Luminance meter

Japanese: 輝度計 - きどけい（英語表記）luminance meter

A device for measuring the luminance of a light source. In this case, a light source does not only refer to a source that emits light itself, but also includes so-called secondary light sources, such as those that scatter and transmit light, such as ground glass, and those that diffusely reflect light, such as ordinary objects. Luminance is calculated by dividing the luminous flux (amount of light per unit time) emitted from a tiny surface of the light source within a tiny solid angle including the observation direction, as shown in Figure 1, by the apparent area and solid angle of that tiny surface. The principle of a luminance meter, as shown in Figure 2, is to use a lens to focus an image of a tiny portion of the surface to be measured on a light receiver, and then use an aperture to appropriately limit the solid angle of the incident light flux to measure light.

Source: Heibonsha World Encyclopedia, 2nd Edition Information

Japanese:

光源の輝度を測定する装置。この場合，光源とは自ら光を発するものだけでなく，すりガラスなどのように光を散乱して透過するものや一般の物体のように光を拡散反射するものなど，いわゆる二次光源も含める。輝度は図1のように光源の微小面から，観測方向を含む微小立体角内に放出される光束(単位時間当りの光量)を，その微小面の見かけの面積および立体角で割ったものである。輝度計の原理は図2のように，レンズにより被測定面の微小部分の像を受光器上に結ばせ，絞りにより入射光線束の立体角を適当に制限して測光するものである。

出典　株式会社平凡社世界大百科事典第２版について　情報

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