The lighting market is flooded today with LED tube replacements for T8 and T5 Fluorescent tubes. 

The Entrepreneur and Business Guru need only walk through an office building, a school, a factory, a warehouse, or almost any commercial installation to see hundreds of Fluorescent tubes in each.  It's natural for them to presume these Fluorescent tubes can be replaced with LEDs.  If one business person thought it was a great idea, there will be many more thinking the same.  Now many companies have set up production to make a retrofit LED T8 product, and there are now way too many brands on the market that cannot live up to claims. 

The truth is emerging.  Over eager sales people, distributors and Reps have assured the end user they can provide a lighting product equivalent to the Fluorescent tube they are replacing.  But eight out of 10 recipients say otherwise.  It turns out that on average, the LED substitute produces only about half the lumens of the Fluorescent T8. Recipients that are pleased with the LED T8 are either overwhelmed by saving half the energy costs, OR, they only needed spot lighting to start with and they might have been over lighted with the original Fluorescent lighting.  Spot lighting is the only way the LED T8 measures up to the Fluorescent, i.e. with a narrow viewing angle.

The pitch may say lower LED wattage is intentional, that's to save you money because you need less Lumens with LEDs.  You can bet the manufacturer wants as many Lumens as possible from that tube, and if they could, they would.

The truth is that a T8 size tube doesn't have sufficient surface area to adequately dissipate the heat produced by the LEDs required to replace Fluorescent T8s.  This is due to LED luminous efficacy, and the LED's Tj-Tc thermal resistance.  The LEDs in a tube light must operate with junctions cooler than 80°C to get a competitive luminous efficacy.  "Competitive" refers to a LED tube light that can be cost effective with the ROI's that have been sold.  Given the LED's internal thermal resistance, that is Tj to Tc in °C/W for the affordable version, that system needs at least 6 square inches of convective surface area per Watt of power.

See the following graphic for this surface area calculation:



This graphic illustrates that a 4-foot T8 form factor, rear half only, can provide 75 square inches of surface area. 
This area divided by 6 inches:
    75 ÷ 6 = 12.5 Watts.

If the modern LED provides 70 lumens/Watt (in the field), then we might have 875 lumens from this T8 LED tube. 
12.5W * 70 L/W = 875

The 4-foot Fluorescent T8 uses about 32 Watts for about 2950 lumens.

This LED math is merely illustrating the Physics.  Some LEDs may have less than 70 L/W, or may require more than 6 sq in per Watt because of higher Tj to Tc resistance.  If the LED T8 aluminum has cooling fins, that can increase the surface area by maybe 2:1, so:
    875 * 2 = 1750 lumens,
about 60% of the fluorescent. 

It is shown that a finned rear housing and high luminous efficacy high quality LEDs are required.  High Quality relates to lowest possible Tj to Tc thermal resistance.  In any case, a finned rear area can allow little more than half the luminous output of a Fluorescent lamp. 

Do you get what you pay for?  An LED T8 without a finned rear heat sink is a design with poor thermal management. 

Applications that actually need the equivalent light will end up needing twice as many LED T8s.  Some purchasers are only trying to save on electric utility billing, and may try anything at the expense of proper lighting.  Remember that commercial spaces are usually required to meet some IESNA lighting requirement, and this means lighting levels of the entire space, not a few spot checks and settling with the highest of those measurements.

Beware those that tell you the LED color temperature makes up the difference.  Beware the 4-ft LED tube that uses more than about 20 watts without a fan because it is not likely to give the promised 50,000 hours of life.  In any case, the DOE (Department of Energy) has tested about a dozen LED T8s and their report is linked below. 

One place that the LED T8 excels in performance is refrigerated display applications where the LED's operating ambient temperature is maintained well below 25°C.  This low temperature makes up for the LED T8's inability to dissipate the heat.

Fluorescent technology has evolved, and competes with LEDs in luminous efficacy and lifetime; Fluorescent costs less and outperforms the LED tube product.  The LED panel light having LEDs mounted on an integrated housing and heat sink DOES compete with Fluorescent ceiling fixtures.  Shop wisely.

See the DOE report: LED T8 Fluorescent Replacement.