Photometric Quantities and Units of Light Measurement
Photometry is the science of measuring light as perceived by the human eye. Unlike radiometry, which measures the total electromagnetic radiation, photometry focuses on the visual spectrum. To ensure clarity and avoid confusion with radiometric measurements, standards organizations recommend using a subscript "v" (for visual) when denoting photometric quantities.
Key Facts
- Luminous Flux is measured in lumens (lm).
- Luminous Intensity is measured in candelas (cd).
- Illuminance is measured in lux (lx).
- Luminance is measured in candelas per square metre (cd/m²), also known as nits.
- Luminous Efficacy describes the ratio of luminous flux to power or radiant flux, measured in lumens per watt (lm/W).
Foundational Concepts of Light Measurement
To understand how light is quantified, we must distinguish between the energy emitted by a source, the intensity of that light in a specific direction, and how that light interacts with surfaces.
Luminous Energy and Flux
Luminous energy (Qv) represents the total light energy emitted, measured in lumen seconds (lm⋅s), a unit sometimes referred to as the talbot. When this energy is measured over time, it becomes Luminous Flux (Φv), also known as luminous power. This is the total amount of visible light emitted by a source per unit of time, measured in lumens (lm), which is equivalent to candela steradians (cd⋅sr).
[ไม่มีภาพประกอบ]Intensity, Luminance, and Illuminance
While flux measures total light, Luminous Intensity (Iv) measures the flux per unit solid angle, expressed in candelas (cd). This describes how concentrated the light is in a specific direction.
When light hits a surface, we measure Illuminance (Ev), which is the luminous flux incident on a surface per square metre, measured in lux (lx). Conversely, Luminous Exitance (Mv) refers to the flux emitted from a surface, also measured in lumens per square metre.
Finally, Luminance (Lv) describes the luminous flux per unit solid angle per unit projected source area. Measured in candelas per square metre (cd/m²), this is the quantity often referred to as the nit in display technology.
[ไม่มีภาพประกอบ]Efficiency and Density
Beyond basic brightness, photometry evaluates how efficiently a source converts energy into visible light. Luminous Efficacy (K or η) is the ratio of luminous flux to either radiant flux or power consumption, measured in lumens per watt (lm/W). To determine the relative performance of a source, the Luminous Efficiency (V) is used, which is the efficacy normalized against the maximum possible efficacy.
Other specialized measurements include Luminous Exposure (Hv), which is time-integrated illuminance (lx⋅s), and Luminous Energy Density (ωv), measured in lumen seconds per cubic metre (lm⋅s/m³).
Summary of Photometric Units
| Quantity | Symbol | Unit | Unit Symbol | Definition/Note |
|---|---|---|---|---|
| Luminous energy | Qv | lumen second | lm⋅s | Also called the talbot |
| Luminous flux | Φv | lumen | lm | Luminous energy per unit time |
| Luminous intensity | Iv | candela | cd | Luminous flux per unit solid angle |
| Luminance | Lv | candela per square metre | cd/m² | Also called the nit |
| Illuminance | Ev | lux | lx | Luminous flux incident on a surface |
| Luminous efficacy | K / η | lumen per watt | lm/W | Ratio of flux to radiant flux or power |
Frequently Asked Questions
What is the difference between a lumen and a lux?
A lumen measures the total amount of visible light emitted by a source (luminous flux), whereas a lux measures how much of that light is actually hitting a specific surface area (illuminance). One lux is equal to one lumen per square metre.
What is a nit in light measurement?
A nit is a common term for one candela per square metre (cd/m²). It is the unit used to measure luminance, which is the intensity of light emitted from a surface in a particular direction.
What does the subscript "v" signify in photometric symbols?
The subscript "v" stands for "visual." It is used to distinguish photometric quantities (which are weighted for human vision) from radiometric or photon quantities.
How is luminous efficacy different from luminous efficiency?
Luminous efficacy is the absolute ratio of luminous flux to power consumption (measured in lm/W). Luminous efficiency is a dimensionless value representing the efficacy normalized by the maximum possible efficacy.
What is a talbot?
A talbot is an alternative name for the lumen second (lm⋅s), which is the unit used to measure luminous energy.