Multifunctional sky camera system for total sky imaging and spectral radiance measurement

• US 9,565,377 B2
• Filed: 04/30/2013
• Issued: 02/07/2017
• Est. Priority Date: 04/30/2013
• Status: Expired due to Fees

• Current Assignee: International Business Machines Corporation
• Original Assignee: International Business Machines Corporation
• Inventors: Hamann, Hendrik F., Lu, Siyuan
• Primary Examiner(s): Anderson, II, James M
• Application Number:
  US13/873,653
• Publication Number:
  US 20140320607A1
• Time in Patent Office: 1,379 Days
• Field of Search: None
• US Class Current: 1/1
• CPC Class Codes: G01W 1/12 Sunshine duration recorders measuring intensity of sunshine G01J
• CPC Class Codes: G06T 2207/10021 Stereoscopic video; Stereoscopic image sequence
• CPC Class Codes: G06T 5/94 based on local image properties, e.g. for local contrast enhancement
• CPC Class Codes: G06T 7/20 Analysis of motion motion estimation for coding, decoding, compressing or decompressing digital video signals H04N19/43, H04N19/51
• CPC Class Codes: G06T 7/215 Motion-based segmentation
• CPC Class Codes: G06T 7/593 from stereo images
• CPC Class Codes: G06T 7/85 Stereo camera calibration
• CPC Class Codes: H04N 13/239 using two 2D image sensors having a relative position equal to or related to the interocular distance H04N13/243 takes precedence
• CPC Class Codes: H04N 23/60 Control of cameras or camera modules
• CPC Class Codes: H04N 23/69 Control of means for changing angle of the field of view, e.g. optical zoom objectives or electronic zooming
• CPC Class Codes: H04N 23/698 for achieving an enlarged field of view, e.g. panoramic image capture
• CPC Class Codes: H04N 23/75 by influencing optical camera components
• CPC Class Codes: H04N 25/621 for the control of blooming

Abstract

A multifunctional sky camera system and techniques for the use thereof for total sky imaging and spectral irradiance/radiance measurement are provided. In one aspect, a sky camera system is provided. The sky camera system includes an objective lens having a field of view of greater than about 170 degrees; a spatial light modulator at an image plane of the objective lens, wherein the spatial light modulator is configured to attenuate light from objects in images captured by the objective lens; a semiconductor image sensor; and one or more relay lens configured to project the images from the spatial light modulator to the semiconductor image sensor. Techniques for use of the one or more of the sky camera systems for optical flow based cloud tracking and three-dimensional cloud analysis are also provided.

Claims

1. A method for cloud tracking, the method comprising the steps of:

(a) obtaining a time-lapsed series of sky images using a sky camera system, wherein an image of the sun is included in the time-lapsed series of sky images, the sky camera system including (i) an objective lens having a field of view of at or greater than 170 degrees, (ii) a spatial light modulator at an image plane of the objective lens, wherein the spatial light modulator is configured to attenuate light from objects in images captured by the objective lens, (iii) at least one anti-blooming semiconductor image sensor, and (iv) one or more relay lens configured to project the images from the spatial light modulator to the anti-blooming semiconductor image sensor, wherein the spatial light modulator is used to attenuate the image of the sun such that the image of the sun can fit into a dynamic range of the anti-blooming semiconductor image sensor along with the time-lapsed series of sky images, and wherein a response of the sky camera system, prior to field operation, has been calibrated indoors using a calibration light source of known radiation intensity, and

(b) computing I) a velocity of one or more pixels in the time-lapsed series of sky images using optical flow analysis, wherein pixels in the time-lapsed series of sky images corresponding to cloud regions have a finite velocity and pixels in the time-lapsed series of sky images corresponding to non-cloud regions have a zero velocity, and II) total solar irradiance by integrating a signal from all pixels in the time-lapsed series of sky images multiplied by an attenuation factor at corresponding pixels of the spatial light modulator and the response of the sky camera system.

