Experiment: Can LCD skylight and window simulators provide functional lighting?

Jan 15, 2019

Jianchuan Tan LC, WELL AP, LEED GA

earned his PhD and Master’s degrees in Architectural Lighting at the Lighting Research Center at Rensselaer Polytechnic Institute, and now works in Philadelphia as a product developer for Lightglass Lighting.

At both Lightglass and Luxtech, there is a deep interest in people-centric lighting, the relationship between lighting products, human biology and psychology. There are many LCD-based artificial window and skylight products on the market, but in what ways are they useful? In October 2017, I conducted a series of experiments on LCD screens to better understand the functionality of the light they provide.

It is not difficult to understand the value of LCD artificial windows and skylights. Using high definition photos and videos, they create the impression that there is a window to the outside where there really is not. Perhaps created as a replacement for placing a translucent film with an image in front of a down light, LCD displays provide a simple and practical way to enhance windowless spaces. However, I wanted to know if in addition to the aesthetic enhancements, LCD lighting products also provided functional task lighting.

EXPERIMENT:I disassembled a common LCD display to isolate the 90+W LED backlight array used to illuminate the product. I chose a series of images to display on the screen in front of the LED array, using white as the maximum output, and measured the light using a photometer. In the “Backlight” column is the data for the LEDs without any screen in front. The image at the bottom is what I chose to display on the screen while taking the measurements, the first is just a completely white screen. In the “CCT” column is the measured Correlated Color Temperature on top of the illuminated screen. The final column is a Circadian Stimulus measurement, showing percentage of melatonin suppression possible. Below are the results:

RESULTS: During the all-white screen test, the maximum possible light output for the LED array, the data shows ~450 lumen output, and the efficacy is ~ 5 Lumens per Watt. This lumen output is too low to be used for task lighting, and is far lower than the lumens provided by a real window or skylight. The efficacy of 5 lm/W is also very low, where most LED lighting products can reach upwards of 120 lm/W and a real window consumes no energy on its own.