LED Linear T8 Lamp Design Basics for high performance

This guide is both useful for producing LED linear lamps and for end-users.  It explains the factors to be considered and how pricing and technical elements can change the answer to the question "Is installing LED tubes a good decision?"

Introduction

Linear lamps are the most used light source in our office and commercial buildings. Due to their form and function, their interiors have a high requirement of artificial lighting, as it is where most of us work or spend time away from home.

With billions of linear lamps across the world, operating 11 hours on average each day of the year, the office and commercial sectors account for near half of the lighting electricity consumed on the planet.

Our homes, with more lamps (usually incandescent or halogen), lag far behind, with only a quarter of the lighting electricity consumed, as we use them less than 2 hours per day on average.

To reach the goal of environmental friendly lighting, most of the linear lamps used should be as durable and energy efficient as technologically and economically possible. With around 97% of the installed linear lamps consisting of fluorescent lamps, a decades old lighting technology,  there is a lot of room for improvement from the much younger white LED technology. 

How to build a LED linear lamp superior to fluorescent

Step 1: Define "superior product"

Before building a superior product we need to clearly define what we want to improve and from what base. As straight forward as it might sound, many do not know where a fluorescent lamp stands in this aspect and hastily jump on the "upgrade with LED" band-wagon simply because of word of mouth about how good, green and lost lasting the new technology is.

In reality, a switch from fluorescent to LED can be either an upgrade, an downgrade or simply make no difference at all.

According to our experience,  DOE tests and other literature, the performance of the most popular form of fluorescent tube, the T8,  is:

* while in the manufacturer datasheets the luminous flux of the bare lamp is specified, that value is seldom reached in an actual installation. Two important factors have a negative impact: ballast factor and the light loss from converting a 360 degree lamp in a 180 degree (or less) lighting fixture. From an average of datasheet values, a 0.87 ballast factor correction and a 17% further lumen loss was applied.

Resulting that a LED T8 replacement has to exceed the following performance targets:

Minimum Lumens target explained:

Because of the faster depreciation of the the luminous output of LED compared to fluorescent, over the course of its useful lifetime, LED linear lamps should have 20% more lumens.

LEDs are considered at end of life when they have lost 30% of the initial brightness. In contrast fluorescent lamps loose on average 6% of their light output until their end of life, at which point they fail and do not emit light at all.

Below:  a sample lifetime of an 50.000 hours LED compared with other lighting technologies. 

Minimum Energy Efficiency Explained:

"a 50% increase in energy efficiency" or alternatively "a 50% cut in energy costs" is relevant from an economic and environmental point of view and a convincing argument for factors of decision in the business sector.

Lifetime requirement explained:

LED lamps are complex products with a higher manufacturing cost and environmental foot print of their production.  Compared with a fluorescent linear lamp that has an average life of 25.000 hours, LED linear lamps should have minimum twice that.

Step 2: Choose between the "Henry Ford" and the "Apple" approach to manufacturing

In the 1920s, Henry Ford envisioned that total control over the supply chain behind his Ford Motor company is the way forward. For this purpose he purchased iron mines and coal mines to supply the material for his steel mills,  rubber plantations for his tire factories and even a railroad company to transport his cars.  

Today Apple builds the Iphones, Ipads entirely from parts produced by someone else (Arm, Samsung and so on) in a factory that it does not own, only visits. That factory is in China, of the company Foxcom.

In other words, the LED linear lamp can be made from a majority of parts built in house or outsourced.

It is our opinion that using outsourced parts is the recommended course of action. Further more, for this particular product these parts should be almost entirely off  the shelf versions, not custom build.  A LED tube is about function and results and not looks.

Step 3: Know what parts you need to build a LED tube

As the above shows, the parts of a LED tube are:

• LED module -  one or more rows of LEDs on a circuit board/PCB (E,F)
• Cover  - A polycarbonate or arcyilic sheet for protection and light distribution purposes (G)

• Aluminum heat sink (under the LED module and/or on the back of the tube, C, H) - required to protect the LEDs from their overheating, a phenomenon that will result in a whole array of unwanted effects.
• Power supply - needed to power the low voltage LEDs from the mains 110/230V. (A)
• G13 end caps - to make the LED tube compatible with the fixtures in which it is to replace the fluorescent tubes (1,2,3)
• Other small parts: wires, glue, thermal paste, screws - to put the assembly together.

Step 4: Choose the LED module for superior performance

For superior lifetime compared to linear fluorescent the LED module should have at least 30.000 hours L70 lifetime, with an optimum of 50.000 hours. The LED manufacturer must have lifetime data testing of 10.000 hours with various currents and temperatures. A sample is below

As it can be noticed, lifetime depends on temperature and current. This means that a certain LED lifetime does not guarantee a similar product lifetime.

For superior performance the luminous flux of the LED module has to be 30% higher than fluorescent of the same length while having an energy efficiency of at least 95 lumens per watt, with 110 being an optimum, equivalent with 50% energy savings.

The increased flux is required to attenuate the light loss due to the cover and the gradual lumen depreciation over time.

Step 5: Ensure proper heat management for reliable performance

Even the best LEDs will fail if heat is not managed well. Further more, until mechanical failure, an entire host of negative effects occur, as detailed in this article. For superior, reliable performance very good thermal management is a key condition.

The heat management system for a LED linear tube is made of three key elements which have to remove the Heat from the LEDs to the ambient:

• Circuit Board (PCB)
• Thermal paste or tape 
• Heat sink

A combination of aluminum PCB, high quality thermal tape and a finned aluminum heat sink constitute the optimum thermal solution. Ample testing was done to confirm this fact.

Step 6: Choose a reliable power supply and decide where to put it

The power supply must have at least the lifetime of the LED module, otherwise a LED tube with even the best components and design will be a downgrade from a linear fluorescent installation.  This aspect is many times overlooked by manufacturers and customers a like. Too often the focus is on LED lifetime and not the lifetime of the "weakest link".

The location of the power supply is also a key element. Currently there are three main approaches:

1. Small, internal. Jam its components on a small pcb and place it inside the LED tube, usually at its end. The components could also be placed on the LED PCB itself.

2. Long, internal. Spread its components on a long narrow pcb and place it inside the tube, on the bottom.

3. External, enclosed. The power supply is a standalone, external unit with its own case and thermal management, connected to the tube with wires.

They key  advantages and disadvantages of each approach are:

Step 7: Do not ignore the quality of the other parts

The poly-carbonate cover is important also. It  must has adequate shock protection and deliver a very good  optic result.

A poly-carbonate diffuser with perfect uniformity, no hot spots and at least 75% light transmission should be used. The diffuser will also increase the view angle of the LED module from 120 degrees to at least 160, a requirement for replacing fluorescent tubes in many applications.

While a transparent cover is used by some manufacturers,  the higher light transmission of  90% does not offset the significantly increased glare factor, multiple shadows effect or decreased view angle. A LED tube with a transparent cover actually has a very narrow application scope.

The glue, wiring, connectors, screws and end sockets must be selected carefully as each and any of them can be the "weakest link".

Step 8: Present the product in a professional manner

Ample literature about performance testing standards exist. The product must be superior in real life applications, not on the datasheet only. Independent evaluation organizations can be used for confirmation of product performance. Two examples are The DesignLights Consortium™ and LED Lighting Facts.

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Replacing a well tried and tested fluorescent lighting technology with LED makes economical and environmental sense only if effective illumination level and quality is  maintained while reducing the energy bill and increasing the interval between lamp replacements.