Application Note

Light Emitting Diode

Key Characteristics ofLight - Brightness and Color

Table of contents
1. Overview ................................................... 2
2. Properties of Light .................................... 2
3. Brightness ................................................. 2
4. Color Evaluation Methods ........................ 8
5. Summary ................................................. 10

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Application Note
1. Overview
LEDs are used in products in various fields such as luminaires, home appliances, and automobiles.
Since the environments/conditions where the LEDs are actually used vary depending on different
applications, LEDs are required to have various performances. Nichia offers a wide variety of LEDs
designed for different types of applications to maximize the required performance.
When designing luminaires, it is necessary to use design and evaluation methods that are suitable forthe chosen application. In order to select the most suitable methods, customers should fully understandthe concepts of light and use suitable photometric properties and units for the chosen application.
This application note explains the most basic concepts of light which are “Brightness” and “Color”.

Light Emitting Diode

2. Properties of Light
The basic concepts of light properties are “Brightness” and “Color”.
“Brightness” indicates the intensity/amount of light emitted from a light source and light reflected off
an object (e.g. floor or wall). In darkness, the absence of any light, humans cannot see objects or movefreely. For any environment or circumstance, a suitable level of brightness is essential for humans tolive.
“Color” represents the appearance of an object’s surface such as “an apple is red.” Humans perceive
the difference in intensity of each wavelength of light reflected off an object surface as color. Coloris dependent on not only the wavelength components of irradiated light but also the reflective
characteristics of an object’s surface.
White light is generally used in homes, offices, etc. White light consists of various colors(wavelengths) and is the color appearance that allows humans to perceive the color of an object
without discomfort. On the other hand, monochromatic light such as red or blue light is used toaccentuate a particular color for decorative and display purposes. Sections 3 and 4 explain photometricproperties and units.

3. Brightness
3.1. Luminous Flux (Total Luminous Flux)
3.1.1. Luminous Flux – Photometric Property and Unit
Luminous flux is a measure of the total amount of light emitted from a light source and is the mostcommonly used photometric property to indicate the brightness of LEDs, luminaries, etc. The SI unit
for luminous flux is the lumen (lm). Nichia uses luminous flux to sort most of Nichia’s LEDs into thesorting ranks for each specification by measuring the brightness of the LEDs1.
Lumen
(lm)

Figure 1. Image of Luminous Flux
Some of Nichia’s LEDs are sorted by luminous intensity. The sorting ranks vary depending on the product type. See the applicable specification forthe details.
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Application Note
3.1.2. Definition of Luminous Flux
In order to obtain the luminous flux, it is necessary to measure the amount of radiant energy that isemitted from the light source and received by a certain surface per unit of time.
A Certain Surface

This radiant energy is referred to as radiant flux and its SI unit is the watt (W). The luminous flux is
obtained by multiplying the amount of radiant flux within the wavelength range of visible light(380nm to 780nm) with the spectral luminous efficiency function (human eye’s spectral sensitivity).
Generally, radiant flux is used to represent the light emitted from a light source as energy, andluminous flux is used to represent the light emitted from a light source as brightness withconsideration of the human eye’s spectral sensitivity.
The equation for calculating luminous flux is shown in Equation 1, and an image of the calculation isshown in Figure 3.
Equation 1:780

Φv = Km ∫ 380 Φe,λ(λ)・V(λ) dλΦv = Luminous flux (lm)Km = The maximum spectral luminous efficacy (lm/W)Φe,λ(λ) = Spectral radiant flux in wavelength (W/nm)
V(λ) = Spectral luminous efficiency function
dλ = Wavelength difference (nm)Km refers to the maximum spectral luminous efficacy, which is Km=683lm/W at a wavelength of
555nm (507nm in scotopic vision) where the spectral luminous efficiency function is 1.0.
Φe,λ(λ) and V(λ) represent the spectral radiant flux in wavelength and the spectral luminous efficiency
function at a given wavelength, respectively.
Integrated in the Visible Wavelength Range

380

480

580
680
Wavelength λ (nm)

780

1
0.8

×Km =

0.6
0.4
0.2

Km×Φe,λ(λ)×V(λ)×

Spectral Luminous
Efficiency Function V(λ)

1.2Spectral Radiant Flux
Φe,λ(λ)Light Emitting Diode

Figure 2. Definition Image of Luminous Flux

0
380

480

580
680
Wavelength λ (nm)

780

380

480

580
680
Wavelength λ (nm)780

Figure 3. Image of Luminous Flux Calculation
The term “luminous flux” is sometimes called “total luminous flux” to emphasize the fact that it is
the total for all directions from the light source. The term “luminous efficiency” usually refers to thevalue obtained from dividing the total luminous flux by the input power (see Equation 2).

