PURPOSE: To detect an index luminous signal while using a solid light receiving element, which is small-sized, highly reliable, cheap and requiring no high voltage, by guiding a light to the light receiving element while converting wave length thereof through a focusing plate filled with a specific solution.

CONSTITUTION: A cell in the shape of a transparent plate is made of 1mm thick glass plate, while a phosphor coloring matter brilliant.sulfoflavin solution fills the inside of the cell to be sealed. A focusing plate thus obtained is made to be sample for measurement. For comparison, the focusing plate filled with the solution, wherein rohdamine 6G and 4-MU serving as a phosphor materials are dissolved in ethylalcohol respectively, is also made by the same method to measure conversion efficiency as the focusing plate. As a result of measuring a size of a luminous signal in the focusing plate, wherein the concentration of the phosphor coloring matter is changed, the output of the luminous signal in the case (c) of the focusing plate containing a brilliant.solfoflavin solution is bigger than in the case (a) of the focusing 7 plate containing 4-MU and in the case (b) of the focusing plate containing rohdamine 6G.