What unit is light intensity measured in?
The lux (symbol: lx) is the SI derived unit of illuminance and luminous emittance, measuring luminous flux per unit area. It is equal to one lumen per square metre. In photometry, this is used as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface.
One candela per steradian is termed a lumen, which is the measure of light intensity people are most familiar with. However, what matters most in terms of measuring light intensity is the number of lumens falling on a surface, which is expressed as lux.
- Light intensity is one of the factors affecting the rate of photosynthesis. Other factors are concentration of carbon dioxide, temperature and to a lesser degree, water. Light intensity directly affects the light-dependent reaction in photosynthesis and indirectly affects the light-independent reaction.
- Luminous flux is the quantity of the energy of the light emitted per second in all directions. The unit of luminous flux is lumen (lm). One lumen is the luminous flux of the uniform point light source that has luminous intensity of 1 candela and is contained in one unit of spatial angle (or 1 steradian).
- Unit of light flow or luminous flux. The output of artificial lights can be measured in lumens. Unit of illumination equal to one lumen per square metre. The metric equivalent of foot-candles (one lux equals 0.0929 footcandles).
In physics, intensity is the power transferred per unit area, where the area is measured on the plane perpendicular to the direction of propagation of the energy. In the SI system, it has units watts per square metre (W/m2).
- The more photons emitted per unit time, the greater the intensity of the light. A single photon has wavelength and speed. Therefore the energy of a single photon is not measured by its amplitude. However, the amplitude of a light wave depends on the number of photons per second being emitted.
- Thus, sound intensity is the power per square meter. The common unit of power is the watt ( 1w = 1 Joules/s). The unit of SL is called the decibel (abbreviated dB). I is the intensity of the sound expressed in watts per meter and Io is the reference intensity defined to be 10-12 w/m2.
- Instead they talk about the intensity of the light. Intensity is like brightness, and is measured as the rate at which light energy is delivered to a unit of surface, or energy per unit time per unit area.
- Candela (cd) Unit of luminous intensity of a light source in a specific direction. Also called candle.
- Footcandle (fc or ftc) Unit of light intensity, measured in lumens per square foot.
- Lumen (lm) Unit of light flow or luminous flux.
- Lux (lx) Unit of illumination equal to one lumen per square metre.
- The corollary of this is that electrical charge is not a fundamental unit, but defined in terms of the ampere and the second. The speed of light is currently defined as "exactly 299,792,458 metres per second".
- Intensity in different conditions
Illuminance Example 120,000 lux Brightest sunlight 111,000 lux Bright sunlight 20,000 lux Shade illuminated by entire clear blue sky, midday 1,000 - 2,000 lux Typical overcast day, midday
- The total luminous intensity per square meter is measured in nits. (1 nit = 1 candela/sq. meter). A 300 nit laptop screen with about 600 cm^2 of area would yield = 300*0.06 = 18 candela. Summary: Candela is the standard SI unit for measuring the intensity of a light source for photography.
Updated: 2nd November 2019