Part21 ~ What is the difference between brightness and illuminance? ~

What is brightness?

Luminance is a physical quantity that describes the intensity of light emitted or reflected by an object.
Luminance is an index for quantifying the intensity of light emission or reflection, and indicates the amount of light energy emitted per unit area.
Brightness is determined by measuring how much the total energy of light emitted by a light source is dispersed over the surrounding area.

Intuitively, it tells you how bright an object appears to the human eye.
This quantity indicates how strong the light emission from an object's surface is, and thus determines how bright the object appears.
Luminance can also depend on the direction of the observer's line of sight.
The brightness changes depending on the light source and the direction of the object's surface, so the appearance and brightness of the object differs depending on the viewpoint.

The unit of brightness is "candela per square meter (cd/m²)" in the International System of Units (SI).
One candela corresponds to the intensity of a particular light source emitting light of a particular wavelength in a particular direction.
In the notation of brightness, "cd/m²" indicates how much light a light source emits per unit area.

This brightness measurement quantifies how bright a light source or object appears, and plays an important role in a variety of situations.

Brightness is an important element in forming visual impressions. For example, when designing televisions, computer screens, or lighting equipment, it is necessary to select an appropriate brightness to provide a comfortable visual experience.

The brightness of an object also affects its color and texture, with different brightness depending on the reflectance of light and the nature of the object's surface.
Luminance is therefore an important concept in optics and visual science, and is an essential element in understanding the properties of light and the appearance of objects.

How to measure brightness

One way to measure brightness is to quantify the intensity of light emitted or reflected by an object using optical instruments or sensors.

1.Luminance meter

A luminance meter is a device that measures the luminance of a light source and weights the wavelength of light according to the sensitivity of the human eye.
Also, brightness is not limited to light sources.
The brightness of objects such as walls and floors can also be expressed by their luminance.
The brightness of an object is measured by measuring how much light is reflected by the object when light from a light source hits it.
Special sensors and optical equipment are used to measure the reflected light.

Generally, the brightness of a light source such as a television or mobile phone is expressed by luminance, but the brightness of something used to illuminate the surroundings, such as a lamp, can also be expressed by luminance.

2.Brightness measurement using a spectrophotometer

A spectrophotometer is a device for measuring color and brightness, and is used to analyze the brightness of objects.
These devices measure the wavelength distribution of light emitted by an object and calculate brightness based on the results.

3.Measurement using image processing and digital technology

Another method is to use a camera or sensor array to capture the brightness of an object's surface as image data and analyze it to estimate the brightness.
By using image processing techniques, it is possible to spatially map the brightness of objects.

The accuracy of brightness measurement depends on the measurement equipment and method used, the light irradiation conditions, the characteristics of the object, etc.
By selecting an appropriate measurement method and performing accurate measurements, it is possible to evaluate the brightness and light characteristics of objects and utilize them in a variety of application fields.

What is illuminance?

Like brightness, illuminance is an index for quantifying brightness as a psychophysical quantity.
Illuminance indicates how well the light emitted by a light source is distributed over a particular area, quantifying how brightly that surface is illuminated.
In other words, it is an index that shows the amount of light and luminous flux density per unit area.

Illuminance generally refers to the amount of light energy that reaches per unit area.
Therefore, the unit of illuminance is expressed as "lux" in the International System of Units (SI).
1 lux indicates the uniform distribution of 1 lumen of luminous flux over an area of 1 square meter, and is used as a unit to express the brightness of light.
Lumen is a unit of luminous flux and represents the total amount of energy of light emitted in a particular direction.

Luminance is a numerical value that represents how much light reaches the human eye from a surface light source or light-receiving surface, whereas illuminance represents the amount of light that reaches a certain surface per unit area, and it is a value that represents the amount of light that reaches a certain surface per unit area, and is a value that represents the amount of light that reaches the human eye from a surface light source or light-receiving surface. It is calculated from the distance.

This concept quantifies how much light intensity brings to a particular area and is an important indicator of light irradiation.
Illuminance is affected by the distance from the light source, the irradiation angle, and the reflectance of the object surface.
Even if the brightness of the light source is constant, if the distance that the light is emitted from is far, the illumination intensity will be low.

Conversely, if the distance from the same light source is short, the illuminance will be high even if the light source is the same.
The reflectance of an object's surface also affects illuminance.
For example, a surface with a high reflectance will produce a higher illuminance from the same light source.

