There are two sets of light intensity measurement units:

**Photometric **units and **Radiometric*** units.

The **Photometric** units measure the intensity of **visible light**** as it is perceived by the human eye, and the **Radiometric** units measure the intensity of **electromagnetic radiation*****, which is the broader physical phenomenon of light, including the whole spectrum of radiation beyond **visible light** **(like x-rays and infrared radiation for example).

**Light intensity is generally measured in three ways:**

**1.** The directional intensity received from a light source as it is measured from a point in space. i.e **Luminance** in **Photometric** units or **Radiance** in **Radiometric** units.

**2.** The total light intensity output a light source emits to all directions i.e **Luminous Flux** in **Photometric** units or **Radiant Flux** in **Radiometric** units.

**3.** The amount of light intensity received by a surface from all directions i.e **Illuminance** in **Photometric** units or **Irradiance** in **Radiometric** units.

Similarly to the way measurement of kinetic power is based on the power of an ideal horse, the **Photometric** measurement units base the scale of **light intensity** on the light emitted by an **ideal candle**.

**Luminance (Candela):**

When measured from any point in space, the **Luminance** of an ideal candle seen from that point is measured as **1** ‘**Candela**‘ i.e. **1** ‘**CD**‘.

**>** In 3D rendering, a photometric IES file describes a light source’s light beam pattern by listing the **Luminance** or CD of the light source in different directions.

For light emitting surfaces like LCD screens **Luminance** is measured as **Candelas** per **1 square meter of surface** i.e. **CD/m2**. Typical LCD computer monitors for instance, have a **Luminance** of about 250 CD/m2. imagine your computer screen displaying pure white and extended to an area of 1m x 1m, the light intensity perceived from it would be as if about 250 candles were spread on the whole area.

**Luminous Flux (Lumen):**

The amount of light emitted by an ideal candle through **1 solid angle** ‘**Steradian**‘**** conic beam distribution is measured as **1** ‘**Lumen**‘ or **1** ‘**lm**‘. the total **Luminous Flux** of the candle in all directions is **4 x PI lumens** i.e 12.56 lumens which is simply the whole surface area of the unit sphere.

**>** The total amount of light produced by different kind of light bulbs is usually specified by **Luminous Flux **measurement i.e how many **Lumens** does the light source output.

**>** If sn optical reflector is placed next to a light source, focusing all it’s light output to a narrow direction, it wont change the light source’s **Luminous Flux (Lumen) **output, but since the same **Luminous Flux** will be focused to a narrower beam, it will have a higher **Luminance (CD)** measured from that direction, and therefore surfaces at that direction will be receive a brighter **Illumination (Lux) **(see below).

**Illuminance (Lux):**

A 1 m2 (meter squared) area surface, receiving illumination of 1 lumen has a measured **Illuminance** of **1 lux** or **1 lx**. **Illuminance** is measured by how many lumens a surface receives per square meter.

**>** In photography, the amount of **Illuminance** at which a surface is lit is important for determining the proper **photographic exposure** for the picture.

**The inverse-square law**:

As a light beam travels through space it’s distribution covers a larger and larger area, therefore it’s energy per area is reduced. the light energy a candle emits through 1 solid angle steradian, in a distance of 1 meter will cover an area of 1 meter squared, therefore the area of **1 meter** squared will receive **1 lumen** of light energy and will be illuminated with an illuminance of **1 lux**. as that 1 lumen of light energy travels another 1 meter further, to a distance of **2 meters** from the candle, it spreads and covers an area of **4 meter** squared. each square meter of the 4 now receives just 1/4 of a lumen, so it’s illuminated by only **1/4 lux**. as that 1 lumen of light energy travels another 1 meter further, to a distance of **3 meters** from the candle, it spreads and covers an area of 9** meter squared**. each square meter of the 9 now receives just 1/9 of a lumen, so it’s illuminated by only** 1/9 lux**. after a distance of **4 meters**, the same 1 lumen on light energy will be spread on an area of **16 meter squared**, so each square meter will be illuminated by **1/16 lux**. you can already see the emerging pattern, the illumination intensity is **inversely proportional** to the **square of the distance** to the light source. This phenomenon is referred to as ‘**The inverse-square law**‘, and in practical terms it means that surface illumination is greatly influenced by it’s distance from the light source.

**Notes:**

*** Radiometric** units measure light intensity using **Watt** light energy units.

note that this isn’t the **Watt** measurement units of electric consumption we’re used to for classifying electric light sources with, but a Watt measurement of the actual energy in the light itself.

** **Electromagnetic radiation** of wave lengths that stimulate the human eye.

*** Also referred to as ‘**light**‘ in physics.

**** A ‘**Steradian**‘, also referred to as ‘square radian’ is a measurement unit of 3D conic span or ‘solid angle’. a solid angle of 1 Steradian beginning at the center of a unit sphere covers exactly an area of 1 squared on the surface of the sphere. (the whole surface area of the sphere being 4 PI). The Steradian can be thought of as the Radian’s 3 dimensional ‘cousin’.

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