Application Note

Light Emitting Diode

Precautions for Designing
LED Tubes

Table of contents
1. Overview ..................................................................................... 2
2. LED Tubes ................................................................................... 2
3. Impact of Lamp Cover on Optical Characteristics ....... 4
4. Advantage of Reflective Sheet ............................................ 7
5. LED Pitch and Light Diffusion ............................................. 9
6. Junction Temperature and Luminous Flux .................. 10
7. Summary ................................................................................. 11

The part number NFSW757D-V1 in this document is the part number ofNichia’s product, and does not have any relevance or similarity to othercompanies' products that may have trademark rights.

This document contains tentative information, Nichia may change the contents without notice.
SP-QR-C2-210933-1
1/12Oct. 8, 2021

Application Note
1. Overview
Recently LEDs have been increasingly introduced into lighting fixtures as the LED performance advances.
LED tubes have been substituted with conventional tube fluorescent lamps in the marketplace.
This application note describes some important factors to design LED tubes with some evaluation results.

2. LED Tubes
2.1. LED Tube Structure
Figure 1 shows the typical LED tube structure which does not incorporate a power supply. Table 1summarizes the features of the component materials.
The LED performance can be affected by some component materials depending on their properties;

Light Emitting Diode

therefore, customers should evaluate the component materials and confirm whether they would affectthe LED performance.
Cover
PCB

LED

End Cap
Reflective Sheet
Heat Sink
Figure 1. LED Tube Structure
Table 1. Features of Component Materials
Component Material
CoverNoteProtects the LEDs and diffuses the light.
Made of glass and resin such as polycarbonate.
The cover with high transmission rate can contribute to minimizing thedepreciation rate of the lamp’s luminous flux; on the other hand, it reduces
the light diffusion, and the individual LED’s emission light can be visible
through it.

PCB

Printed circuit board with LEDs, protection device, etc. mounted on
In general, aluminum, glass epoxy (FR-4), glass composite board (CEM-3),
etc. are used for PCBs.
Thermal conductivity rate: aluminum > CEM-3 > FR-4
To uniformly dissipate the heat generated from LEDs, large copper foil
patterns should be formed on the PCB.
High reflective ink should be used for the solder resist to increase the lamp’s
luminous flux. (cf. Section 4)
This document contains tentative information, Nichia may change the contents without notice.
SP-QR-C2-210933-1
2/12Oct. 8, 2021

Application Note
Reflective Sheet

Increases the lamp’s luminous flux like the high reflective solder resist

Heat Sink

Helps to dissipate the heat generated from LEDs and secure the housing’s
strength.
In general, the LED-mounted PCB is attached to the heat sink with adhesive
tape and/or screws. The heat dissipation can be insufficient and the heat
distribution can be uneven depending on the attachment method.

End Cap

Electrically and mechanically connects the LED tube with the control system.

2.2. Luminous Flux Decrease and Color Shift of LED Tubes
The LED tube’s optical characteristics can be affected by the following factors:

Light Emitting Diode

1) Lamp cover2) Reflective sheet3) Relation between the LED pitch and the light diffusion
4) Relation between the LED junction temperature and the luminous flux
2.3. LED Tube Specifications for Evaluation
Table 2 and Figure 2 show the LED tube specifications to be used for the evaluations herein. We evaluatedthe impact of each factor (cf. Section 2.2) on the LED tube.
Table 2. LED Tube Specifications
ItemSpecificationSize

1,200mm

LED Model

NFSW757D-V1

Quantity of LED

160pcs.

LED Tube Luminous Flux

2,400lm

LED Tube Electrical Current

350mA

LED Tube Voltage

45V

LED Tube Power

16W

LED Tube Efficacy

150lm/W

Color Temperature

5,000K

General Color Rendering
Index (Ra)Notes
LED Pitch: 7mm
16 series, 10 parallelIncluding the luminous flux decrease due tothe lamp cover(10%) and thermally saturated.
35mA per 1 LED

Lamp Luminous Flux / Lamp Power80

This document contains tentative information, Nichia may change the contents without notice.
SP-QR-C2-210933-1
3/12
Oct. 8, 2021

Application Note
Bridge Diode for
Reverse Polarity Feed
Protection
Device (Fuse)

LED Pitch: 7 [mm](Distance between the centers ofan LED and its neighboring one)
Sensing
ResisterLight Emitting DiodeFigure 2. LED Tube Structure

