient light in the environment, while reducing system cost and space, as well as attenuating substantial power consumption.
According to the embodiments, the present invention provides an image sensor with an integrated light meter for controlling display brightness. Since the photodiode of an image sensor is continuously exposed and collecting a light signal, the light signal is measured using the proposed light meter when the image sensor is not imaging. While the sensor is imaging, the light intensity can be approximated using the required Exposure and Gain Product (EGP) calculated by the on-chip Automatic Exposure (AE) Control Loop. For an image sensor without on-chip AE Control Loop, the sensor's exposure and gain setting, calculated and applied by off-chip AE Control Loop, can be characterized and approximated to light intensity. The sensor chip can include dedicated light intensity measurement circuits capable of quantifying light intensity when the exposure and gain of the sensor can no longer be adjusted. The relationship of light intensity, measured by a dedicated measurement circuit and/or approximated by the EGP value and the integrated light meter, is correlated with the illuminance (or luminance) of the ambient light, and is utilized to control the display brightness.
With respect to the embodiment illustrated in
When the image sensor 12 is capturing and outputting image data, signals generated by light falling on the sensor photodetectors of the image sensor 12 are amplified and read out of the image sensor 12, and the digital equivalent is then output by analog signal chain 13. The output digital signals are usually further processed by the (digital) image signal processor (ISP) 14, and are then forwarded to a video port or video bus 15. Subsequently, a display controller 16, a main circuit in a video signal generator responsible for the production of the video signal, directs the digital signal to a LCD driver 17, which drives and displays the image on the display 11.
While the image sensor 12 is capturing and outputting image data, the ambient light intensity of the scene can be directly measured from the image data. In the embodiment, the AE control loop 18A utilizes analog gain, digital gain and integration time to control the exposure time (i.e., how long the image sensor 12 is exposed to incident light), as well as the amount of (analog and/or digital) gain applied to the signals out of the image sensor 12. The AE control loop compares the measured light intensity to a programmable target light intensity and calculates the exposure time and gain (analog and/or digital) required to converge the measured and target light intensity. An AE exposure gain product (EGP) is thus obtained by multiplying the exposure time by the total gain (e.g., analog and digital). A typical EGP curve showing the relationship between the EGP and illuminance (or luminance, or light intensity) is illustrated in
In this alternative embodiment, an EGP curve is also obtained as shown in
Referring back to
When the image sensor 12 is not used for capturing and outputting image data, the analog signal chain 13, the ISP 14, the video port 15, and the AE control loop 18A/B become inactive, as illustrated in
In this mode, an intensity curve is obtained as shown in
For the embodiments discussed above, the output signal of the block 20 may be connected to, for example, an INTERRUPT pin of a device controlling the backlight 10. Accordingly, whenever the threshold criteria in the block 20 have been met, the measured light intensity of the ambient light is read from the block 19 and utilized to generate an interrupt signal for adjusting the backlight 10. Alternatively, according to another embodiment, a conventional pulse width modulation (PWM) circuit is used instead. Whenever the threshold criteria in the block 20 have been met, the measured light intensity of the ambient light is read from the block 19 and utilized to generate a PWM signal for ultimately adjusting the backlight 10.
The embodiments disclosed above may be used, in other embodiments, to measure separate color channels of the image sensor 12, in order to provide an image sensor with an integrated color light meter for performing color light sensing functions. The measured separate color channels may be utilized to respectively control, for example, separate color lighting elements in the backlight 10.
According to the operations in Mode I and Mode II discussed above, the image sensor 12 with an integrated low-power light meter can be used to automatically adjust the display brightness based on the ambient light in the environment, both when the image sensor 12 is imaging and is not imaging. Accordingly, a low-power image sensor with an integrated ambient light meter is obtainable, and the system space and cost can be substantially reduced compared to a conventional system with a discrete ambient light photo sensor.
Although specific embodiments have been illustrated and described, it will be appreciated by those skilled in the art that various modifications may be made without departing from the spirit and scope of the present invention, which is intended to be limited solely by the appended claims.
