Fluorescent Tubes: Kind of History!
Fluorescent lamps produce light in an indirect manner. Mercury vapour in the tube is electrically excited to give off powerful ultraviolet radiation. Which impinges on the phosphor which is coated on the inside of the tube, the phosphor converts the UV to visible light, in this case the rather sickly blue glow which we have all come to know and love as fluorescent illumination.
The Frenchman Georges Claude is usually credited; the year is often given as 1911 or 1910
For example: a neon-argon or neon-argon-helium mixture can produce Blue light.
These lights to begin with were only really used to light film sets. Like all unfiltered electric arcs, they were very rich in harmful ultraviolet and produced a burning sensation in the the actors eyes. This was known as "klieg eyes". As a protection, Hollywood actors began wearing sunglasses all the time on the set. This started the whole trend! Stars, starlets, and groupies started wearing sunglasses day and night.
A light source is spoken of as "cold" or "cool" if its equivalent black-body "characteristic" colour temperature is very much higher than its actual operating temperature. The yellowish-green of fireflies has a characteristic temperature of many thousands of degrees, yet the little bugger is barely warm, so efficient is its tiny beacon. Very little energy is required to produce light if the conversion efficiency is high, the energy of a pea falling an inch, if completely converted into light, would produce a faint glimmer for every man, woman, and child who ever lived. For the record, 1 watt equals 680 lumens at the wavelength of maximum luminosity for the human eye, 555 nanometers. This is called "the least mechanical equivalent of light," and no light source can ever be more efficient than this.
Actually, a 40 W incandescent bulb has an absolute efficiency of about 1% and a 100 W bulb runs about 2%. The efficiency goes up as the wattage goes up. So a single 200 W bulb gives more light than two 100 W bulbs. Five-thousand-watt incandescent bulbs, which are seldom seen in homes, run as high as 4.66 % efficiency. Light output should never be measured in watts, contrary to the boxes in which some bulbs are sold: "Light of a 150 watt bulb from only 135 watts!" etc. Responsible manufacturers give the electric power consumption in watts and the light output in lumens, so lumens per watt is the rational measure here.
This is for an 8-foot industrial slimline T8. A more typical figure, for a 40 W "standard warm white" household fluorescent tube, would be about 9%. The efficiency depends on the colour: a pure green fluorescent tube, such as you might see at a Fair Grounds, runs over 12%. A pure red tube is a pitiful 0.53%. This is the reason that fluorescent tubes tend toward the green and the crap - it's cheap! (Several companies offer so-called "full-spectrum" fluorescent tubes, which claim to have a colour approximating daylight.)