The author tries to obtain the brightness distribution curve of a fluorescent lamp more easily by solving matrices of interreflection derived by Prof. O' Brien by means of a computer.
First of all, he divides a 20 watts fluorescent lamp into 20 parts and measures their brightness to take the ratios of them to that of the central part. And also he measures transmittances and reflectances of phosphors and a glass, radiation of ultraviolet and visible mercury rays.
Then getting shape factors between divided parts of a tube, he forms the matrices, and tries to coincide the calculated results with the measured ones changing the values of matrices, especially of the initial luminous radiances.
After many trials, he finds that inside the fluorescent lamp, ultraviolet rays interreflect with the reflectance factor about 0.7 and the consequent radiant flux density is quite high.
Next, he derives a following formula which contains the transmittance of the phosphor and the glassy and verifies it practically.
Li·out=τg [L_oi·out+τpΣ^m_j=1F (i, j) Lj·in]

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Average illumination is one of the most important indices in illuminating engineering. The lighting design often includes a prediction of the average illumination on the working plane or on the floor and the illumination and luminance distributions within the room. But, to calculate the value of it theoretically is quite difficult becase of the complexity of the calculatin. conditions. In that case, measuring is only way to obtain the values.
For the calculations of average illumination, the authors used average illumination method in which readings taken at specified positions are weighted according to their positions.
In this paper, to estimate the errors of the average illumination method and know its practicality and usefulness the following process was taken.
1. Obtain average illumination by interreflection method and regard it as standard illumination.
2. Obtain illumination distributions at specified positions-by-point by point method and using them get approximate value of average illumination.
3. Compare average illumination by interreflection method with the above values for various room conditions.
In case of luminous ceiling and panel lighting, the authors confirmed the practicality of average illumination method under various conditions.

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