Lately the present author has reported in this journal the newly method to calculate the orthogonal projection curve of surface source concerning the unit sphere in calculating the illumination due to light from it.
In this report the approximate solution of the brightness increase on the internal surface of toroidal light source due to interreflection has been calculated by the above author's method and the increase of light intensity on the direction of the axes of symmetry (θ=0) has been also calculated.
As an example, for the dimmension of 20W toroidal type fluorescent lamp on the market, the maximum brightness increase on the internal surface is about 10.1%-11.2%, and the brightness distribution on the surface is represented approximately by the algebraic expression of the 4 thorder. Then the iucrease of light intensity on the direction θ=0 is about 1.7%
Next the difference of the light distribution and the total flux with and without internal interreflection has been calaulated approximately, provided that the reference has been as the ideal toroidal light source without thickness.
For the dimension of 20W lamp, the error to calculate the total flux is estimated about 0.7%.
From the present report, it has been made clear that the effect to the light distribution and the error of total flux due to interreflection is insignificant in spite of rather large brightness increase on the internal surface of toroidal source.
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