How Light Meters OperateEngineering360 News Desk | July 09, 2015
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.
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: β cones perceive light within 400-500 nm, Υ cones between 450-630 nm, and ρ 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.
A 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.
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.
Photodetectors are sensitive to all colors of visible light, including wavelengths not identified by the eye. Therefore, after exposure to a sample, photodetectors need to apply a correction factor to readings. Different light sources require different correction factors. Many commercial lux meters are preconfigured to register incandescent light, but have problems reading high intensity discharge, metal halide, high pressure sodium, and cool white fluorescent lights. Meters with preconfigured correction factors can provide accurate lux measurements for these sources. More advanced light meters are tuned to particular light sources with optical filters and lenses, removing correction factor uncertainty.
Most lux meters are handheld devices and are easily transported to the job site. Articulated and tethered photodetectors may require both hands to optimally position the photodetector and the module, but they also provide measurement flexibility. Some handheld models may include a stand or mounting structure, such as a tripod.
Cosine correction: light entering the photodetector at an angle can refract and cause a measurement error. Some photodetectors correct for this misreading.
Range: the dynamic detection range of the light meter, expressed in lux or foot candles. Many meters offer the user an adjustable detection scale.
Accuracy: the tolerance of meter readouts; most meters range between ±3-7%. Regular calibration minimizes tolerances, and laboratory and reference meters are calibrated for absolute measurements. Many operational factors can reduce instrument accuracy.
- Resolution: the minimal interval between readout numerals, usually adjustable based on measurement scale.
- Operating temperature: maximum ambient temperature to ensure effective operation. Many photodetectors will be permanently damaged above 120° F.
Auto-off: electronic models usually incorporate an automated power-down feature if the device is left idle.
Backlit: the LCD display uses backlighting to enable low-light measurement reading.
Battery indicator: low battery output can cause fluctuations in reading. It is recommended to measure samples twice, with interval, to compare results.
Hold: the user is able to lock-in or save a readout with this function.
Filters: internal or external mechanisms can remove certain wavelengths, such as blacklight, from the measurement that would skew the result
Outdoor: meter is suitable for outdoor applications, which experience the greatest differences in light intensity and environmental variables.
Over-range indicator: the meter informs the user the current detection scale is inadequate.
- Memory: an internal or external memory option allows users to store measurements.
- USB: a USB port allows the device to interface with computers, and may be used to recharge the battery.
Photodetector calibration is imperative for useful measurements. To calibrate a lux meter, a standard calibration-certified lamp with a known luminous intensity is placed opposite a meter that has been oriented for uniform illumination on the sensor. Measurements are taken at five different distances to determine a reference illumination, and the average of these measurements is summed with the correction factor of the reference lamp and the correction factor of the meter's resolution. This value is aligned with the meter's scale.
Most industrial lux meters do not provide an absolute readout—instead they provide a representative value in respect to their tolerance and resolution. Laboratory-calibrated lux meters are available for such applications, but will eventually need to be recalibrated. Some manufacturers recommend removing the photodetector cover several minutes before measuring a sample to reduce potential adaptation sensitivity in the sensor.