How Light Meters Work

Most light meters consist of a body, a photo cell and a readout. The light that falls on the photo cell has energy. This energy is transferred by the photo cell into electric current. The amount of current generated depends on the amount of light striking the cell. The meter then reads the electrical current and calculates the appropriate value of either Lux or foot-candles.

A key thing to remember about light is that it is usually made up of many different types (colors) of light at different wavelengths. The reading, therefore, is a result of the combined effects of all the wavelengths. A standard color can be referred to as color temperature, and is expressed in degrees Kelvin. The standard color temperature for calibration of most light meters is 2856 degrees Kelvin, which is more yellow than pure white.

Different types of bulbs burn at different color temperatures. Sper Scientific meter readings will, therefore, vary with different light sources of the same intensity. This is why some lights seem “harsher” than others. See the chart to the below for suggested lighting levels for various applications.

Light Meter Units of Measure

Light can be quantified in many ways, i.e., Lux, Lumens, foot-candles, candle power, candelas, and so on. The two most popular scales are Lux, which is the European measure, and foot-candles, which is the U.S. scale. Lux is a unit of illumination of one square meter, which is one meter away from a uniform light source. 1 candela = 1 Lux. Foot candles are a unit of illumination of one square foot which is one foot, away from a uniform light source.


Convenient Lighting Levels for Various Applications
Offices
100 to 300 Lux General workrooms, corridors, stairs and restrooms
300 to 750 Lux Conference and computer rooms
750 to 1500 Lux Technical offices, rooms for drawing and calculating
Factories
300 to 750 Lux Winding, steel work and welding
750 to 1500 Lux Inspection, welding, and heavy machinery operation
1500 to 3000 Lux Inspection and testing operations, selection areas, machine tool areas
Schools
75 to 300 Lux Lecture rooms, assembly halls, corridors, stairs and toilets
200 to 750 Lux Classrooms, demonstration rooms, gymnasiums
300 to 1500 Lux Precision drawing rooms, experimental laboratories, libraries, and reading rooms
Shops
120 to 500 Lux Stairs, elevators, toilets and corridors
750 to 1500 Lux Display windows and sales areas
4000 to 8000 Lux Jewelers and goldsmiths


 

Light Measurement Conversion Factors
To Convert From Into Multiply By
Lumens Candela steradian 1.0
Lumens Canle power (spherical) 0.07958
Lumens Watts 0.0015
Lumens per sq cm Lamberts 1.0
Lumens per sq cm Lux 10000
Lumens per sq cm Phots 1.0
Lumens per sq ft Foot-candles 1.0
Lumens per sq ft Foot Lamberts 1.0
Lumens per sq ft Lumens / sq meter 10.76391
Lumens per sq ft Lux / sq meter 10.76396
Lumens per sq meter Foot-candles 0.0929
Lumens per sq meter Lumens / sq ft 0.0929
Lumens per sq meter Phots 0.0001
Lumens per sq meter Lux 1.0
Lux Foot-candles 0.0929
Lux Lumens / sq meter 1.0
Lux Phots 0.0001


 

All models have a fast and accurate response, and are CE approved. The sensors are cosine and color corrected and hermetically sealed to ensure long term stability. All Sper Scientific light meters come ready to use with probes, 9V battery, carrying case, instructions, and a 5-year warranty.

Sper Scientific light meters have been checked and calibrated using N.I.S.T. traceable light sources and standards. The light source is a color correct bulb that burns at 2856 degrees Kelvin. The meters have a 5-year warranty. They are used in many applications, from testing the reflectance of porcelain pigments in a QC lab to pharmaceutical degradation studies.