Photometer - light meter

In the narrow sense, it is a device that measures the luminous intensity of a light source. In the broad sense, it is a device that measures illuminance, brightness, transmittance, concentration, etc. It is also called a photometer. Until recently, the method of measuring luminous intensity was to adjust the distance from a standard light source with known luminous intensity and an unknown light source to the measurement point, visually determine whether the brightness is equal at that position using the human eye, and calculate the luminous intensity of the unknown light source from the distance relationship. However, since this method lacks objectivity, it is now common to use a photoelectric detector with a filter that matches the wavelength sensitivity of the human eye, and to calculate the luminous intensity from the measurement of the illuminance at a point some distance away from the known and unknown light sources. A photometer that uses a photoelectric detector is called a photoelectric photometer. With a suitable detector, a photoelectric photometer can measure light energy in the ultraviolet and infrared ranges as well as the visible range. In addition, when combined with a spectrometer, it is possible to measure the energy distribution of a light source for each wavelength and the transmittance distribution of a sample.

[Tanaka Shunichi]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

狭義には光源の光度を測定する装置。広義には照度、輝度、透過率、濃度などを測定する装置。測光器ともいう。光度を測定するのに、しばらく前までは、光度が既知の標準光源および未知の光源から測定点までの距離を調節し、その位置で輝度が等しくなるのを人間の目を使って視覚判定し、距離の関係から未知光源の光度を求める方法が用いられた。しかしこの方法は客観性に欠けるので、現在はフィルターで波長感度を人間の目のそれとあわせた光電検出器を使用し、既知光源、未知光源からある程度離れた点での照度の測定から光度を求める方法が普通用いられる。光電検出器を用いる光度計を光電光度計という。光電光度計では適当な検出器を使用すると、可視域だけでなく紫外・赤外域での光のエネルギーなどの測定も行える。また分光器と組み合わせると、各波長についての光源のエネルギー分布や試料の透過率分布の測定も可能になる。

［田中俊一］

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