3. Impact of Lamp Cover on Optical Characteristics
We evaluated the impact of several kinds of lamp cover on the optical characteristics.
3.1. Evaluation Method
We measured the luminous flux/color/color rendering of the LED tubes with Covers A, B, C, and no coverto evaluate the impact of the covers on each characteristic. We also evaluated the light diffusion (lightevenness through each cover). Each item was measured just after the LEDs were operated to eliminate
the thermal impact due to the cover.
Table 3. Lamp Cover for Evaluation
Cover ACover B

Cover CNo Cover

Resin

GlassResin-576777-

Diameter(mm)252533-Thickness(mm)111-Appearance

Property
Light Transmission
Rate(%)1

Note:
1Rate of light which vertically penetrates the cover and passes through it

This document contains tentative information, Nichia may change the contents without notice.
SP-QR-C2-210933-1
4/12Oct. 8, 2021

Application Note
3.2. Evaluation Result
Table 4 shows the evaluation results.
Figures 3, 4, and 5 (Page 5) show the relation between the transmission rate and the luminous flux
depreciation, the color shift, and the changes in the general color rendering index, respectively. Theluminous flux depreciation rate, color shift, change of Ra were calculated on the basis of the measurement
values of the LED tube with no cover.
Table 4 Evaluation Results of Lamp Covers
Cover A

Cover B

Cover CNo Cover

2,692

2,779

2,868

2,958

9.0

6.1

3.0

-

x

0.34530.34430.3436

0.3423y0.35800.35700.35590.3546

Δx+ 0.0030+ 0.0020+ 0.0013-Δy+ 0.0034+ 0.0024+ 0.0013

-

82.2

81.8

81.8

82.0

+ 0.2

- 0.2

- 0.2

-

Lamp’s Luminous
Flux(lm)
Luminous FluxColorColor Shift

General Color
Rendering Index (Ra)Change of Ra
Light Diffusion(Unevenness)(LED Pitch; 7mm)

10Luminous Flux Decrease [%]Light Emitting Diode

Depreciation Rate(%)

86420405060708090100

Light Transmission Rate [%]Figure 3. Light Transmission Rate – Luminous Flux Decrease

This document contains tentative information, Nichia may change the contents without notice.
SP-QR-C2-210933-1
5/12
Oct. 8, 2021

Application Note
0.38

Black Body Line

0.37

0.36yNo Cover
Cover A (Light Transmission Rate: 57%)
Cover B (Light Transmission Rate: 67%)
Cover C (Light Transmission Rate: 77%)

0.35

Rank: sw50

0.34

0.33
0.32

0.33

0.34

0.35

0.36

0.37

x100
90

80
70
CRI

Light Emitting DiodeFigure 4. Color Shift due to Covers

60

No Cover50

Cover A (Light Transmission Rate: 57%)
Cover B (Light Transmission Rate: 67%)

40

Cover C (Light Transmission Rate: 77%)

30
20
100
R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15

Figure 5. Impact of Covers on Color Rendering Properties
Based on the evaluation results, the highest luminous flux was obtained with Cover C which had thehighest light transmission rate; the luminous flux was decreased by 3.0% with Cover C.
On the other hand, the individual LED’s light under Cover C was easily noticeable when the tube wasoperated. The luminous flux decrease was the highest with Cover A; however, the individual LED’s light
was not noticeable but the light was evenly diffused, like a fluorescent lamp’s light. We found that thelight diffusion contradicts the luminous flux decrease.
Moreover, the lower the light transmission rate was, the more the color shifted.
As shown in Figure 4, using the milky white covers, the color shifted toward yellow in the + direction of xand y.
Figure 5 shows slight decreases in the figure of R12 (Blue) with Covers A, B, and C; however, R12 does notaffect the General CRI (Ra).

This document contains tentative information, Nichia may change the contents without notice.
SP-QR-C2-210933-1
6/12Oct. 8, 2021

Application Note
4. Advantage of Reflective Sheet
Using the reflective sheet or the resist ink containing a highly reflective material on the board can enhance
the light extraction efficiency, resulting in higher luminous flux (Figure 6).

More light can beextracted thanks tothe reflective sheet.

