Illuminance

Illuminance is a scientific measurement used in photometry to describe light. It measures the total luminous flux that lands on a specific surface area.

This measurement is important because it tells us how much light actually hits an object. It is not the same as luminance, which describes how bright a surface looks to our eyes. Instead, illuminance focuses on the light arriving at the surface. Scientists use a luminosity function to weight the light by its wavelength. This ensures the measurement correlates with how humans actually perceive brightness.

To understand how this works, we must look at how light interacts with surfaces. When light hits an area, it is called incident light. The amount of this light per unit area determines the illuminance. There is also a related concept called luminous emittance, or luminous exitance. This describes the luminous flux that is emitted from a surface rather than hitting it.

If a surface is a perfectly diffuse reflector, it is known as a Lambertian reflector. In this case, the luminance is isotropic, meaning it is the same in all directions. The relationship between the light received and the light reflected depends on the surface's reflectance.

Scientists use several different units to measure these light levels. In the International System of Units (SI), illuminance is measured in lux (lx). One lux is equivalent to one lumen per square metre (lm·m⁻²). Other systems use different names for these values. The CGS system uses a unit called the phot. In photography, people often use a non-metric unit called the foot-candle.

Using the correct unit helps scientists and photographers communicate clearly about light levels.

History shows that our language for light has changed over time. In the past, people often used the word "brightness" to describe illuminance. However, this caused confusion with other terms like luminance. Because of this, scientists now avoid using "brightness" for quantitative descriptions. They reserve that word for describing physiological sensations or how a person perceives light. Using precise terms like illuminance makes scientific data much more accurate.

Light levels in our world vary across a massive scale. The human eye can detect a range of light spanning over 2 trillion-fold. For example, sunlight is very bright at 100,000 lux. You can read large text at 10⁸ lux, which is 1,000 times brighter than direct sunlight.

However, such extreme brightness can cause discomfort or long-lasting afterimages. On the much dimmer end, a full moon provides only 0.1 lux. Starlight is even lower at 0.001 lux, yet we can still see white objects under it.

Astronomy provides a great example of illuminance in action. Astronomers measure the illuminance that stars cast on the Earth's atmosphere. This helps them determine how bright a star appears to us. They often use a scale called apparent magnitudes in the visible band.

There is a mathematical formula that allows scientists to convert these V-magnitudes into lux. This connection allows us to use light measurements to study the distant universe.

Illuminance also relates to the physics of light sources. When a light source is far away, it can be treated as a point source. In these cases, illuminance is linked to luminous intensity. This relationship uses the cosine law and the inverse-square law.

The formula shows that illuminance depends on the intensity of the source, the angle of incidence, and the distance. By understanding these connections, we can calculate exactly how much light will reach any surface in space or on Earth.