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In this article, Phuc Gia would like to send you an overview as well as help you distinguish between terms that are easily confused, namely: Radiance, Irradiance, Luminance, and Illuminance.
DISTINGUISHING IRRADIANCE AND RADIANCE
Before learning about these two terms, we first must understand a more basic term. That is “radiant flux”.
What is Radiant Flux – Radiant Power?
“Radiant power” (also known as “radiant flux”) is the radiant energy emitted, reflected, transmitted, or received per unit of time (usually seconds).
The energy mentioned here is the energy of Photons traveling in light. According to the quantum theory of light, each photon carries energy. Therefore, when there is light (a stream of photons) passing through a unit area over time, we consider it a stream of radiant energy.
From any radiation source, the radiant energy per unit of time is called “radiant power”.
Therefore, the unit of radiant power is Watt (W), or also written as Joule/second (J/s).
*Symbol: Փ.
Assuming a radiation stream (light) has energy Q (J) passing through a unit area over time t (s), then:
dՓ = dQ / dt (W)
I. WHAT IS IRRADIANCE?
1. Definition
“Irradiance” is the radiant power (W) incident upon a certain surface per unit area (m2).
Therefore, it can be said that “Irradiance” is the “Irradiation power density”.
*Symbol: E = Փ/A
or E= dՓ/dA
*Unit: W/m2.
2. Spectral Iradiance
Is the “radiant power” dՓ(λ) incident upon a certain surface per unit area (dA) per unit wavelength (dλ) (measured in nanometers)
Lλ = dՓ(λ) / dA / dλ (W/m2/nm)
3. Examples:
Suppose we have a physical plane (A), area m2, and photons from different directions fly in and hit it, then:
– The number of photons hitting it in 1 second is “Flux” (W).
– If we divide that plane (A) into unit regions (m2) then that “Flux” becomes “Irradiance”. (W/m2)
– If we divide that plane (A) in half into (B) and (C), then the “Flux” is halved, but they still have the same Irradiance. (density)
+ (A)=(B)+(C)
+ Flux (B) = Flux (C)=1/2 Flux (A)
+ Irradiance (A)= Irradiance (B)= Irradiance (C)
4. Consequences
Along with those “Flux” flying in and hitting the given plane, there is also “Flux” bouncing off the surface, flying in the opposite direction. It could be reflected light, fluorescent light, or metal melted to a glowing temperature.
This “Flux” is called “Radiant Exitance” (Radiosity/Flux leaving a surface – per unit surface area).
Also known as “Radiant power leaving a surface” (possibly due to emission, reflection, or transmission) per unit area.
M = B = Փ/ A or = dՓ/ dA
But this does not indicate how bright an object looks. Because, basically, if the light at that surface is only emitted in a certain direction but not the direction you are looking at, then that surface cannot be seen as glowing.
In another way of understanding, suppose you see the Sun from Jupiter, or see the Sun from Earth, that is because space is completely empty and it does not absorb any light. And they both look exactly as bright; it is only because of the further distance that it looks smaller, so the amount of photons entering your eye is also smaller.
But that is not because the Sun does not look brighter, it is only because the viewing angle from your position relative to the Sun is smaller. So we need to find a quantity that explains that angle of yours, and that is called the Solid Angle.
Therefore, we need a definition of that “Solid angle” before we talk about Flux per Solid angle (Radian).
II. SOLID ANGLE
“The solid angle of a cone directed from a point” is the area (A) that it intercepts on a “unit sphere.”
If we have a sphere (I) with a radius of 1, and a concentric sphere (II) with a radius of r, then the area (A1) of the intercept on sphere (I) is r2 times smaller than the area (Ar) of the intercept on sphere (II):
Ar/ A1 = r2
If the plane (II) is tilted by an angle α the area (Ar) becomes (A). In that case:
A1 = Ar/ r2 = A.cos α/ r2
The unit of solid angle is the steradian (sr), similar to the radian (rad) in a two-dimensional (2D) plane.
*Symbol: dΩ
1 steradian is defined as the solid angle of a projected surface with an area ofr2 on a sphere with radius r from the center of that sphere.
III. RADIANCE
1. Example
Suppose we have a beam of light coming from the Sun, and a physical plane (A) perpendicular to that beam. In this case, we receive “Flux” per unit area on the incident surface within a solid angle (sr).