Without Reflective SheetWith Reflective Sheet

Light Emitting Diode

Figure 6. Comparison of Light Extraction
4.1. Evaluation Method
We measured the luminous flux and evaluated the light diffusion with two kinds of reflective sheet and ahighly reflective resist, respectively.
We compared the luminous flux by using the three lamp covers (A to C), and compared the light diffusion
by using Lamp Cover C with the least light diffusion. The luminous flux was measured just after the LEDs
were operated to avoid the thermal impact.
4.2. Evaluation Result
The evaluation results are given in Table 5.
Figure 7 shows the relation between each reflective sheet’s reflectance and the increase rate of theluminous flux. The increase rate with each reflective sheet was obtained on the basis of the luminous fluxof the LEDs attached to the highly reflective resist layer (Reflectance: 80.2%).

This document contains tentative information, Nichia may change the contents without notice.
SP-QR-C2-210933-1
7/12Oct. 8, 2021

Application Note
Table 5. Evaluation Results of Reflective SheetsReflective Sheet A

Reflective Sheet B

Highly Reflective(Reflectance: 98.8%)(Reflectance: 94.2%)

Resist Layer(Reflectance: 80.2%)

2,757

2,753

2,6922.42.3-

2,825

2,820

2,779

1.7

1.5-

2,885

2,880

2,868

0.6

0.4

-

Cover A

Luminous Flux [lm](Light Transmission

Increase Rate ofRate: 57%)

Luminous Flux [%]Cover B

Luminous Flux [lm](Light Transmission

Increase Rate ofRate: 67%)

Luminous Flux [%]Cover C

Luminous Flux [lm](Light Transmission

Increase Rate ofRate: 77%)

Luminous Flux [%]Light Diffusion (Unevenness)
LED Pitch: 7mm,3 Lamp Cover C
Luminous Flux
Rate ofIncrease
Fluxof Luminous
Increase Rate[%][%]Light Emitting DiodeAppearance

2.5231.5
2.5
120.5
1.5
0180

Cover A (Light Transmission Rate: 57%)
Cover B (Light Transmission Rate: 67%)

85

0.5

90
95
Reflectance [%]100

Cover C (Light Transmission Rate: 77%)
Cover A (Light Transmission Rate: 57%)

Cover B (Light Transmission Rate: 67%)

080

85

90
95
Reflectance [%]100

Cover C (Light Transmission Rate: 77%)

Figure 7 Relation between Reflectance and Increase Rate of Luminous Flux
Based on the measurement data, the higher the reflectance of the reflective sheet was, the higher theluminous flux became in each lamp; however, the higher the transmission rate of the lamp cover was, thelower the increase rate of the luminous flux was. For example, the increase rate of the luminous flux under
Cover C was below 1% in each lamp with Reflective Sheet A/B. This is because much light is originally
extracted through the covers with high light-transmission rate. Therefore, the reflective sheets are lesseffective to increase the luminous flux through the covers with high light-transmission rate.
In other words, the lower the light transmission rate of lamp covers is, the more effective the reflective
sheets become. There was little difference in the light diffusion under Cover C among the strings.
This document contains tentative information, Nichia may change the contents without notice.
SP-QR-C2-210933-1
8/12
Oct. 8, 2021

Application Note
5. LED Pitch and Light Diffusion
The individual LED’s light may be noticeable with a lamp cover in an LED tube. The symptom can be
reduced by changing the LED pitch and the distance between the LEDs’ emitting surfaces and the lampcover.
5.1. Evaluation Method
We evaluated the light diffusion (how much less noticeable the individual LED’s emission light is) of theLEDs under the lamp cover by changing the LED pitch, the input electrical current, the distance between
the LEDs and the cover (cf. Table 6).
The input electrical current in each LED pitch was calculated on the basis of the lamp length (1,200mm)
and the lamp’s luminous flux (2,400lm). We used a milky white cover in every condition.

Light Emitting DiodeLHFigure 8. LED Pitch and Distance between an LED and a Lamp Cover5.2. Evaluation Result
Table 6 shows the evaluation results.
Table 6. Evaluation Results
L : LED Pitch
H : Distance between
LEDs and cover

10mm

13mm

15mm

17mm

20mm

608093108

13010mm
15mm
18mm

Input Current[mA]

The individual LED’s emission light can be noticeable on the red-encircled tubes. Based on our findings,
the smaller the LED pitch is, also, the larger the distance between the LEDs and the cover is, the more thelight can be diffused. Thus, the more the light is diffused on an LED tube, the less noticeable the individual
LED’s light is.

