Led & Solar - Science
Light and human perception
Understanding how the human eye originally developed allows us to understand what is the "optimum" light in our modern lives. The eye gives good vision for hunting during the day, and for defence during the night. During the day the eye is able to adjust both to see clearly in the shadow of the forest and, with more difficulty, in the intense light of the open plains. The ability of the iris to adjust to the amount of light is very effective, there may be as much as 100 times as much light on the plain as in the forest, but within a few seconds the eye has adjusted.
Indeed the light in the forest seems more comfortable than that on the plains. Many experience headaches when spending a day in excessively bright light and strong shadows; we do not wear sunglasses just to look cool! Studies show that the wavelength (colour) and flicker of light are also important to creating a comfortable light.
Traditionally many building designers have concentrated in making space as "light" as possible, as cheaply as possible. Indeed "brighter is better" was a myth that grew but has been disproved by study after study (See Veitch & Newsham - Determinants of Lighting Quality II: Research and Recommendations for an excellent summary) Similarly the efforts to achieve brightness were made without regard to the wavelength or flicker of the light. Many fluorescent lighting systems have been installed that flicker and have poor wavelength.
Fortunately LED lighting allows us to make major improvements to how we perceive light in many settings. The wavelength, glare control and distribution of light are much more important to how light is perceived as "good". These are all aspects of lighting where LED help.
Measurement of light - Why a high Lux is not necessarily best
Light is one type of radiant energy, and travels as photons exhibiting wave characteristics - the wavelength determines the colour that we see. The radiant energy arriving at a surface can be measured in watts. However the eye will not sense energy outside the range of wavelengths of "visible" light, so the International Commission on Illumination have created a "standard" response of how the eye will "see" visible light.
This response recognises the ability of the eye in daylight conditions to detect photons with the wavelength we perceive as yellow light, at 560 nanometres (nm), more efficiently than either green (510nm) or orange light (610nm), and much more efficiently than either red or blue light. Indeed one watt of energy at 560nm measures a luminous flux of 685 lumens, whilst one watt at 510nm would only measure 342 lumens. The simplest device for measuring light is an illuminance meter measuring lux, (the number of lumens per square meter). Such a meter uses the Commission's curve to determine the lux relative to wavelength.
Before electric light became widely available most rooms had too little light, so creating a higher lux reading become the goal. Different types of lamps emit different wavelengths of light depending on the materials used. Manufacturers would concentrate on making lamps with the output concentrated at the yellow 560nm, so as to appear to create a maximum efficiency lamp in terms of lux per watt. This yellow light is not necessarily the most comfortable light for the brain, which prefers a broader spectrum of light, daylight. Experiments show that working under a good spectrum of light is better than working under a higher lux reading of light centred on the yellow wavelength.
The Eye Structure
The eye developed to allow good vision for hunting during the day, and for defence during the night. The eye uses two different types of receptor to capture the light; 125 million Rods and 6 million Cones. See human eye shape jpg The brain uses the cones predominantly during the day. Cones come in three types focusing on light at 430, 535 and 590 nanometres (blue, green and orange/red respectively).
The brain combines the signals to "see" a white light when the three colours are combined. Not surprisingly tests have shown that people feel most comfortable when the mix of light
wavelengths they see matches that of natural daylight.
Originally the brain used the Rods more for defence. Rods are excellent for determining shape, for peripheral vision, and for very low levels of light (scotopic vision). Rods "see" best at lights around 500nanometres (a pale blue colour), in effect as light fades the brain only sees in black and white. See right, Unfortunately many streetlights use sodium lamps, strongest at 600 nanometres; many only emit a small energy at 500 nanometres where the rods see best.
Light that has strength evenly across the range from 430 to 590 nanometres provides light that they eye finds easy and natural to use. LED lights can be constructed of many microelements to select a mix of wavelengths, or use a phosphor coating to stimulate a range of wavelengths.