Definition of Illuminance
In photography, the first issue encountered is "light intensity", which determines the amount of exposure during shooting. How do we express the amount of light at a certain distance from the light source? Usually, this is described as "illuminance", and the principles applied in the field of metering are based on the concept of "illuminance". Before discussing "illuminance", we must first define the meanings of the following two terms.
(a) Standard Light Source. The traditional standard light source is the light emitted by a candle made from whale oil. The modern definition involves heating a black body radiator (a one-cubic-centimeter black object) to a temperature close to melting, where the amount of light emitted is termed the standard light source.
(b) Candela. This is the unit of light emitted by the standard light source, with another unit being lumen.
Illuminance refers to the amount of light distributed over a unit area at a certain distance from the light source. Therefore, its unit is defined as "foot-candle" (foot-candle) or lumen/ft², and it is now also expressed in lux (1 lux = 1 lumen/m²). 1 lumen/ft² = 10.76 lux. For convenience, we will discuss using "foot-candle" as the unit.
In the field of metering, questions related to "illuminance" often involve the "inverse square law," which states:
This means that the illuminance at a certain distance from a light source is directly proportional to the brightness of the source and inversely proportional to the square of the distance. For example, if the brightness of the point light source is 1 candle, the illuminance at a distance of 1 foot from the source is 1 foot-candle; if the distance increases to 2 feet, the illuminance becomes 1/4 foot-candle; if it increases to 3 feet, the illuminance becomes 1/9 foot-candle. Thus, as the distance increases, the illuminance decreases in proportion to the square.
Note that this formula discusses point light sources (e.g., flashlights, photography bulbs), while sunlight is treated as an area light source. The illuminance of sunlight remains the same whether at high altitude or at sea level.
The diagram above illustrates the "inverse square law." It indicates that if the point light source remains unchanged, the illuminance decreases with distance according to the inverse square relationship. Therefore, if the camera's aperture is f/16 when the light source is one meter away, when the light source distance increases to two meters, the required aperture should change to f/8, not f/11, because the illuminance becomes 1/4 of the original instead of 1/2.
When a surface is illuminated, the incident light flux per unit area is referred to as "illuminance," measured in lux, where one lux equals one lumen of light flux per square meter:
Illuminance (lux) = Light Flux (lumen) / Area (square meters)
Typically, adequate lighting for reading is about 500 lux. The lighting in classrooms and offices should reach at least 300 lux. Below are several representative illuminance levels:
| Bright Sunlight | 100,000 lux |
| Cloudy Day | 8,000 lux |
| Drawing | 600 lux |
| Reading | 500 lux |
| Nighttime Baseball Field | 400 lux |
| Office, Classroom | 300 lux |
| Street Light | 5 lux |
| Full Moon | 0.2 lux |
| Starlight | 0.0003 lux |
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