If we then move twice as far away from the Sun:
– The solid angle will be 4 times smaller.
– The amount of light (Irradiance) will also be 4 times smaller.
However, the Radiance remains constant because it is the power per unit solid angle.
2. Definition
“Radiance is the radiant power (W) incident perpendicularly onto a unit area (m2) per unit solid angle (sr)”.
Therefore, it can be said that Radiance is the “Radiant power density per unit solid angle” or also known as “Radiant illuminance.”
The larger the “Radiance,” the brighter the object appears
*Symbol: L
*Unit: W/m2/sr
L = dՓ/ dA/ dΩ (W/m2/sr)
If the incident plane is not perpendicular but tilted by an angle α:
L = dՓ/ dA/ dΩ/ cosα (W/m2/sr)
3. The relationship between Radiance (L) and Irradiance (E)
From: L = dՓ/ dA/ dΩ/ cosα
=> L. dΩ. cosα = dՓ/ dA = E
This is a type of cosine factor found in Lambert’s Law: “A diffuse surface emits light proportional to the amount of light incident upon it.” It is the product of the light’s brightness (L) (Radiance) and the solid angle (dΩ) (Solid angle). This is why the Sun’s luminous intensity remains constant, but the solid angle decreases as your distance increases.
Therefore, the product of Radiance and Solid Angle shows the Flux density incident on the plane per unit area (perpendicular to the beam).
The result is Irradiance (E).
Thus, if a surface is a perfect diffuse reflector, Radiance (L) is proportional to Irradiance (E).
4. Spectral radiance
Is the radiant power (Light) incident perpendicularly onto a unit area (m2) per unit solid angle (sr) per unit wavelength (nm).
L = dՓ(λ)/ dA/ dΩ/ dλ (W/m2/sr/nm)
DISTINGUISHING ILLUMINANCE AND LUMINANCE
To better understand these two quantities, we first need to learn about their common units…
1. Lumen (lm)
Lumen is the basic unit of Luminous flux used to measure the total amount of visible light emitted from a source, typically generated by an LED chip.
Lumen differs from Flux (Radiant power) in that Lumen includes all emitted electromagnetic waves, whereas Flux is measured according to a model (“luminosity function”) of human eye sensitivity to different wavelengths. Lumen is related to lux in that one lux is one lumen per square meter:
1lm = 1lx. 1m2
If a light source emits one candela of uniform Luminous intensity over a solid angle of one steradian, the total Luminous flux emitted into that angle is one lumen:
1lm = 1cd. 1sr
A source emitting 1W of light power at the color the eye perceives most efficiently (wavelength of 555 nm, in the green region of the spectrum) has a luminous flux of 683 lumens. Therefore, one lumen represents at least 1/683 W of visible light energy, depending on the spectral distribution.
But more importantly, evaluating luminous flux is merely a measurement of the general potential of light without accounting for technical product design stages such as shape, size, reflectivity, lens clarity, etc.
In terms of power, Lumen can be understood like the horsepower of a car engine.
For example, with two cars having the same horsepower rating, but assuming one is a jeep as heavy as a tank, it will not necessarily run as fast as a Toyota with the same horsepower rating, simply because their weights differ.
Similarly, with two lamps having the same luminous flux (lm) index but differing in electrical efficiency, lens clarity, or differences in the size and shape of the reflector, different light outputs will be produced.
2. Lux (lx)
The difference between the units “lumen” and “lux” is: “lux” accounts for the area over which the luminous flux is spread or covered.
“1 lux is the illuminance of a surface with an area of 1 square meter having a luminous flux of 1 lumen.”
A light stream of 1000 lm, concentrated on an area of 1m2, will illuminate that 1m2 with an illuminance of 1000 lx.
The same 1000 lm, when spread over an area of 10m2, will produce a dimmer illumination of only 100 lx.
Achieving an illuminance of 500 lux is possible in a kitchen with a fixed fluorescent lamp having a capacity of 12,000 lumens. To illuminate a factory floor with an area tens or hundreds of times larger than a kitchen, dozens or hundreds of such lamps are required.
Therefore, illuminating a larger area (m2) while maintaining the same illuminance value (lx) requires more lumens (lm).
Mathematically, 1 lx = 1 lm/m2.