This document contains tentative information, Nichia may change the contents without notice.
SP-QR-C2-210933-1
9/12
Oct. 8, 2021

Application Note
6. Junction Temperature and Luminous Flux
Most of Nichia’s white LEDs emit white light by the combination of LED dice which emit blue light andyellow phosphor. As the LED die generates higher temperature, the light output from the die is reduced,
decreasing the output excited by phosphor. Therefore, the higher the junction temperature gets, the lower
the luminous flux becomes.
Customers may not be able to realize the designed luminous flux, if the junction temperature is high.
Moreover, when there is variation in the junction temperatures among the LEDs, the emission light fromthe LED tube cannot be even.
Customers are required to optimize the thermal design for the LED tubes by making it possible to dissipate
heat.

Light Emitting Diode6.1. Evaluation Method
We measured the heat distribution over the LED-mounted board and the LEDs’ junction temperature
with/without heat dissipation system through a heat sink. A thermograph was used to evaluate the heatdistribution. The LEDs’ junction temperature was calculated by the following formula after measuring thesoldering temperature with a thermocouple.
The measurements were done while the temperature was saturated within the closed assembly with alamp cover (Ta=25°C).
The lamp’s luminous flux was measured just after the LEDs were operated and while the temperature wassaturated, and then, those values were compared to obtain the depreciation rate.

𝐿𝐸𝐷 ′ 𝑠 𝐽𝑢𝑛𝑐𝑡𝑖𝑜𝑛 𝑇𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 𝑇𝐽 = 𝑇𝑆 + 𝑅𝜃𝐽𝑆 × 𝑊

Equation 1

TS = Temperature at the soldering area (cathode): ℃
RθJS = Thermal resistance between the LED die and Ts measurement point: ℃/W
W = Input power (IF ˣ VF) applied to each LED: W

6.2. Evaluation Result
Table 7 shows the measurement results. The luminous flux depreciation rate was calculated by theluminous flux measured just after the LEDs were operated and while the temperature was saturated.

This document contains tentative information, Nichia may change the contents without notice.
SP-QR-C2-210933-1
10/12Oct. 8, 2021

Application Note
Table 7 Evaluation Results (Ta=25°C)
With Heat SinkWithout Heat Sink

49.7

59.3

ThermalDistribution

Junction TemperatureLight Emitting Diode(℃)
Lamp’s Luminous

Just after operation

2,679

Just after operation

2,702

Flux(lm)

Thermally saturated

2,625

Thermally saturated

2,615

Luminous Flux
Depreciation Rate(%)

2.0

3.2

In each tube, the heat was uniformly distributed all over the board, regardless of the existence of the heatsink. Therefore, there was no problem in the heat dissipation in either tube; however, the LEDs’ junction
temperature without the heat sink was approx.10°C higher than that of the ones with the heat sink. In thetube without the heat sink, the luminous flux depreciation rate was approx. 1% higher.
The findings show that decreasing the junction temperature contributes to minimizing the decrease of
the lamp’s luminous flux.
Please refer to “Thermal Design of LEDs” for precautions and evaluations for the thermal design of LEDs.

7. Summary
This application note includes very critical information about the lamps’ optical characteristics such as
luminous flux decrease. Customers should evaluate and design their LED products, with reference to theevaluation results described herein.

This document contains tentative information, Nichia may change the contents without notice.
SP-QR-C2-210933-1
11/12
Oct. 8, 2021

Application Note

Light Emitting DiodeDisclaimer
This application note is a controlled document of Nichia Corporation (Nichia) published toprovide technical information/data for reference purposes only. By using this applicationnote, 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/orapplications for the product. Nichia does not provide any guarantees or grant any licenseunder or immunity from any intellectual property rights or other rights held by Nichia or
third parties.
 Nichia makes no representation or warranty, express or implied, as to the accuracy,completeness or usefulness of any information contained herein. In addition, Nichia shallnot be liable for any damages or losses arising out of exploiting, using, or downloadingor otherwise this document, or any other acts associated with this document.
 The content of this application note may be changed without any prior or subsequent
notice.
 Copyrights and all other rights regarding the content of this document are reserved by
Nichia or the right holders who have permitted Nichia to use the content. Without priorwritten consent of Nichia, republication, reproduction, and/or redistribution of thecontent of this document in any form or by any means, whether in whole or in part,
including modifications or derivative works hereof, is strictly prohibited.

491 Oka, Kaminaka-Cho, Anan-Shi,
TOKUSHIMA 774-8601, JAPAN
Phone: +81-884-22-2311 Fax: +81-884-21-0148

NICHIA CORPORATION
http://www.nichia.co.jp

This document contains tentative information, Nichia may change the contents without notice.
SP-QR-C2-210933-1
12/12
Oct. 8, 2021