08-17-2017, 12:28 AM
lux meter basic principle pdf
Luxmeter
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.
Lux meters, sometimes called light meters, measure the intensity of illumination as distinguished by the human eye. This value does not correlate to an objective value of energy radiated or reflected, as different wavelengths within the visible spectrum are perceived with varying sensitivity by the eye, and lux meters evaluate light intensity in consideration of this variable.
Measurement
The human eye distinguishes colors of light according to two complementary models of visual physiology. Trichromatic theory states that each of the three types of cones in the eye are activated by a certain range of wavelength: cones perceive light within 400-500 nm, cones between 450-630 nm, and cones between 500-700 nm. Opponent process theory states that colors are perceived by rods and cones antagonistically: black vs. white, blue vs. yellow, and red vs. green.
The result is an eye that perceives certain colors more accurately the average color perceptibility of the eye is represented by the CIE chart. More shades of green are identified than any other color and this is the primary reason night vision equipment amplifies green light reflection. Visible light intensity accounting for these inherent biological preferences is known as luminous flux. Lux meters cannot compensate for individual visual deficiencies or variances. Total power output is measured as radiant flux.
Lux light meter values popular areasA unit of luminous flux is known as a lux (lx). It is equal to one lumen per square meter and lux values directly correlate to brightness in consideration of distance, orientation, and environment. One lumen is the true visible light irradiance of a source it is equivalent to one candela per three-dimensional angle measured, or one candela per steradian. Essentially, it is the output of one common candle. More precisely, candela is specified as "a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian." A foot-candle is an alternative measure of illumination primarily used in the United States; one foot-candle is equivalent one lumen per square foot and approximately 10.76 lx.
Operation
Most lux meters register brightness with an integrated photodetector. The photodetector is positioned perpendicular to the light source for optimal exposure many lux meters use an articulated or tethered photodetector for this purpose. Readouts are presented to the user via analog instrument or digital LCD. Digital types often include basic operator inputs. Many digital types can save measurements and have an adjustable detection range.
Photodetectors composed of selenium or silicon determine brightness photovoltaically. Generated current is proportional to the photons received. Silicon-based detectors need to amplify the voltage generated by light exposure. Selenium-based detectors convert photons to a high enough voltage that they be directly connected to a galvanometer, but have difficulty determining lux measurements for light sources below 1,000 lumens.
Photodetectors that measure brightness via photoresistance are composed of a ceramic substrate doped with cadmium sulfide. An electronic switching current is supplied to the cell and resistance increases as more photons are detected to ultimately provide a proportional readout. Legislation curtails the availability of cadmium devices in certain territories.