Luminous energy facts for kids
In photometry, luminous energy is how much energy of light we can actually see. It's sometimes called the quantity of light. Luminous energy is different from radiant energy, which is the total energy of light, whether we can see it or not.
This is because our human eye can only see light within a certain range, called the visible spectrum. Our eyes are also more sensitive to some wavelengths (colors) of light than others. For example, when it's bright (photopic vision), our eyes see light best at a wavelength of 555 nm, which is a greenish-yellow color. So, light with the same total energy will have more luminous energy if it's 555 nm than if it's a different color. Light outside the visible spectrum, like ultraviolet or infrared, has zero luminous energy for us, even if it has a lot of radiant energy.
The official unit for luminous energy is the lumen second. It's also sometimes called the talbot, named after William Henry Fox Talbot.
What is Luminous Energy?
Luminous energy measures the total amount of light energy that our eyes can actually detect over a period of time. Think of it like this: a powerful flashlight might send out a lot of light energy (radiant energy), but if much of that light is infrared (heat) or ultraviolet (like in a blacklight), we won't see it. The luminous energy is only the part we perceive as brightness.
Our eyes have special cells that are more sensitive to certain colors. This is why a green laser pointer might seem brighter than a red one, even if both have the same total power. The green light is closer to the 555 nm wavelength that our eyes are most sensitive to.
Luminous energy is calculated by adding up the luminous flux (how much light we see at any moment) over a certain time. So, if a light source is on for a long time, it will produce more luminous energy than if it's on for a short time, assuming the brightness stays the same.
Units of Luminous Energy
The SI unit for luminous energy is the lumen second (lm⋅s). This unit combines the "lumen," which measures how much light we see, with "seconds," which measures time.
Another name for the lumen second is the talbot (T), named after the scientist William Henry Fox Talbot. This name is not officially part of the SI system but is often used.
See also
- Radiant energy
| Quantity | Unit | Dimension | Notes | |||||
|---|---|---|---|---|---|---|---|---|
| Name | Symbol | Name | Symbol | Symbol | ||||
| Luminous energy | Qv | lumen second | lm⋅s | T J | The lumen second is sometimes called the talbot. | |||
| Luminous flux, luminous power | Φv | lumen (= candela steradian) | lm (= cd⋅sr) | J | Luminous energy per unit time | |||
| Luminous intensity | Iv | candela (= lumen per steradian) | cd (= lm/sr) | J | Luminous flux per unit solid angle | |||
| Luminance | Lv | candela per square metre | cd/m2 (= lm/(sr⋅m2)) | L−2J | Luminous flux per unit solid angle per unit projected source area. The candela per square metre is sometimes called the nit. | |||
| Illuminance | Ev | lux (= lumen per square metre) | lx (= lm/m2) | L−2J | Luminous flux incident on a surface | |||
| Luminous exitance, luminous emittance | Mv | lumen per square metre | lm/m2 | L−2J | Luminous flux emitted from a surface | |||
| Luminous exposure | Hv | lux second | lx⋅s | L−2T J | Time-integrated illuminance | |||
| Luminous energy density | ωv | lumen second per cubic metre | lm⋅s/m3 | L−3T J | ||||
| Luminous efficacy (of radiation) | K | lumen per watt | lm/W | M−1L−2T3J | Ratio of luminous flux to radiant flux | |||
| Luminous efficacy (of a source) | η | lumen per watt | lm/W | M−1L−2T3J | Ratio of luminous flux to power consumption | |||
| Luminous efficiency, luminous coefficient | V | 1 | Luminous efficacy normalized by the maximum possible efficacy | |||||
| See also: SI · Photometry · Radiometry | ||||||||
