All lamps have a burn life profile, the amount of light the bulb makes over its lifetime. We don’t notice it, but all bulbs grow dimmer as they burn. The concept of burning is accurate, as the electrical energy excites the filament in an incandescent bulb or the cathode in a metal halide or florescent bulb. Over time, the electricity and heat erodes filament or cathode until they fail. The surface temperatures of the filaments in traditional bulbs can reach 3,300 degrees kelvin (5,480 degrees F), while the anode temperatures of a florescent bulb will reach over 2,000 degrees kelvin. In all cases, the heat evaporates the materials, thinning the filaments and anodes. The only reason these materials do not burn up in a flash is the inert gas in the bulb.
As the filament or cathodes evaporate away, they create less light. Different technology bulbs have different burn life profiles. Incandescent bulbs dim slowly, losing about 10% of their brightness over the 700 – 800 hours of life until the filament finally erodes at a hot spot and breaks.
Metal halide and sodium vapor bulbs also gradually lose light output, dimming as much as 50% before filament erosion reaches the point of failure. While these lights have a long life, in excess of 10,000 hours in some applications, the light output in the last quarter of life is just more than 50% of what the bulb made when new.
Florescent tubes work differently. In place of a filament, florescent tubes use an arc between two cathodes at each end of a tube filled with low-pressure mercury vapor gas. The cathodes heat the mercury into a vapor until an arc forms between the cathodes. The excited gas in the tube emits invisible light that reacts with the phosphors in the coating on the inside of the tube. The coating fluoresces, creating the light. The only time the electricity erodes the cathode is the first strike of the arc. In a continuous burn, the erosion of the cathode is so slow that the bulb can burn for over 20,000 hours. A florescent bulb loses about 10% of their output in the first few hundred hours, and then level out through the rest of the life of the bulb until the cathode fails, or the mercury gas breaks down.
Saving Power & Wearing Bulbs
While florescent bulbs can have a marathon life, they also have an Achilles heel. The initial starting strike of the arc erodes the cathodes, wearing away at the life of the lamp. The more times the lamp cycles on and off, the more cathodes erode.
There are a number of ways to start a modern florescent lamp. If you have a CFL lamp, you might notice that it may flicker a few times when you first turn it on. Depending on the kind of CFL you purchased, you might notice that the lamp does not make a lot of light at the initial start, but builds up to full power after a minute. This behavior is typical of rapid start or program start florescent lights. There are also florescent lights, called instant start lamps, which reach full power immediately after the power turns on. This design excites the gas in the tube by applying a high spike voltage that strikes an arch through the gas in the lamp, bringing the lamp close to 100% output within seconds.
Florescent lights (Metal halide and sodium vapor lights too) are actually systems of two basic components, the lamp, and the ballast. The ballast is a device that converts typical high line voltage power of 120 or 277 volts into the ultra-high voltages required to create the arcs that excite the gasses. The ballast type must match the type of bulb used, so a instant start ballast must be matched with a tube made to work with instant start ballasts.
Occupancy sensors, electronic devices that detect the movement of heat, provide another way to reduce energy consumption. First deployed as integrated packages with the T5HO high bay fixtures, warehouses with extensive pick modules now install occupancy sensors to help curb the electricity consumption in the modules. In California, (and some other states) regulatory measures further restrict the number of kilowatts used for lighting, and mandate the use of occupancy sensors to turn off the lights when there is no human activity in a room.
That is where a serious rub occurs. There are only a few events in the lift of an electric lamp. There is the OFF state, where no energy flows and no wear happens. There is the ON state, where energy flows, and the filaments, cathodes and anodes slowly wear out. Then there is the STOP state, when the energy stops flowing through the lamp, and the lamp components cool. Finally, there is the START state, where most of the wear happens with a lamp.
In an incandescent lamp, the filament must react to the sudden, instant application of energy flow. In less than a quarter of a second, the filament changes from room temperature to over 3,000 degrees F, creating a strain on the tungsten filament. When the power switches off, the filament metal rapidly cools, though not as fast as it heats up. As with any lamp, if you turn on an incandescent lamp and leave it alone, it will burn, releasing material from the filament until a weak spot forms that cannot handle the current, or where vibration breaks the metal and cuts the circuit. The more often the lamp is turned on, and then off, the more stress placed on the filament.
Have you noticed how often you see an incandescent light blow when you first switch on the power? That failure is the shock of the instant heating of over 3,000 degrees, where a weak spot finally give up. The more you turn on and off the lamp, the more the heating stresses wear out the filament. Flashing lamps, like the ones for special lighted signs, traffic lights, brake and turn signal lamps use thicker filaments designed with more flexibility and material to help extend the life of the bulb.
Fluorescents, under extreme conditions of high frequency on and then off function, have a lifespan much shorter than cheap incandescent lamps. Each start cycle slightly erodes the electron-emitting surface of the cathodes. When all the emission material is gone, the lamp can’t start with the available ballast voltage. There are special fixtures intended for flashing lights (such as for advertising) that use a ballast that maintains cathode temperature when the arc is off, preserving the life of the lamp. However, these are higher cost special ballasts that use matched lamps, not practical for warehouse or office applications.
The extra energy used to start a fluorescent lamp is equivalent to a few seconds of normal operation. While it is more energy-efficient to switch off lamps when not required for several minutes, the relative cost of the energy might be far below the long-term cost of lamp replacement and disposal. Disposal is a problem, because all fluorescent lamps contain a small amount of mercury, creating a environmental disposal challenge.
Trouble with T8 in Warehouses
Using the old T12 systems, most pick modules used 2 bulb fixtures to provide +30FC to the pick areas. In fact, the old 2 light T12 systems could deliver as much as 45 – 50 FC of light, making the pick module a very bright place. Warehouse managers who remembered the building specifications remembered the 30FC requirements, and many assume what they saw in their pick module was 30FC, when in reality the light level was much higher.
With the higher output of T8 fixtures, electrical contractors substituted single bulb strip fixtures for the old T12 systems. The fixtures cost less, used fewer bulbs, and consumed less energy. What should be a home run isn’t, because a single lamp T8 does not put out enough light energy. T8 fixtures, when installed without a reflector at the typical 10’ above the floor only put out about 26 – 28 FC of light. With a gloss white reflector, they may reach 30 FC, but not much better.
Warehouse managers used to the bright lights of the old twin tube T12 modules look into a new module with single bulb T8 fixtures and think the space is dark. Compared to what they know and remember, they are right. The new lights create 28FC of light, not the +40FC they may have seen in their old facility.
T8 single bulb fixtures will never produce enough light energy to meet a 30FC specification.