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 |
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.