How does L70 affect IS 1944 road lighting compliance over time?

IS 1944 requires the road to meet specified minimum lux at all times. If you design to initial lux (100% output), the road is over-lit initially and below spec at end of life. Correct design: calculate using L70 maintained lux as the target - the road exceeds spec when new and just meets spec at end of LED life. This is called maintenance factor design.

Can I measure lumen depreciation on my installed street lights?

Yes, approximately. Measure lux at the road surface at a fixed reference point (same time of night, no moon, same weather conditions) at installation and annually. Track the decline. A 15% lux reduction over 5 years indicates normal L80+ performance. A 30% reduction in 3 years indicates poor quality LEDs or overdriven chips.

What is the difference between L70 and the street light's warranty period?

L70 is a performance metric (when does light output decline to 70%). Warranty period is a commercial commitment (the manufacturer will repair or replace failures within the warranty period). A 2-year warranty does not mean the light fails in Year 3 - it means the manufacturer accepts responsibility for manufacturing defects for 2 years. A quality light with L70 = 50,000 hours will continue performing well for 15+ years - far beyond any warranty period.

What is Lumen Depreciation and L70 Rating in LED Lights?

What Lumen Depreciation Is

All light sources lose output over time. Incandescent: rapid depreciation, visible yellowing within months. Sodium: loses 30% output in the first 4,000 hours (2 years at 6h/night). LED: very gradual depreciation - a quality LED street light loses only 10–15% of initial output over 30,000 hours (10 years at 8h/night). This gradual decline is called lumen depreciation (or lumen maintenance). The rate of depreciation depends on: LED chip quality, junction temperature (higher temperature = faster depreciation), current density (over-driven LEDs depreciate faster), and driver quality (unstable current accelerates degradation).

L70 - The Standard Metric Explained

L70 is the number of hours at which an LED source retains 70% of its initial lumen output. If a street light produces 5,000 lumens when new, an L70 rating of 50,000 hours means it still produces 3,500 lumens (70%) after 50,000 hours of operation. At 8h/night: 50,000 hours = 17 years. This is the standard end-of-life definition for LED road lighting - at L70, the light is still functional but is due for replacement in the next maintenance cycle. L80 = still at 80% (better). L90 = still at 90% (premium). For IS 1944 road lighting design, always design using maintained lux at L70 values - not initial lux - to ensure the road meets the lux standard throughout the design life.

How to Use L70 in Procurement

Specify in tender: 'LED luminaire shall demonstrate minimum L70 ≥ 50,000 hours per LM-80 test data, with TM-21 projected life calculation.' LM-80 (IES LM-80) is the test standard for measuring LED chip lumen maintenance over time (minimum 6,000 hours of actual testing). TM-21 is the calculation methodology for projecting long-term lumen maintenance from LM-80 data. A supplier claiming L70 = 50,000 hours without an LM-80 report is making an unverified claim. A supplier with an LM-80 report from a NABL or internationally accredited lab is providing verifiable data. Xera Tech can provide LM-80 data for its LED module range on request.

Why Cheap LEDs Depreciate Faster

Cheap LED lights depreciate faster because they: over-drive the LED chips (run at higher current than rated - more lumens initially but much faster degradation). Use low-grade phosphor that degrades in UV and heat. Have inadequate heat sinking - the LED junction runs hotter, accelerating chemical degradation of the phosphor. Have poor driver stability - current ripple causes micro-stress on chip junctions. In practice: a cheap 40W LED producing 5,200 lumens initially may drop to 3,100 lumens (L60) after 25,000 hours - the road that was adequately lit in Year 1 is dangerously dim in Year 9. A quality 40W LED still at 3,900 lumens (L75) after 50,000 hours - safely lit throughout its design life.