Lifetime Comparisons: Fluorescents vs. LED Lighting

Lighting Calculations in the LED Era
James R. Benya, PE, FIES, FIALD

Paper Topics:
• The differences between relative photometry (conventional) and absolute photometry (SSL)
• Lighting calculation methods
• Description of light loss factors for conventional and SSL products
• Light loss factor recommendations for conventional and SSL products
• Comparisons of SSL systems to conventional systems in offices and schools

Author Biography

Jim Benya is a professional lighting designer and consultant with 35 years of experience. He is a Registered Professional Electrical Engineer (California PE E12078), Fellow of the Illuminating Engineering Society of North America (FIES), a Fellow of the International Association of Lighting Designers (IALD), and is Lighting Certified by the NCQLP (LC). His firm, Benya Lighting Design, is a member of the US Green Buildings Council and an IESNA Sustaining Member.

Read Jim's complete biography here.

Summary of Key Findings

Lifetime of LFL
Fluorescent manufacturers are promoting super T8 lamps with very long lifetimes. This paper will teach the reader how to compare fluorescent lifetime to SSL in an apples to apples way. It details two issues that result in the effective life of fluorescent lamps being far less than the published number:

Issue #1:  Fluorescent lamps specify average rated life– when 50% have failed. 
• Effective usable life is at first relamp (around 70% of rated life) 
• This means the usable life of a 30,000 hour lamp is closer to 20,000

Issue #2: The usage and type of ballast used can have a big affect on lamp life. 
• "The life of fluorescent lamps is now carefully presented as a function of ballast type and average operating period per start. For example, the life of a premium lamp with an instant start ballast might be 35,000 hours at 12 hours per start but only 18,000 hours at 3 hours per start. The same lamp might only survive 7,500 hours at 45 minutes per start on this ballast. Hence the lamp life being used must first be carefully determined from lamp, ballast and operating situation data." (Page 7)

Recommended Light Loss Factors
In this white paper all potential light loss factors are discussed and specific recommendations are made for offices and schools. In addition an important new factor is considered, the Lamp Burnout Factor (LBF). This factor considers the lamps that are burned out when group relamping takes place.


       * A 1.0 ballast factor is assumed. Total LLF for fluorescents will increase or decrease based upon this.
• LLD: Lamp Lumen Depreciation
• TAF: Thermal Application Factor
• RSDD*LDD: Room Surface Dirt Depreciation x Luminaire Dirt Depreciation
• LBF: Lamp Burnout Factor

Comparison of Conventional to LLF
This paper includes comparisons of conventional technology to SSL technology in typical school or office scenarios. Based on these calculations, the white paper concludes the following:

“Solid-state lighting has been long regarded as the future king of efficient lighting. This paper indicates that that foretold future is now here for one of the harder applications of solid state lighting, the general lighting of common commercial spaces.” (Page 16) 

In a classroom or open office setting, the LED troffer solution was found to deliver equivalent light levels for 20 to 25% less energy than state of the art T8 and T5 systems.

In a corridor, the LED downlights were found to deliver equivalent light levels for 40% less energy than state of the art compact fluorescent downlights.