Light Loss Factor Explained!

The Light loss factor, commonly referred to as LLF, is the multiplying factor which light output can be can be adjusted by due to environmental impacts and/or product degradation.  Lamp Lumen Depreciation (LLD), Lamp Dirt Depreciation (LDD), Ballast Factor (BF), Luminaire Ambient Temperature Factor (LATF) are the more common factors which adversely affect the light output. To keep this a little shorter we will only focus on these main four factors.

All light sources, even LED’s, are effected by a LLF. Some more than others and some much more than others. The amount of light delivered to a work plane, where it’s needed, can be reduced by as much as 80% or worse. I’ve seen worse… LED fixtures are a unique light source in that they are typically only affected by the LLD and occasionally LATF.  

Light Loss Factor

The Luminaire Lamp Depreciation (LLD) factor is typically set by each source’s manufacturer. Most manufacturers publish this information in the lamp catalogs. Incandescent, HID, Fluorescent, Induction and LED sources all have an LLD factor. The table below shows an example of a Probe-Start lamps lumen depreciation, mean/initial lumens.


HID lamps LLD have improved considerably over the last decade with a majority of these improvements in the lower wattage products. The upper wattage lamps, 750w-2000w, have seen little improvement. The greatest improvement came with the development of Pulse-Start technology replacing the original Probe-Start products.


This development brought forth a much longer lasting lamp with better color stability of the life of the lamp. Most Metal-Halide Probe-Start HID products had a LLD factor of 0.65. The newer Pulse-Start technology decreased this factor to 0.80. Oddly, some of the oldest HID technology is still putting up a fight against LED’s. Of course I’m talking about High-Pressure Sodium (HPS) and its predecessor Low-Pressure Sodium (LPS). LPS developed back in the 1920’s by Author H. Compton at Westinghouse, produces up to 200 lm/w initially.

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Some of the best LED’s in production today are less than 150 lm/w. LPS is known most for its monochromatic light and is still preferred today around observatories because of the simplicity to filter out that color allowing telescopes to see further into space.


LPS lamps are also greatly affected by LLD with an average factor of 0.75. HPS lamps are almost as efficient as LPS delivering 130 lm/w initially. This technology is still widely used today but more so because of its relatively low initial cost and low maintenance requirements. Its LLD factor is only 0.90. This technology come to fruition in 1964 at GE. Both of these technologies are still in production today. As the technology of LED’s continue to develop and efficiencies improve, HPS will slowly begin to fade away.

Fluorescent lamps have also undergone improvements over the last few decades. Their improvements are attributed to better phosphor coatings, improved chemical mixtures and electronic ballast technology just to name a few. The original T12 Fluorescent lamps are all over the map with regards to their LLD. This factor ranges, on average, from 0.72 to 0.88 depending on what lamp and manufacturer you’re looking at. With the release of T8 technology and combined with electronic ballasts, fluorescent manufacturers improved this range to 0.85 to 0.95. The most recent technological breakthrough for fluorescent lamps came with the release of the T5 lamp. These 5/8” diameter 4ft lamps pack a punch with an average initial lumen package of 5000 and only a 0.92 average LLD.


Another change in performance came with the switch from traditional, magnetic ballasts with a typical Ballast Factor ranging between 0.65 and 0.88 to electronic ballast giving the manufacturer a much higher range. These factors range on average from the traditional output of 0.88 up to 1.18. Note that by overdriving any light source, you can rapidly reduce the expected life of that source. This is most evident in the case with 1500w Metal Halide lamps. These are essentially 1000w lamps driven at 1500w. Their life expectancy goes from 18,000 hours (1000w) to only 3,000 hours (1500w). The amount fluorescent lamps are being overdriven by is not as dramatic as in this case but you get a good since of the degradation caused by overdriving lamps. LED’s are a little more forgiving as long as their Thermal Junction Temperatures (Tj) are properly managed.

Incandescent and Halogen lamps have undergone very little changes other than improvements in the manufacturing process creating much more uniform life expectancy from every lamp and slight variations in the gasses used in the lamps improving their efficiency by fractions of a percent.

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A fixtures Lamp Dirt Depreciation (LDD) is very much dependent on the construction of the fixture. Fixtures are constructed with barelamps, enclosed lamps, and various levels of sealed housing. For clarification we can refer to its Ingress Protection rating if the manufacturer has provided it.


The volume of debris and the size of debris a fixture can collect can greatly affect the performance of a luminaire. Even a fixture designed to be sealed can potentially allow air in and out. Assuming a light fixture is sealed relatively well, upon turning the light source is on, the fixture begins to heat up causing the air inside the fixture expands and potentially be pushed out. Once the light is turned off, it cools and the air inside begins to contract, creating a vacuum, pulling air in along with dust and debris. Over time the accumulation of dust will affect the fixtures performance. Adding to the losses, if the fixture is reflector based, the light will have to pass through this particle accumulation twice, once prior to being reflected and once after being reflected.
Another common type of fixture uses bare lamp(s), this type of fixtures can have large losses if the environment is anything other than a clean room.


The Luminaire Ambient Temperature Factor (LATF) effects all sources except Incandescent. Incandescent lamps like it hot or cold. Their output will not be affected by these variants.

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Fluorescent lamps on the other hand do not like the cold or the heat in extremes (if you call below 50° or above 95°F extreme!?!). These lamps perform at their peak at 77°F. If operated at any other temperature, they will be performing at less than 100%.
Metal Halide lamps struggle to strike in very cold or very hot conditions but once they have ignited, they will operate effectively at almost any temperature within reason.
A product I haven’t mentioned to this point is Induction. This product does not like to operate in moderate or hot climates. In these environments induction lamps will experience reduced lumen output and shortened life spans. In mild or cooler applications, induction has a life that rivals LED (100,000hrs), but due to the size of the source, is very hard to control optically.
LED’s like the cold and can be designed to operate in desert climates if needed. These products are tested at 25°C (77°F). If they operate in an environment cooler than that, their life will be extended and their output is increased. If operated in higher ambient temperatures, the opposite happens, though most manufacturers have taken into consideration their fixtures potential operation in warm climates and will publish data supporting their operation as high as 50°C (122°F).

Lights operated in normal conditions and serviced only when they fail are likely to have a total Light Loss Factor of 0.40 to 0.70. LED fixtures are the exception in most cases. LED fixtures are primarily subject to LATF more than any other factor.

While LED’s do have an LLD, their life cycle is so much longer than that of any other source that comparing LED’s at their L70 (0.70 LLD) to any other source shouldn’t be considered, speaking fairly. I personally will only include any type of LLF for LED’s if mandated by an AHJ (Authority Having Jurisdiction) or if extreme environments warrant it.


Reference: Lighting Research Center, | Edison Tech Center, | | Philips Lighting, | |

Written by: Lee Braddock, LC, MIES | Lighting Applications Engineer