Illuminance is closely related to the perception of brightness and comfort.
For example, when designing interior lighting for living spaces, offices, factories, etc., it is important to ensure an appropriate level of illuminance.
Ensuring sufficient illuminance can improve work efficiency and reduce people's visual fatigue.
On the other hand, insufficient illumination can reduce work efficiency and cause visual impairment and stress.

Illuminance is also important in agriculture and plant cultivation.
Appropriate illuminance is necessary for plant growth and photosynthesis, and ensuring appropriate illuminance according to plant type and growth stage affects crop yield and quality.

Illuminance is also important in visual science and optics research.
Measuring illuminance allows us to understand human visual sensitivity and properties such as the hue and contrast of objects under lighting conditions.
The concept of illuminance is essential for scientifically elucidating the properties and effects of light.

Illuminance quantifies the intensity of light and is widely used in a variety of fields, from daily life to industry and science and technology.
By measuring the amount of light irradiation, it is possible to create a more comfortable environment and work more efficiently.

How to measure illuminance

Illuminance measurement is a technique for quantifying the intensity and brightness of light, and there are various methods. Some common illumination measurement methods are described below.

1.Measurement using a luminometer

A luminometer is a device for measuring illuminance, quantifying the intensity or brightness of light.
It measures the amount of light in a specific area and expresses the result in units of lux.

A light meter is a device with a light sensor that measures illuminance by detecting the amount of light hitting a surface.
A sensor receives the intensity and brightness of the light, and based on that information, a light meter numerically indicates the illuminance in a specific area.

This device is used to measure the brightness of indoor and outdoor lighting, as well as to evaluate lighting design and work environments.
They are used in a variety of locations, including offices, schools, factories, roads, and public spaces, and play an important role in ensuring proper illuminance levels.

A light meter is simple and easy to use, and allows you to quickly measure the light intensity in a specific location or environment.
The advantage of a light meter in light measurement is that it can accurately measure the intensity of light in a particular direction or angle.
This allows you to evaluate the intensity of light from a specific direction and changes in illuminance due to the angle of the light source, which is useful for design and evaluation of lighting levels.
Light meters are used as important tools in lighting design, environmental assessment, and evaluating the efficiency of light sources.
To ensure uniform lighting and appropriate brightness, luminance meters are useful for evaluating lighting installations and improving the environment.

2.Measurement using cameras and sensors

There is also a method of estimating illuminance by using a digital camera or light sensor and utilizing image processing technology.
It is possible to measure illuminance by acquiring the intensity of light in a specific area or object surface as image data and analyzing the brightness.

3.Calculations using mathematical models and optical simulations

There are also methods for predicting and calculating illuminance by mathematically modeling the characteristics of object surfaces and illumination configurations and performing optical simulations.
Illuminance can be calculated by considering parameters such as object shape, reflectance, and light source placement.

These methods are used to measure illuminance in different situations and environments.
Accurate illuminance measurement requires selection of appropriate measuring instruments and methods, as well as an understanding of light characteristics and environmental conditions.
Illuminance measurement is important in lighting design, architecture, agriculture, and industry, helping to maintain and evaluate appropriate lighting levels.

Brightness and illuminance to improve comfort and efficiency

Brightness and illuminance are different concepts that describe the properties of light.
Luminance indicates the brightness of light emitted by a light source or object, quantifying how bright the object or light source itself appears.
Illuminance, on the other hand, refers to the amount of light that reaches a particular surface or place, indicating how bright that place is.

In other words, brightness indicates the properties of the light source itself, while illuminance indicates the brightness of the place where the light reaches.

Due to this difference, brightness refers to how bright a light source or object reaches the eye, while illuminance refers to the amount of light that falls on a particular location.

Brightness and illuminance are essential elements in understanding the impact of light on our lives and improving comfort and efficiency.
Appropriate brightness and illuminance adjustment plays the role of reducing visual stress, ensuring safety and comfort, and improving quality of life.





Written by: BARREL Editorial Department
Supervised by: Yuki Oe (Lecturer, Department of Lifestyle Design, Faculty of Human Culture, University of Shiga Prefecture)
Lecturer at the University of Shiga Prefecture, Faculty of Human Culture (Department of Lifestyle Design).
She specializes in architectural environments, light environments, lighting environments, and color environments.
She is a member of the Architectural Institute of Japan/Illumination Institute of Japan.
After completing her doctoral course at Nara Women's University Graduate School of Humanities, she worked in technology planning at Sharp.
She worked as an assistant professor in the Faculty of Science and Technology at Tokyo University of Science and as a part-time lecturer at Osaka Jonan Women's Junior College before reaching her current position.