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INTEGRATING SPHERE AND FISHEYE CAMERA SYSTEM FOR GONIOMETRIC MEASUREMENT OF LIGHT SOURCES

Integrating spheres are commonly used for measuring the total luminous/radiant flux of light sources due to their advantages in measurement speed and overall cost compared to goniophotometers. However, the integrating sphere measurements fail to reveal any information on the luminous intensity distribution of the light source, which becomes more and more important for solid-state lighting products due to a large variety of designs and applications. Furthermore, the overall measurement uncertainty of the integrating sphere method is limited by the error resulting from the sphere's spatial non-uniformity, which is often unknown and difficult to correct. Correcting this error is not possible due to the lack of information on the intensity distribution of the light sources. In some cases, the typical intensity distribution of a light source is available from the manufacturer. However, a correction is still not possible because the exact intensity distribution of the light source is unknown while they are mounted in an integrating sphere (due to unknown angular alignment relative to the sphere). In practice, the spatial non-uniformity is inevitable because of the difficulty in applying the sphere coating or in making uniform diffuse reflective material and the use of baffles inside a sphere. To minimize the measurement error from the spatial non-uniformity of a sphere, a standard light source from a national institute of metrology (NIM) that has a similar intensity distribution is typically required so that errors from the sphere's spatial non-uniformity are mostly cancelled. But the types of standard sources are very limited.

It has been discovered that a differential goniophotometer provides a fast measurement of an angular intensity distribution of a primary light source. Instead of using a conventional goniophotometer, the differential goniophotometer includes an integrating sphere or hemisphere in combination with a fisheye lens for measurement of primary light, total luminous flux, and total radiant flux.  A camera is placed on the fisheye lens in which the fisheye lens has a large field of view to measure relative luminance or radiance distribution of the primary light source over an entire interior wall of the integrating sphere. Curvilinear images acquired by the fisheye lens are used to produce a luminous or radiant intensity distribution of the primary light source, based on a characterization of the integrating sphere for a point spread function and spatial non-uniformity. The absolute angular intensity distribution of the primary light source is obtained with calibration of the primary light source for total luminous flux or total radiant flux. The measured angular intensity distribution of the primary light source provides a correction of measurement error in total luminous flux, total radiant flux, and total spectral radiant flux, which can result from a spatial non-uniformity of an integrating sphere.

The new sphere method can be implemented into any existing integrating system for simultaneous fast measurements of total flux and luminous/radiant intensity distributions of light sources with little increase in cost ($1K).  Additionally, the luminance/radiance distributions of light sources can be measured inside an integrating sphere even with the highest coating reflectance of 98 %. Measurement of a luminance/radiance distribution inside a sphere with a lower coating reflectance such as 90 % is much easier because the difference of luminance/radiance inside the sphere is much larger. Also, a sphere with a low coating reflectance is more stable and less susceptible to the self-absorption error, but it tends to be less uniform. However, the error from the spatial non-uniformity of the sphere can be corrected using the newly developed sphere method.

 

Abstract: 

A new and fast method for measuring the angular intensity distributions of light sources is described. Instead of using a slow conventional goniophotometer this new method uses an integrating sphere that is commonly used for the measurement of light sources, but only for fast measurement of total luminous flux or total radiant flux. A fisheye camera with a large field of view is used to measure relative luminance/radiance distribution of a light source over the entire surface of the integrating sphere, which is then used to derive the relative luminous/radiant intensity distribution of the light source based on the characterization of the integrating sphere for its point spread function and spatial non-uniformity. The absolute angular intensity distribution of the light source is obtained with the calibration of the light source for total luminous/radiant flux. The measured angular intensity distribution of the light source not only provides additional valuable information but also makes it possible to correct the measurement error resulting from the spatial non-uniformity of an integrating sphere, which is often the dominant source of uncertainty and is the fundamental limit of achievable uncertainty using the integrating sphere method.

Benefits: 

The measured angular intensity distribution of the light source not only provides additional valuable information but also makes it possible to correct the measurement error resulting from the spatial non-uniformity of an integrating sphere, which is often the dominant source of uncertainty and is the fundamental limit of achievable uncertainty using the integrating sphere method.

Inventors: 
Yuqin Zong
Patent Number: 
9958317
Technology Type(s): 
Dimensional, Advanced Manufacturing Processes, Manufacturing, Optical Properties and Infrared Technology, Optical Technology, Precision Measurement
Internal Laboratory Ref #: 
15-029
Patent Issue Date: 
May 1, 2018
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