Calibrated image-sensor-based ambient light sensor

• US 8,872,093 B2
• Filed: 04/18/2012
• Issued: 10/28/2014
• Est. Priority Date: 04/18/2012
• Status: Expired due to Fees

• Current Assignee: Apple Inc.
• Original Assignee: Apple Inc.
• Inventors: Lee, Chiajen, Sharma, Anup, Fan, Xiaofeng
• Primary Examiner(s): LEE, JOHN R
• Application Number:
  US13/449,842
• Publication Number:
  US 20130278576A1
• Time in Patent Office: 923 Days
• Field of Search: 250/214.AL, 250/214.R, 250/208.2, 250/208.1, 356/221, 356/222, 356/229, 356/213, 348/191, 348/226.1, 348/227.1, 345/207
• US Class Current: 250/214.AL
• CPC Class Codes: G09G 2320/0626 for control of overall brightness
• CPC Class Codes: G09G 2360/144 the light being ambient light
• CPC Class Codes: G09G 3/20 for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
• CPC Class Codes: H01L 27/14623 Optical shielding

Abstract

An electronic device may be provided with an image sensor for capturing digital images. The image sensor may be used as part of image-sensor-based ambient light sensing circuitry for producing ambient light sensor readings. The image-sensor-based ambient light sensing circuitry may include a reference array. The reference array may be formed from an array of light sensor elements that are matched to elements in the image sensor but that are covered with a light blocking material. Control circuitry can measure current flow into the reference array and the image sensor array and can use current measurements from these arrays in producing a calibrated ambient light sensor reading. The control circuitry may make current measurements by measuring a decay time associated with the voltage of a discharging capacitor. A comparator, pulse generator, and switch may be used in periodically recharging the capacitor. The capacitor may be adjusted to ensure accurate readings.

Claims

1. Image-sensor-based ambient light sensing circuitry, comprising:

an image sensor array;

an optically-black reference sensor array; and

control circuitry that produces an ambient light sensor reading based on data from the image sensor array and the reference sensor array, wherein the control circuitry is configured to measure current flow into the image sensor array and the reference light sensor array and is configured to produce the ambient light sensor reading based at least partly on how much current flows into the reference sensor array and how much current flows into the image sensor array.

2. The image-sensor-based ambient light sensing circuitry defined in claim 1 wherein the control circuitry includes a capacitor and wherein the control circuitry is configured to discharge the capacitor into the image sensor array to measure current flow into the image sensor array, and is configured to discharge the capacitor into the reference sensor array to measure current flow into the reference sensor array.

3. The image-sensor-based ambient light sensing circuitry defined in claim 2 wherein the reference sensor array comprises a plurality of light sensor elements that are covered with a light blocking material to prevent ambient light from reaching the light sensor elements.

4. The image-sensor-based ambient light sensing circuitry defined in claim 3 wherein the image sensor array has M light sensing elements, wherein the reference light sensor array has N light sensing elements that are covered with the light blocking material, and wherein the control circuitry is configured to produce the ambient light sensor reading based at least partly on the ratio of M to N.

5. The image-sensor-based ambient light sensing circuitry defined in claim 2 wherein the capacitor comprises a variable capacitor that is adjusted by the control circuitry.

6. The image-sensor-based ambient light sensing circuitry defined in claim 5 wherein the control circuitry is configured to adjust the variable capacitor to ensure accuracy in performing current flow measurements.

7. The image-sensor-based ambient light sensing circuitry defined in claim 2 wherein the control circuitry comprises a counter for measuring a voltage decay time associated with discharge of the capacitor during current flow measurements.

8. The image-sensor-based ambient light sensing circuitry defined in claim 2 further comprising at least a first switch and a second switch, wherein the control circuitry is configured to discharge the capacitor into the image sensor array during measurement of current flow into the image sensor array by closing the first switch and opening the second switch and is configured to discharge the capacitor into the reference light sensor array during measurement of current flow into the reference light sensor array by closing the second switch and opening the first switch.

9. The image-sensor-based ambient light sensing circuitry defined in claim 2 wherein the control circuitry comprises a switch that bridges the capacitor.

10. The image-sensor-based ambient light sensing circuitry defined in claim 9 wherein the control circuitry is configured to close the switch to charge the capacitor.

11. The image-sensor-based ambient light sensing circuitry defined in claim 9 wherein the control circuitry includes a comparator that compares a capacitor voltage associated with the capacitor to a threshold voltage.

12. The image-sensor-based ambient light sensing circuitry defined in claim 11 wherein the control circuitry includes a pulse generator.

13. The image-sensor-based ambient light sensing circuitry defined in claim 2 wherein the control circuitry comprises:

a switch that bridges the capacitor; and

a pulse generator having an output that supplies a control signal to the switch.

14. A method of calibrating image-sensor-based ambient light sensor circuitry, comprising:

closing a second switch to supply current to an image sensor;

while the second switch is closed, measuring an image sensor current for the image sensor while the image sensor is exposed to ambient light at an ambient light level;

opening the second switch and closing a first switch to supply current to a reference array of light sensor elements that are covered with light blocking material;

while the first switch is closed, measuring a reference current for the reference array; and

producing an ambient light reading representative of the ambient light level based at least partly on the measured image sensor current and the measured reference current.

15. The method defined in claim 14 wherein the image-sensor-based ambient light sensor circuitry comprises a capacitor and wherein measuring the image sensor current and the reference current comprises monitoring a capacitor voltage associated with the capacitor as the capacitor is discharged.

16. The method defined in claim 15 wherein monitoring the capacitor voltage comprises using a counter to measure a voltage decay time associated with discharging the capacitor from a first predetermined voltage to a second predetermined voltage.

17. The method defined in claim 15 wherein the image-sensor-based ambient light sensor circuitry comprises a third switch, the method further comprising:

charging the capacitor to a predetermined voltage by closing the third switch.

18. The method defined in claim 17 wherein the capacitor comprises a variable capacitor, the method further comprising adjusting the variable capacitor using the control circuitry.