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Light has been an indispensable part of life throughout human history. When early humans discovered fire, it not only provided warmth but also allowed them to see their surroundings. Since then, many methods have been developed, from lighting tools such as candles and gas lamps to today’s LED, fluorescent, and bulb technologies.
Light intensity, one of the fundamental concepts for understanding lighting systems, plays an important role especially in interior and exterior design, energy efficiency, and lighting engineering. In this article, we will provide a comprehensive answer to the question, "What is light intensity?"
Light intensity refers to the amount of light emitted by a light source in a specific direction. Technically, it is defined as the luminous flux emitted per unit solid angle (steradian). In other words, a light source can emit varying amounts of light in different directions, and light intensity measures this directional distribution.
Not all lighting devices produce the same light intensity. For example, the illumination capacity of a simple bulb differs from that of an LED panel. This difference is largely due to the technology they use and their respective light intensities.
The unit of light intensity in the International System of Units (SI) is the Candela, represented by the symbol “cd”. It is also one of the seven base units in the SI system.
The word “Candela” means “candle” in Latin. Historically, the light emitted by a candle was considered approximately 1 candela. Therefore, the basis of measuring luminous intensity is derived from the candle.
Light intensity is fundamentally calculated using the following formula:
I = E × d² / cos(θ)
This formula is used to understand how effective a light source is in a specific direction. When performing the calculation, parameters such as distance and angle must be measured accurately.
Nowadays, specialized devices called goniophotometers are used to measure light intensity. These devices rotate the light source along both horizontal and vertical axes to measure light intensity from different directions.
Measurements are used to create a light intensity distribution curve. This curve shows the angles at which the light is more intense and serves as an important reference for lighting design.
Thanks to light intensity measurement:
Additionally, this measurement reveals not only the total light energy but also the light intensity in a specific direction.
Light intensity measures the amount of light emitted by a light source in a specific direction. This measurement is different from the total amount of light.
Light intensity is measured in the SI unit candela (cd).
Illuminance (lux) measures the total amount of light falling on a surface, whereas light intensity measures the light emitted by a source in a specific direction.
It is determined by measuring the light distribution of a light source at various angles using a goniophotometer. The results are given in candelas.
