We have to measure light in many tasks. For this, the Lux meter should be used. To measure the light accurately, a lux meter is a very suitable device. It is calibrated to read directly in lux.
The meters measure illumination in terms of luxes or foot-candles are known as Lux meters or Light meters.
A lux is equal to the total intensity of light that falls on a one square meter surface that is one foot away from the point of source of light.
A foot-candle is equal to the total intensity of light that falls on a one square foot surface that is one foot away from the point source of light.
Lux Meter Working Principle
Most of the lux meters consist of a body, photocell or light sensor, and display. The light that falls on to the photocell or sensor contains energy that is converted to electric current. In turn, the amount of current depends on the light that strokes the photocell or light sensor. Lux meters read the electrical current, calculate the appropriate value, and shows this value on its display.
It is the basic lux meter working principle. Analog luxmeters display values on a dial usually with a needle or pointer whereas digital devices display values as numbers letters. A Lux meter may be portable or bench type.
Since light usually contains different colors different wavelengths, the reading represents the combined effects of all the wavelengths. Typically, standard colors or color temperatures are expressed in degrees Kelvin (K). The standard color temperature for the calibration of most lux meters is 2856°K, an amount that is more yellow than pure white.
Selecting luxmeters or light meters requires certain performance specifications include photocell, illumination range, Lux resolution, operating temperature and foot candle resolution. Special features include low battery Indicators, low voltage, alarms, remote light sensors, built-in memory, auto power off, zero function etc.
Lux meters are used to measure levels of light in schools, hospitals, production areas, laboratories. They are also used to monitor light-sensitive displays in museums, art galleries etc.
Illumination (symbol E)
The luminous flux falling on a surface per unit area is known as illumination and is expressed in lux. It is also expressed in lumens per square feet.
Lux or Metre Candle: It is defined as the illumination produced by a uniform source of 1 C.P. on the inner surface of a sphere of 1-meter radius.
Foot Candle or Lumens / Sq. Foot: It is defined as the illumination produced by a uniform source of 1 C.P. on the inner surface of a sphere of 1-meter radius.
1 Foot Candle = 10.7608 Lux
1 Lux = 0.09293 ft. Candle
For practical purposes, illumination in lux may be divided by 10 to obtain illumination in lumens/square foot. An idea of illumination levels can be had from the following data:
|Summer midday in open||
|Winter midday in open||
|Sunrise and sunset||
Full moon and bright sky||0.25 Lux|
Required Values of illumination
Average living room||50 to 60 Lux|
|Corridors and passages||
10 to 30 Lux|
General offices||80 to 110 Lux|
|Office with good lighting||
600 to 800 Lux|
200 to 250 Lux|
|Hospital operation table||
1000 to 2000 Lux|
To Measure the Intensity of Light Source by a Flux meter
- keep the light source one foot away from the light sensor,
- now switch on the lux meter,
- note down the reading from Lux meter.
No shadows should be allowed to fall on the light sensor during measurements, and the light sensor should be held perfectly horizontally and stationary for accurate results.
How much lux do you need in a room?
Illumination requirement in a room depends upon its type and use. For example, in a bathroom, it is 100 lux whereas in a study room it is 300 lux. Because in a study room we need more illumination to read and write properly. You can measure the illumination level in a room at a particular place with the help of a flux meter and increase or decrease the number of light sources ( CFL or LED) accordingly.
Average illumination over a large area is measured by subdividing the whole area into small zones and measuring the illumination at the center of each zone. The dimensions of this area should not exceed ¼ the height of light fitting from the working place.
Thanks for reading about lux meter working principle.