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Application Note
Equation 2: η = Φv / P
η = Luminous Efficacy (lm/W)
Φv = Total Luminous Flux (lm)P = Input Power (IF × VF) (W)IF = Forward Current (A), VF = Forward Voltage (V)
3.2. Luminous Intensity

Light Emitting Diode3.2.1. Luminous Intensity – Photometric Property and Unit
Luminous intensity is a measure of the amount of light emitted from a light source in a particular
direction. The SI unit for luminous intensity is candela (cd). For a light source that has directivity
characteristics (i.e. the brightness varies depending on the illumination direction) such as a flashlight,
luminous intensity is used to evaluate whether the intended brightness is emitted in the intended
direction.
LEDs also have directivity characteristics. The directivity characteristics of Nichia’s LEDs varydepending on the LED type. For some of Nichia’s LEDs, the amount of light in the direction
perpendicular to the light emitting surface has been defined as the maximum luminous intensity(typical value) and stated in the specification. Refer to the applicable specification for the details.

Candela(cd)

Figure 4. Image of Luminous Intensity
3.2.2. Definition of Luminous Intensity
Luminous intensity is defined as the luminous flux emitted by a light source in a particular direction
per unit solid angle (see Figure 5). The solid angle ω is defined as the value obtained from dividing
the surface area A (m2) on the entire sphere of a radius r (m) by r2 (see Equation 3). The SI unit forthe solid angle ω is steradian (sr).
1 Steradian (sr)Measurement Direction

Figure 5. Definition Image of Luminous Intensity
Equation 3: ω = A / r2
ω = Solid Angle (sr)A = Surface Area on the Entire Sphere of a Radius r (m2)r = Radius of the Sphere (m)This document contains tentative information, Nichia may change the contents without notice.
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Application Note
The surface area of the entire sphere is 4πr2 (m2) and its solid angle is 4π (sr). For a light source withuniform brightness in all directions, the luminous flux (the intensity of light) per steradian, which iscalled “luminous intensity”, can be obtained from dividing the total luminous flux by 4π. (seeEquation 4).
Equation 4: IV = Φv / 4π
IV = Luminous Intensity of a Non-directional Light Source (cd)Φv = Total Luminous Flux (lm)
4π = Solid Angle of the Surface Area of the Entire Sphere (sr)Light Emitting Diode

When a light source has directivity characteristics and its luminous intensity is not uniform, theluminous intensity cannot be calculated from the total luminous flux by using Equation 4; however,
the total luminous flux can be obtained by measuring the luminous intensity in each segment of theentire surface and summing them up.
3.3. Illuminance3.3.1. Illuminance – Photometric Property and Unit
Illuminance is a measure of the luminous flux incident on a surface, per unit area and its SI unit is thelux (lx). Unlike luminous flux and luminous intensity, illuminance is not a measure to indicate theperformance of a light source, but it is commonly used to evaluate the spatial lighting.
For example, when designing office lighting, the number of luminaries and the installation interval
are determined to meet the desk illuminance of 750lx recommended by JIS (Japanese Industrial
Standard).
lux
(lx)

Figure 6. Image of Illuminance3.3.2. Definition of Illuminance
Illuminance is defined as the luminous flux incident on a surface, per unit area (1m2). In the case of
the point light source2 shown in Figure 7, as the distance from the light source becomes farther, theamount of light falling on the unit area decreases, leading to low illuminance.
1m2

Figure 7. Definition Image of Illuminance
The actual light source has a certain emitting area. When the illumination distance is larger than the emitting area, it can be considered to be a point
light source.
2

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Application Note
Figure 8 explains the relationship between the illumination and the illumination distance, where thelight source has a uniform light intensity in all the directions and the total luminous flux is 1lm. The
illuminance at the illumination distance of 1m can be obtained using a sphere with a radius (r) of 1mand an area A (unit area) of 1m2. In this case, the solid angle ω is calculated as follows:
A = 1m2
r = 1mω = 1srA = 1m2
r = 2mω = 1/4sr