*Relationship between illuminance and power
Illuminance is not a direct measurement of light energy, but rather the perception of the human eye.
Therefore, the conversion factor will vary according to the wavelength composition or the color temperature of the light. At a wavelength of 555 nm, in the middle of the spectrum, 1 lux is equivalent to 1,46 mW/m².
3. Candela
Candela truly helps us measure the volume, intensity, and density of light, whereas lumen refers to the total amount of light generated by a light source and Lux refers to the amount of light incident on a surface.
Candela refers to the intensity of that light in a specific direction.
Now, the major difference between lumen and Candela is: lumen refers to the total visible light output, while Candela refers to the output or intensity of light in a specific direction.
Candela is a base unit used in measuring light source parameters; it is the energy emitted from a light source in a specific direction and is calculated as follows:
1 candela is the intensity with which a light source emits 1 isotropic lumen within a solid angle.
A 1-candela light source will emit 1 lumen over an area of 1 square meter at a distance of one meter from the center of the light source.
1 cd = 1lm/ 1sr.
An ordinary candle emits light with a luminous intensity of approximately one candela. If emission in certain directions is blocked by an opaque barrier, this light source still possesses an intensity of approximately one candela in the directions that are not obscured.
“Candela” means “candle” in Latin, as well as in many modern languages.
I. LUMINANCE
1. Definition
Luminance is an extremely important component of the display screen industry. If you do not understand luminance, you will never fully understand how to measure a screen accurately.
Luminance refers to the amount of light passing through, emitted from, or reflected from an object.
Luminance can be measured using a luminance meter.
A luminance meter determines the amount of light transmitted through an object, allowing experts to determine the extent to which light affects that object.
*Unit: Cd/m2 – which is luminous intensity per specific unit area.
*Note:
Luminance is often confused with brightness. While the two concepts are quite similar, there are some striking differences between them.
Luminance is something that can be measured with a luminance meter; it is objective and a very clear fact that can be tested repeatedly.
However, brightness is something perceived only by the human eye; it cannot be measured by any formula.
2. Application
We encounter luminance more often than we think.
Luminance plays an important role in daily life.
For example, we encounter luminance every time we look at a screen. The whiter the screen, the greater the luminance we are exposed to. If we are exposed to too much luminance, it can become an issue harmful to the eyes and sometimes cause vision problems. However, most phone and computer devices allow you to change the level of luminance exposure in the device settings.
Luminance also plays an important role in the field of photography.
If the luminance level is too high, it can affect the quality of the photos you take.
One way to adjust the luminance level in your photos is to edit them through photo editing tools. These applications allow you to adjust the luminance of the photo, making it look brighter or darker as you see fit.
II. ILLUMINANCE
Illuminance is another important term in the display screen industry.
It is essential to understand the difference between the two terms Illuminance and luminance so that you can fully measure a display.
While luminance refers to the amount of light passing through or penetrating an object, Illuminance refers to the amount of light incident on a certain surface area.
The best way to explain the difference between these terms is through an example. Imagine a street lamp being lit.
– The amount of light passing through the light bulb relates to luminance.
– On the other hand, the amount of light shining down on the road below is illuminance.
*Unit: lux (lx)
Illuminance is measured in lux using an illuminance meter.
You must understand the difference between Luminance and Illuminance if you want a deeper understanding of the display industry. Most people who are unfamiliar with these terms refer to them generally as screen brightness.
However, it is important to know that luminance and illuminance are not synonymous with brightness.
You cannot measure brightness; it is something the human eye perceives.
Since both Luminance and Illuminance are quantifiable terms measurable with specific tools, it is inaccurate to use them interchangeably with brightness.
Thus, through this article, Phuc Gia hopes to have provided you with a more comprehensive overview and helped you distinguish between these easily confused terms.
For more details, please contact us at:
PHUC GIA LABORATORY CORPORATION
PHUC GIA CERTIFICATION CENTER
PHUC GIA INSPECTION TESTING CENTER
Address: ICD Long Bien, No. 01 Huynh Tan Phat, Sai Dong B Industrial Park, Long Bien Ward, Hanoi City, Vietnam.
Hotline: 0965996696 / 0982996696 / 02477796696
E-mail: lab@phucgia.com.vn/cert@phucgia.com.vn/info@phucgia.com.vn
Website: phucgia.com.vn
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