Interesting question, I guess that it may be due to two things: that the camera vignetting is not corrected, or that the illumination of the sphere is not uniform (that is illuminated by the window shown in the figure).
I guess that the surface of the sphere is a Lambertian scatterer, so the cos4 law applies.
While rotating the camera by 90/180/270 degrees, nothing happened to the picture, still being an even sphere, and not rotate at all.
Where is your lens focus ?
Does the camera have micro lenses on the sensor ?
Looks to me there are micro lenses involved or you are focusing on the back wall of the sphere. In either case, you will have fall-off at the edges.
I'm sorry for not having explained the problem clearly
1. The focus place of the lens is about 0.5m, and the Intergral sphere has a diameter of 0.5m.
And besides, I have tried to make the focus being infinite which makes little difference.
2. The lens used in the application is circumferential symmetry, tests also done as follows to test if it is Circumferential symmetry.
Rotating the camera for 360 degrees with the lens focusing on the same area of the intergral sphere, little affection on the image.
3. The even sphere has an angle uniformity of about 98% within 60 degrees (The camera's field of view is 23 degrees), and experiment as follows also done for the verification
Rotating the lens for 30 degrees that focuses on the different areas of the intergral sphere, little differences can be found on the
4. The sensor used is bought from Sony, and yes, micro lenses are involved, of which is 3.45 micros for each.
5. One important phenomenon is that, every camera's shooting of the integral sphere,of which the illumination distribution of the picture is even sphere, and always vertical direction being higher in brightness for the same image height than the horizontal direction. Just different degrees of even sphere.
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