25-04-2017, 02:29 PM
26-04-2017, 02:51 PM
The lux (symbol: lx) is the SI unit of illuminance and luminous emittance, which measures the luminous flux per unit area. It is equal to one lumen per square meter. In photometry, it is used as a measure of the intensity, as perceived by the human eye, of light striking or crossing a surface. It is analogous to the radiometric unit watts per square meter, but with the power at each wavelength weighted according to the luminosity function, a standardized model of human visual brightness perception. In English, "lux" is used as a singular and plural form.
Operation
Most lux meters record brightness with an integrated photodetector. The photodetector is located perpendicular to the light source for optimum exposure. Many lux meters use a photodetector articulated or tied for this purpose. The readings are presented to the user through an analogue or digital LCD instrument. Digital types usually include basic operator inputs. Many digital types can save measurements and have an adjustable sensing range.
Photodetectors composed of selenium or silicon determine the photovoltaic brightness. The current generated is proportional to the received photons. Silicon-based detectors need to amplify the voltage generated by exposure to light. Selenium-based detectors convert photons to high enough voltage to connect directly to a galvanometer, but have difficulty determining lux measurements for light sources below 1,000 lumens.
The photodetectors that measure the brightness through the photoresistance are composed of a ceramic substrate doped with cadmium sulfide. An electronic switching current is supplied to the cell and the resistance increases as more photons are detected to ultimately provide a proportional reading. The legislation restricts the availability of cadmium devices in certain territories.