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a portable instrument for measuring illumination; a type of photometer. The simplest luxmeter consists of a selenium photocell that converts luminous energy into the energy of an electric current, which is measured by a pointer-type microammeter with scales calibrated in luxes (Ix). Different scales correspond to different ranges of the illumination being measured; scale changes are made by a switch that changes the resistance in the electric circuit. For example, a Iu-16 luxmeter has three measuring ranges: up to 25, up to 100, and up to 500 Ix. Higher illuminations may be measured by using a light-diffusing attachment on the photocell, which attenuates the incident radiation by a certain factor that is constant over a wide range of wavelengths.

The curves for the relative spectral sensitivity of a selenium photocell and the average human eye are not the same; consequently the readings of a luxmeter are a function of the spectral composition of the radiation. The instruments are usually calibrated with an incandescent lamp, and when simple luxmeters are used to measure the illumination produced by radiation having a different spectral composition, such as daylight or fluorescent lighting, a correction factor determined by calculation is introduced. The measuring error of such luxmeters is at least 10 percent of the indicated value.

More accurate luxmeters are equipped with correcting light filters that make the spectral sensitivity of the photocell close to the sensitivity of the eye; there is an attachment to reduce the errors when measuring illumination produced by obliquely incident light and a checking attachment for measuring the sensitivity of the instrument. The spatial characteristics of illumination are measured by means of luxmeters having spherical and cylindrical attachments. Luxmeters also exist with accessories for measuring brightness. The measuring accuracy of the best luxmeters is on the order of 1 percent.


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