Home Lighting BasicsLuminous Intensity in Our Daily Life and for Home Use
Luminous Intensity in Our Daily Life and for Home Use

Luminous Intensity in Our Daily Life and for Home Use

What is Luminous intensity and Candela

We all know that luminous intensity is light intensity. In photometry, luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle, based on the luminosity function, a standardized model of the sensitivity of the human eye. The SI unit of luminous intensity is the Candela (cd) and the symbol of it is I.

Put it in an easiest way, the light intensity is the intensity of lumen emitted by a light source.

1. Luminous intensity can be classified as Central Intensity and Average Intensity
Central intensity means the measured intensity which on the axes of light beam

Average intensity means the average intensity of different directions in a fixed space.

2. Candela (cd) is the SI unit of luminous intensity, 1cd=1000mcd.


luminous intensity

The relationship among Luminous Intensity, Luminous Flux, Luminance and Illuminance

Name Unit Symbol Definition 
Luminous Intensity cd(Candela) I =Φ/Ω The Luminous Intensity means the intensity of lumens that emitted by a light source per unit solid angle in the fixed direction.
Luminance cd/m² L Luminance means the brightness of luminous surface and can be calculated as the ratio of luminous surface’s luminous intensity in the fixed direction and the area of luminous surface in the fixed and vertical direction.( cd/m²)
Luminous Flux lm(Lumen) Φ Luminous flux refers to the amount of visible light that light source emitted per second.
Illuminance LX (Lux) E Illuminance is the total luminous flux incident on a surface, per unit area.


luminous flux , intensity and illuminous

The comparison of incandescent bulbs’ light intensity to the light intensity of LEDs

Take a Philips 25W incandescent bulb as example

Angle 0°(central) 30° 45° 60° 90° 120°
 Luminous Intensity (cd) 25W incandescent bulb 15 14.5 15 14 12 12
5W  LED bulb 63 50 43 30 4 0

The effective of general lighting is about 60~90° (in that from is 30~45°). We can see from it that at 30°angle, the light intensity of LED bulb is 3 times higher than 25w incandescent bulb. At 45°angle, it still 2.5 times higher than 25w incandescent bulb.

So the actual lighting affection of this LED bulb is equal to a 50~50W incandescent bulb and it is 11times brighter than that 25W incandescent bulb.

luminous intensity and beam angle

Common application data of Luminous Intensity, Luminance and Illuminance
1. Luminous intensity of some common light source:  unit– cd

Highlight Torch: 10000cd

29W spotlight :   2900cd

5mm superbright LED: 15cd
2. Luminance of some common light source:  unit– cd/m²
The sun: 1.5*10

Fluorescent lamp:(5—10)*10³

Moonlight (full moon): 2.5*10³

The fluorescent screen of a black-and-white TV set: around 120

The fluorescent screen of colour television set: around 80

3. The Illuminance of different environment: unit—lux
Dark night: 0.001—0.02

Moon night: 0.02—0.3

Cloudy day indoor: 5—50

Cloudy day outdoor: 50-500

The sun at noon in summer: around 100000

Reading room: 50—60

Domestic video camera’s standard illuminance: 1400
cd value


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