Unit of Light

Light is not a physical quantity, and therefore, cannot be measured. Hence, when we talk about the unit of light, it does not make much sense in practical terms. However, as light includes several physical quantities like wavelength, speed, intensity, etc., that can be measured, the unit of light can be estimated by using these physical properties. But, before we proceed towards understanding what the unit of light is, let's first have a look at what is light, how can we define it.

Light refers to one of the parts of the electromagnetic spectrum, which ranges from radio waves to gamma rays. As the name suggests, electromagnetic radiations are the fluctuations of electric and magnetic fields, which transport energy from one location to another. Visible lights are not inherently different from other parts of the electromagnetic spectrum but are available with the exception that the human eye can detect them.

Electromagnetic spectrum radiation in another way is described as a stream of photons, the massless particles traveling with wavelike properties, and at the speed of light. A photon is the smallest quantity of energy that can be transported, and it was the realization that light travels in discrete quanta, the origins of Quantum theory.

Visible light often has wavelengths in the range of 400-700 nanometers (nm), or we can say 4.00 x 10-7 to 7.00 x 10-7 m, amid the infrared rays (with longer wavelengths) and the ultraviolet rays (with shorter wavelengths). This specified wavelength means a frequency range of nearly 430-750 terahertz (THz). 

The primary properties of light include propagation direction, intensity, frequency or wavelength spectrum, and polarization, while its speed in a vacuum, 299, 792, 458 meters per second (m/s), is one of the fundamental constants of nature. Light, as with all sorts of electromagnetic radiation (EMR), is found to always move at this speed in a vacuum.


Units and Measures:

Light, in general, is measured with two sets of units: 

  • Radiometry emphasizes measurements of light power at all wavelengths, and

  • Photometry measures light with wavelength weighted for a standardized model of human brightness perception. Photometry is useful in quantifying illumination (lighting) intended for human use.  

Three main types of unit of lights are:

  1. Candela

  2. Lumen

  3. Lux



The candela refers to the base unit of luminous intensity in the International System of Units, i.e., (SI); that is, luminous power per unit solid angle emitted by a point light source in a specified direction. However, luminous intensity is analogous to radiant intensity, but rather than simply adding up the contributions of each wavelength of light in the spectrum of the sources, the contribution of each wavelength is weighted by the standard luminosity function.

The Candela is expressed by taking the fixed numerical value of the luminous intensity of monochromatic radiation of frequency 540 x 1012 Hz, Kcd, to be 683 if expressed in the unit lm W-1, which is equal to cd sr kg-1 m-2 s3. Here, the kilogram, metre, and second are in terms of h, c, and ∆vCs.

The frequency chosen in the visible spectrum is near the green, corresponding to the wavelength of around 555 nanometres (nm). If adapted for bright conditions, the human eye is most sensitive near this particular frequency. But, at other frequencies, the significantly more radiant intensity is needed to reach the same luminous intensity, according to the human eye’s frequency response. The luminous intensity for light with wavelength λ is given as

Iv (λ) = 683.002 lm/W. ӯ (λ). Ie (λ),


Iv (λ) = luminous intensity

ӯ (λ) = photopic luminosity function

Ie (λ) = radiant intensity

If there is more than one wavelength, then one must integrate over the wavelengths spectrum to achieve the total luminous intensity.

For example:

  1. A candle, in general, emits light with the luminous intensity of roughly 1cd.

  2. A 25 W fluorescent light bulb gives out about 1700 lumens, and if that light radiant equally in all directions, it will have a luminous intensity of:

         Iv = 1700 lm / 4 π sr  ≈  135 lm/sr = 135 cd.

  1. If focused on a 200 beam, the same light bulb would have an intensity of around 18,000 cd within the beam.



The lumen is the SI unit of luminous flux, the rate at which light (visible) is emitted from a source. Luminous flux differs from radiant flux (power) in a way that the radiant flux includes all the emitted electromagnetic waves, while flux is weighted as per a model (a luminosity function) of the sensitivity of the human eye to various wavelengths. 

In relation to the candela, the lumen is defined as:

1lm = 1cd · sr.

As a complete sphere has an angle of 4 π steradians, a light source that radiates one candela in all directions holds a total luminous flux of  1 cd x 4 π sr = 4 π cd• sr ≈ 12.57 lumens



The lux refers to the SI unit of illuminance and luminous emittance, measuring flux per unit area, and it is equal to one lumen per square meter. In photometry, it is a measure of the intensity of light that hits or passes through a surface.

A particular amount of light, if it is spread over a comparatively larger area, will illuminate the surface more dimly. Hence, we can say that the illuminance, if the luminous flux is kept constant, is inversely proportional to area. Lux is related to the lumen in a way that one lux is equivalent to a lumen per square meter and is given as:

1 lx= 1 lm/m2 = 1 cd•sr/m2

For example: 

A flux of 1000 lumens concentrated on an area of one square meter lights up that area with an illuminance of 1000 lux. However, spreading out the same flux over 10 square meters would produce a dimmer illuminance of just 100 lux.