2. The method of claim 1, further comprising the step of:

(c) distinguishing the cloud regions from the non-cloud regions in the time-lapsed series of sky images using the velocity of the pixels computed in step (b) and a location of the pixels in the time-lapsed series of sky images.

3. The method of claim 1, further comprising the step of:

(d) forecasting cloud movement using linear extrapolation and the velocity of the pixels computed in step (b) wherein the pixels corresponding to the cloud regions at location (x,y) at time t are forecasted to be at location (x +V_xΔ

t,y+v_yΔ

t).

4. The method of claim 1, wherein the sky camera system comprises multiple anti-blooming semiconductor image sensors, and wherein the sky camera system further includes (v) a dichromatic filtering component between the objective lens and the anti-blooming semiconductor image sensor configured to capture images from multiple desired wavelength windows.

8. The method of claim 1, wherein the calibration light source comprises a calibration lamp on an integration sphere.

9. The method of claim 1, wherein the total solar irradiance comprises direct solar irradiance and diffuse solar irradiance, the method further comprising the step of:

computing the direct solar irradiance by integrating a signal from pixels of the image of the sun in the time-lapsed series of sky images multiplied by an attenuation factor at corresponding pixels of the spatial light modulator and the response of the sky camera system.

10. The method of claim 1, wherein the total solar irradiance comprises direct solar irradiance and diffuse solar irradiance, the method further comprising the step of:

computing the diffuse solar irradiance by integrating a signal from all pixels in the time-lapsed series of sky images excluding the image of the sun multiplied by an attenuation factor at corresponding pixels of the spatial light modulator and the response of the sky camera system.

5. A method for three-dimensional cloud tracking, the method comprising the steps of:

(a) obtaining sky images from each of at least one first sky camera system and at least one second sky camera system, wherein an image of the sun is included in the sky images, and wherein the first sky camera system and the second sky camera system each includes (i) an objective lens having a field of view at or greater than 170 degrees, (ii) a spatial light modulator at an image plane of the objective lens, wherein the spatial light modulator is configured to attenuate light from objects in an image captured by the objective lens, (iii) at least one anti-blooming semiconductor image sensor, and (iv) one or more relay lens configured to project the image from the spatial light modulator to the anti-blooming semiconductor image sensor, wherein the first sky camera system and the second sky camera system are located at a distance of from about 100 meters to about 1,000 meters from one another, wherein the spatial light modulator is used to attenuate the image of the sun such that the image of the sun can fit into a dynamic range of the anti-blooming semiconductor image sensor along with the sky images, and wherein a response of the sky camera system, prior to field operation, has been calibrated indoors using a calibration light source of known radiation intensity;

(b) measuring a position of clouds in the images based on x,y,z axes coordinates of each of the first sky camera system and the second sky camera system;

(c) measuring an x′

,y′

,z′

orientation of each of the first sky camera system and the second sky camera system with respect to zenith and north with solar position as a reference point using the solar position based on a local time and a location of each of the first sky camera system and the second sky camera system with respect to the zenith and north;

(d) determining an orientation of the clouds with respect to the zenith and north using the position of the clouds in the images measured in step (b) and the orientation of each of the first sky camera system and the second sky camera system with respect to the zenith and north determined in step (c); and

(e) calculating I) a position and a vertical height of the clouds in the sky using triangulation based on the orientation of the clouds with respect to the zenith and north from step (d) and three-dimensional coordinates of the first sky camera system and the second sky camera system, and II) total solar irradiance by integrating a signal from all pixels in the time-lapsed series of sky images multiplied by an attenuation factor at corresponding pixels of the spatial light modulator and the response of the sky camera system.

6. The method of claim 5, wherein the orientation of each of the first sky camera system and the second sky camera system with respect to zenith and north in step (c) is determined using measurement instruments.

7. The method of claim 5, wherein the orientation of each of the first sky camera system and the second sky camera system with respect to zenith and north measured in step (c) is further corrected using solar position as a reference point, the method further comprising the step of:

(f) calculating a solar position based on a local time and a location of each of the first sky camera system and the second sky camera system with respect to a zenith and north.