PART V: “LIFETIME”

Lifetime is a critical index for evaluating lighting fixtures. Conventional lighting fixtures are relatively inexpensive, of which the most widely adopted type is the fluorescent light, with only 3000 hours’ longevity. Compared to that, LED lights are normally more costly, but are famous for their long lifetimes, normally over 50000 hours. When it comes to plant growth, a long-life light can greatly save the maintenance cost and help increase profits. This is a key factor for plant factory owners when they choose lighting fixtures.   

An LED’s lifetime is defined as the time it takes until its lumen output reaches 70% of the initial output. LEDs have long lifetimes which are usually tens of thousands of hours. It is unrealistic and uneconomical to measure by lighting it up for that long. Almost all the LED manufacturers are disclosing an estimated value based on certain driving and operating conditions, rather than an actual measured figure. This means, that if we alter any of the conditions, the longevity of the light will be changed accordingly.

Let’s first have a look at how driving conditions would affect an LED’s lifetime. In an experiment we did on an LED model with 60 mA rated current, the LEDs have been continuously lit up at 60 mA, 90 mA, 120 mA, separately. After 6000 hours, (Figure 1), the LED at 60 mA has a less than 3% luminous output attenuation after 6000 hours, while at 90 mA and 120 mA, the decay reaches 35% and 41% respectively. Overcurrent shortens lifetime.

Figure 1. Lumen decay trends of an LED model at current levels of 60mA (black), 90 mA (blue) and 120 mA (red).

Besides, operation conditions also make a big difference. High temperature will not only diminish the LED’s luminous efficacy, but also reduce the activity of the junction area, thus shrinking the LED’s lifetime. Figure 2 shows the lumen output decay trends of a same LED type at ambient temperatures of 85℃ and 105℃ respectively. It is apparent that lumen decay aggravates with the ambient temperature rising.

Figure 2. Lumen decay trends of an LED model at 85℃ and 105℃ ambient temperatures.

In one word, the lifetime of an LED is uncertain and highly dependent on its driving and operation conditions. Modern plant factories, especially vertical farms and plant chambers are normally quite enclosed with little exposure to fresh air. As such, thermal design plays a more critical role in ensuring a longer lifetime of the light.