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Imaging Sphere Technical Briefing

Imaging Sphere IS-SA Schematic

No Smoke & No Magic (But We Do Use Mirrors)

The Imaging Sphere™ is an innovative tool that can be used to measure the luminous intensity of light sources, the view angle performance of displays, or the light scatter from surfaces (bi-directional reflectance distribution function or BRDF). It is based on patented technology jointly developed by Radiant Imaging and Royal Philips Electronics. The exact configuration of the Imaging Sphere™ system depends upon the type of measurement being performed, but, in all cases, it includes a hemisphere with a diffuse, low reflectivity coating, a curved secondary mirror and a ProMetric™ CCD imaging photometer or colorimeter. The hemisphere is attached to a flat, non-reflecting baseplate containing a small aperture at its center. The entire assembly is light tight.

In operation, light emitted by the source or display, or reflected by the surface under test, enters through the aperture in the baseplate and strikes the inner surface of the coated hemisphere, creating an illumination pattern characteristic of the source or material under test. Because the aperture is at the centre of the hemisphere, the illumination on its inner surface is an undistorted map of the angular emission profile of the spot under test – all captured at exactly the same distance.

Imaging Sphere OperationA convex mirror located on the baseplate opposite the imaging photometer enables it to view the entire inner surface of the hemisphere at once. Each of the 512 x 512 or 1024 x 1024 camera pixels sees a small cone of solid angle. The image recorded by the camera thus contains all the information necessary to reconstruct the entire angular intensity profile of the illumination at a resolution determined by the camera’s CCD image sensor. Measurement spot size can be arbitrarily set by selecting the aperture in the baseplate (when measuring the view angle of displays or the BRDF of materials).

A key element of the Imaging Sphere is a novel coating on the inner surface of the hemisphere. This spectrally non-selective, 18-20% reflectance coating is designed to deliver only diffuse (scattered) reflections. The baseplate is coated black to approach zero reflectivity. As a result, the camera sees an intense image of the first order illumination pattern with a very weak, uniform background created by second and higher order reflections. This background level depends on the total light flux and it is typically below 1% of peak image intensity, which is subtracted by the instrument software.

The spherical (convex) secondary mirror is located off axis which results in the camera recording an asymmetric (distorted) image of the illumination pattern on the inside of the dome. Nonetheless, it is a view of virtually the entire hemisphere inner surface. This data is then converted into an undistorted view, providing angular-resolved performance data in industry-standard formats. System software, developed by Radiant Imaging, performs these transformations.   

In terms of absolute calibration, the Imaging Sphere software offers several dimensions of calibration that can be factory set or performed by the end user. Specifically, the software allows the data to be calibrated for dome size, camera size and off-axis location of the mirror (to correct the image distortion). Secondly, it provides flat field or cosine field and offset calibrations to match the brightness and background conditions. Just as important, the software enables quantitative calibration of the system response for luminance, luminous intensity, BRDF and colour in commonly-used color coordinate sytemss such as CIE x y, u' v' and L*a*b*.

Radiant Imaging offers three different products based on Imaging Sphere™ technology, each configured to perform a different task: these are the IS-LI™, used to measure luminous intensity of LEDs and other small light sources; the IS-VA™, intended for view angle measurements of FPDs; and the IS-SA™, which performs surface scatter and BRDF or BTDF measurements.

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