You are here: Information & Resources » Fluorescent Tubes » Principles Of Operation Of Fluorescent Tubes » Phosphors And The Spectrum Of Emitted Light

Phosphors And The Spectrum Of Emitted Light

Phosphors And The Spectrum Of Emitted Light Some people find the color spectrum produced by some fluorescent lamps to be harsh and displeasing. A healthy person can sometimes appear to have an unhealthy skin tone under fluorescent lighting. The extent to which this phenomenon occurs is related to the light’s color rendering index (CRI).

CRI is a measure of how well balanced the different color components of the white light are. By definition, an incandescent lamp has a CRI of 100. Real life fluorescent tubes achieve CRIs of anywhere from 50% to 99%. Fluorescent lamps with low CRI have phosphors which emit too little red light. Skin appears less pink and unhealthy compared to incandescent lighting. Colored objects appear muted. For example a low CRI 6800K halophosphate tube, which is about as visually unpleasant as they get, will make reds appear dull red or brown.

CCT Color temperature is a measure of the whiteness of a light source. Typical incandescent lighting is 2700K which is yellowish-white. Halogen lighting is 3000K. Fluorescent lamps are manufactured to a chosen CCT by altering the mixture of phosphors inside the tube. Warm-white fluorescents have CCT of 2700K and are popular for residential lighting. Neutral-white fluorescents have a CCT of 3000K or 3500K. Cool-white fluorescents have a CCT of 4100K and are popular for office lighting. Daylight fluorescents have a CCT of 5000K to 6500K, which is bluish-white.

High CCT lighting generally requires higher light levels. At dimmer illumination levels, the human eye perceives lower color temperatures as more natural. So, a dim 2700K incandescent lamp appears natural, and a bright 5000K lamp also appears natural, but a dim 5000K fluorescent lamp appears too pale. Daylight-type fluorescents look natural only if they are very bright.

Some of the least pleasant light comes from tubes containing the older halophosphate type phosphors (chemical formula Ca5(PO4)3(F,Cl):Sb3+,Mn2+). The bad color reproduction is due to the fact that this phosphor mainly emits yellow and blue light, and relatively little green and red. To the eye, this mixture appears white, but the light has an incomplete spectrum. The CRI of such lamps is only 60.

Since the 1990’s, higher quality fluorescent lamps use either a higher CRI halophosphate coating, or a triphosphor mixture, based on europium and terbium ions, that have emission bands more evenly distributed over the spectrum of visible light. High CRI halophosphate and triphosphor tubes give a more natural color reproduction to the human eye. The CRI of such lamps is typically 82-100.