1lm

1lm

Illumination distance: 1m

Illumination distance: 2m

Light Emitting Diode

Figure 8. Illuminance Calculation
Equation 3: ω = A / r2
= 1 / 12 Substituting A=1 and r=1 into Equation 3 = 1
The value of ω is 1.
The light incident per unit area through this solid angle (1sr) is called “illuminance” and it is obtained
from dividing the total luminous flux (1lm) by the solid angle of the entire sphere (4π). Hence, theilluminance at the illumination distance of 1m is 1/4πlx.
In the same manner as the above, the illuminance at the illumination distance of 2m can be calculated
by substituting A=1 and r=2 into Equation 3. As a result, the value of ω is 1/4. Since the solid angle
is reduced to 1/4 compared with that at the illumination distance of 1m, the luminance is decreased to
1/4 accordingly.
Therefore, the illuminance at the illumination distance of 2m is 1/16πlx.
When the illumination distance is increased by 2 times (from 1m to 2m), the illuminance decreases
to 1/4 (from 1/4π to 1/16π), which is called “inverse-square law”. Note that the inverse-square law
does not apply when a light source does not have uniform directivity characteristics or the illumination
distance is close to the light source.

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Application Note
3.4. Luminance3.4.1. Luminance – Photometric Property and Unit
Luminance is a measure of the light perceived by the human eye. The SI unit of luminance is candela
per square meter (cd/m2). Luminance is used to quantify the light from a light source with a certain
size of the emitting area viewed by the human eye directly (e.g. display screen). The major difference
from luminous intensity is that luminance is used to evaluate a surface light source, not a point lightsource.
Note that the term “nit” is sometimes used for display screens. Though they are different words, nit
and luminance can be used interchangeably (1 nit = 1 cd/m2).

Light Emitting Diode

cd/m2

Figure 9. Image of Luminance3.4.2. Definition of Luminance
When a light source is a surface light source with a certain size for the emitting area, not a point lightsource, the luminance is defined as the luminous intensity per unit area. Considering that the surface
light source is a collection of point light sources as shown in Figure 10, the luminance is obtained
from dividing the luminous intensity in a particular direction by its apparent area (area B) of theemitting surface.
Equation 5: L = Iv / B
L = Luminance (cd/m2)
Iv = Luminous Intensity (cd)
B = Apparent Area (m2)

1m2

1sr

Measurement Direction

Figure 10. Definition Image of Luminance
The apparent area is the emission area viewed in a particular direction. As shown in Figure 11, theapparent area of the light source with the same size of the emission area varies depending on theviewing direction.
Large Apparent Area

Small Apparent Area

Figure 11. Apparent Areas
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Application Note
3.5. Connection between the Photometric Properties
Figure 12 shows the connection between these photometric properties.
Luminous Flux (lm)/ Solid Angle (sr)Radiant Flux
(W)

Converting theradiant power into
photometricquantities.

Luminous
Flux (lm)Luminous Intensity(cd)
Luminous Intensity (cd)× Solid Angle (sr)Luminous Intensity (cd)/ Area (m2)Luminous Flux (lm) / Area (m2)Luminance3(cd/m2)

Light Emitting Diode

Illuminance(lx)

Figure 12. Connection between the Photometric Properties3

4. Color Evaluation Methods
4.1. CIE 1931 standard XYZ colorimetric system (xy Chromaticity Diagram)
The xy chromaticity diagram of CIE 1931 standard XYZ colorimetric system is the most widely used
to represent the color of a light source. Nichia’s LEDs are sorted into the color ranks using the xy
chromaticity coordinates.
CIE 1931 standard XYZ colorimetric system was developed by International Commission on
Illumination (CIE) in 1931 and the RGB color space was originally created with the three primarycolors of light: red, green, and blue; however, it was realized to have inconveniences such as negative
numbers; therefore, the CIE revised its colorimetric system. The normalized tristimulus values are
represented as the x, y, and z chromaticity coordinates, and when the two parameters x and y are
specified, the parameter z is defined, accordingly. The xy chromaticity coordinates without z values
are mapped into two dimensions, which is the xy chromaticity diagram. This xy chromaticity diagram
represents all colors in nature and its chromaticity coordinates fall within the horseshoe-shaped figureshown in Figure 13. The area near the edge of this figure represents monochromatic colors with ahigher colorimetric purity and the area closer to the center represents colors with a lower colorimetricpurity, showing colors close to white.
As explained above, colors are plotted on the xy chromaticity diagram; however, for the color
difference between two colors, the distance between their chromaticity coordinates does notcorrespond to the difference in the visual perception. For instance, even when the distance between
the chromaticity coordinates of two colors within the near-green region is equivalent to that of nearblue region, the color difference within the near-green region is less perceptible to the human eye thanthat within the near-blue region. In order to improve this color difference inequality, various
colorimetric systems were derived from CIE 1931 standard XYZ colorimetric system; however, thexy chromaticity diagram has continued to be commonly used.

