The Measurement of Lamp Output
One of the most important considerations in selecting a light source is how much light it will generate. The unit of measure used for determining light is the lumen. One lumen equals the amount of light generated by a single standard candle.
While lumens are a measurement of light output from the light source (lamp), it only indirectly correlates to how much usable light there is for a certain application. This is true because visual light is actually how much illuminance is reflected from the surface(s) it hits. Light can be absorbed or reflected in different ratios depending on the surface it touches. This accounts for many other factors involved in lighting an area such as the type of fixture (reflector) used, how dirty or clean the fixture is, the total size of the space (room) to be illuminated, the color (reflectivity) of the walls, floor, and ceiling, among other factors.
To determine how much light actually illuminates a surface, a different measurement, foot-candle (fc.), must be used. A foot-candle is 1 lumen of uniform illuminance over the area of 1 square foot. The metric equivalent to foot-candles is lux (1 lumen per square meter). To convert fc. to lux, multiply the fc. by 0.0929. To convert lux to fc. multiply lux by 10.76.
There are two important measurements used when considering selection of a lamp: initial lumens and mean lumens. Two measurements are needed because, like most devices, lamps lose some light output over their rated life. The gradual loss of efficacy (lumens per watt) is called lumen depreciation. Lumen depreciation in lamps is caused by two main factors, loss of chemical additives in the lamp and wall darkening in the arc tube. In the arc tube are chemicals that are responsible for the generation of light. As the lamp operates, these chemicals react with the arc tube components and eventually can no longer produce light. Also, the standard arc tube contains two tungsten electrodes that are heated to incandescence during lamp operation. When the lamp is started, these electrodes evaporate tungsten which deposits on the inside wall of the arc tube. These black deposits absorb light and cause the amount of light emitted by the lamp to decrease. Both of these are very gradual processes, but the end result is a loss in efficacy for the arc tube. In Ventures new Uni-FormŽ pulse start. technology, the unique formed body arc tube lamp provides greater efficacy and better lumen maintenance than the older, standard pinch body arc tube technology
Initial lumens is a term defined as the amount of light output from a lamp when it is new. For a metal halide lamp, these ratings are averages based on photometry at rated lamp watts after 100 hours of operation. Ratings are made after 100 hours to allow the light-producing chemicals in the arc tube to "settle in to the correct position in order to deliver optimum performance. During the first 100 hours, the instability in chemical location is sometimes seen as rapid shifts in color called flaring. Also, the light given off by the lamp during the initial 100 hours is up to 20 percent higher than the initial lumen ratings. Both of these phenomena are normal for metal halide lamps. Both the light output and flaring will typically stabilize within the first 100 hours.
Universal lamp ratings are based on lamps burning in the base up position. When universal lamps are burned in positions other than base up, light output for these lamps can be significantly lower than the listed ratings. The table below shows how position oriented horizontal (HOR) lamps outperform universal rated lamps when burned horizontally.
Lighting designers and specifiers know that any light source will decrease in light output the longer it is operated (see previous page). To properly light a facility for the useful life of a lamp, they must calculate an "average" lumen output known as mean lumens. These ratings are averages based on photometry at rated lamp watts - at 40% of the lamp life when operated on typical commercial ballasts, and based on the lamps burning at 10 hours per start (except as noted).
Both burning and measurements are conducted in the designed burning position (i.e. horizontal or base down). Universal lamps are burned and measured in the base up position. When a lamp is burned in other than the design position, mean lumens will be lower than the rated base-up operation values, and life will be shortened.
The lumens generated by a lamp will decrease as the lamp ages, a factor known as lumen depreciation. Most often the lumen depreciation that the user sees is a combination of decreased lamp output and a dirt factor. Dirt accumulation on the fixtures reflector and lens can greatly decrease the amount of usable light (in many cases more than lumen maintenance). This is another reason why group relamping (and cleaning) is critical for maintaining a lighting system. Open fixture lamps allow the fixture manufacturer to remove the glass lens, thereby removing a key ingredient for dirt depreciation. Venture Lighting continually conducts research and testing to ensure they have the best lumen maintenance possible. Lumen maintenance ratings are based on 10 burning hours per start (except as noted). Lumens will be lower for shorter burn cycles (i.e. 6% to 8% lower at 5 hours per start). Individual lamps may vary from printed mean lumen ratings and the lumen maintenance curve seen on this page. These are a general guide and refer to the average for a group of lamps.