Linear fluorescent lamps are tubular electric discharge lamps commonly used for general illumination in commercial applications such as offices, retail stores, and warehouses but also in homes. High color rendering index (CRI) linear fluorescent lamps have a CRI of 87 or greater.
Linear fluorescent lamps are subject to national efficiency standards. However, current national standards exempt linear fluorescent lamps with a CRI of 87 or (“high CRI lamps”). When Congress enacted the first standards for linear fluorescent lamps, high CRI linear fluorescent lamps were an expensive niche product used for specialty applications only. However, since then the high CRI exemption has become a loophole and many manufacturers have introduced inexpensive linear fluorescent lamps for general lighting applications that have CRIs above 87 but efficiency levels that are significantly less than the requirements in the national standards.
CRI is a quantitative measure defined by the International Commission on Illumination that compares color rendering by a light source against color rendering by standardized daylight. Daylight provides the “truest” color rending for the human eye and has the maximum possible CRI of 100. Artificial light sources typically have lower CRIs. Most linear fluorescent lamps that comply with current national standards have CRIs between 80 and 87 and there is little perceptible difference in the light quality of compliant, “normal” CRI fluorescent lamps and exempt high CRI linear fluorescent lamps. Popular light emitting diode replacements for linear fluorescent lamps are also not covered by national standards, but tend to have high CRIs, and to be more energy efficient that fluorescent alternatives.
In May 2018, Vermont adopted states standard that close the high CRI loophole by requiring linear fluorescent lamps with CRI of 87 or higher to meet the same efficiency requirements as normal CRI linear fluorescent lamps. Linear fluorescent lamps that comply with national standards are about 60% more efficient than exempt high CRI lamps.
Most linear fluorescent tubes come in either T12 (1.5 inch), T8 (1 inch) or T5 (5/8 inch) diameters. T8 and T5 lamps are significantly more energy efficient than T12 lamps. The national energy efficiency standards for linear fluorescent lamps that went into effect in 2012 should have removed T12 lamps from the market. However, according to NEMA, in the fourth quarter of 2017 T12 lamps (presumably all high CRI, exempt products) still made up 11% of all shipments of linear fluorescent lamps. The persistence of T12 lamps in the U.S. market today is solely due to the high-CRI loophole.