3

Only luminance is measured as a surface light source.

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Application Note
0.9
0.8
0.7
0.6
y

0.5
0.4

0.3
0.2
0.1
0
0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
xFigure 13. xy Chromaticity Diagram4

Light Emitting Diode4.2. Color Temperature
Color temperature is a measure used to describe the hue of white light. The SI unit of color
temperature is kelvin (K). The color temperature is defined as the value of the absolute temperatureof a blackbody 5 for the light emitted when the blackbody is heated to a certain level. As thetemperature increases, the color emitted from the blackbody changes from red to yellow to white toblue as shown in Figure 14. Unlike other colorimetric systems, color can be represented by a single
numerical value, which is kelvin, and this enables an intuitive visualization of colors. For this reason,
the color temperature is often used to simply describe the hue of white light.
For instance, a yellowish light with a low color temperature (2700K to 4000K) is used for residential
lighting, which is called “warm white” or “traditional light bulb color”. The yellowish light creates acalm, inviting, and relaxing atmosphere. On the other hand, a bluish light with a high color
temperature (5000K to 6500K) is called “bright white” or “daylight” and creates a crisp atmosphere.
This bluish light is commonly used in offices and environments that require bright light to performtasks correctly.
For a blackbody with the ideal radiation characteristics, the chromaticity coordinates of the color
temperature are plotted on the blackbody locus, which is also called the “Planckian locus” (see Figure15). However, actual various white lights in nature have different radiation characteristics from theblackbody and most white lights have chromaticity coordinates that do not fall exactly on theblackbody locus. The color temperature is defined to correspond to only the colors on the blackbody
locus; however, for the other colors, the correlated color temperature (CCT) that corresponds to theclosest temperature of the blackbody is used for convenience. Along with the CCT, the deviation (duv)from the blackbody locus is used to describe the white hue more precisely.
HeatedBlackbody
Low
High
Temperature

Figure 14. Image of Colors of Light Emitted from the Blackbody

4
5

The colors on this diagram are illustrated for reference only and differ from the actual colors.
The blackbody is defined as an ideal physical body that can absorb all electromagnetic radiation.

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Application Note
0.7
0.6

3000K

3500K

2700K

4000K

0.5

2000K

y

4500K

duv +0.02
duv +0.01
Blackbody Locus

5000K

0.4

5700K

duv -0.01
duv -0.02

6500K

0.3
0.2
0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

x

Figure 15. Color Temperatures on the xy Chromaticity Diagram

Light Emitting Diode4.3. Color Rendering Index (CRI)
Color rendering index (CRI) is a measure to describe how accurately a light source can reproduce thecolor of an object it illuminates, instead of representing the color of the light. The color reproducibilityof luminaires is indicated by a numerical value based on the color illuminated by a reference lightwith characteristics similar to sunlight. The higher the color reproducibility, the higher the CRI. The
highest CRI is 100. For residential LED lighting, the average CRI (Ra) has generally been about 80;
however, luminaries with ultra-high CRI (the Ra is nearly 100) have become available in recent years.
For the details, see the application note “Color Rendering Evaluation for LEDs for General Lighting”.

5. Summary
This application note explains various photometric properties indicating brightness and color. These
photometric properties are important when designing luminaries. A thorough understanding of theirdefinitions and differences helps to maximize the performance of the LEDs.
Nichia offers a wide variety of LEDs that are ideally suited for applications such as luminaries withhigh luminous flux (luminous efficacy) and display screens with high luminance. If customers require
help selecting the LEDs, contact a local sales representative.

References:-

JIS C 8152 Photometry of white light emitting diode for general lighting-

JIS Z 8113 Lighting vocabulary-

JIS Z 8781 Colorimetry-

JIS Z 9110 General rules of recommended lighting levels

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Application Note

Light Emitting Diode

Disclaimer
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technical information/data for reference purposes only. By using this application note, the user agrees
to the following: This application note has been prepared solely for reference on the subject matters incorporated
within it and Nichia makes no guarantee that customers will see the same results for their chosen
application.
 The information/data contained herein are only typical examples of performances and/or
applications for the product. Nichia does not provide any guarantees or grant any license underor immunity from any intellectual property rights or other rights held by Nichia or third parties.
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