If the lighting system in a parking lot doesn’t render colours accurately, it doesn’t matter — we’ll still find our car. But accurate colour rendition can be extremely important in merchandising, graphic arts, and the inspection of raw materials and finished products. In this article we look at how we perceive colours, and how our perception of colour is influenced by the type of lighting we use.

THE BASICS

Colour vision involves three elements: the eye-brain system, the object being viewed (its reflectance or transmission of colour), and the spectral characteristics of the light source that illuminates the object.

The Part that Eye and Brain Play

As the graph at the top of the next column indicates, the retina of the human eye contains three different types of microscopic light-sensitive “cones.” They are referred to as S, M, and L cones because of their sensitivity at short, medium, and long wavelengths of light. The subjective experience of colour which the brain creates is based on the relative stimulation of the three types of cone. If pure 430 nanometer (nm) light strikes the retina, the S cones respond strongly, the M and L don’t respond, and the brain creates the subjective experience we call violet. If 650 nm light strikes the retina, it is only the L cones that respond significantly, and we experience red. At wavelengths in between, at least two types of cones respond simultaneously, and the brain creates myriad other colour experiences.

The Part the Light Source Plays

Most objects contain opaque pigments that selectively reflect light. A few (such as traffic light lenses and stained glass) selectively transmit light. The mix of light wavelengths that reaches our eyes

Response to Light of Different Wavelengths:

The Eye’s 3 Types of Retinal Cones

and creates a colour experience depends both on

1. which wavelengths the object reflects (or transmits), and on

2. the wavelengths present in the illuminating light source.

THE LIGHT SPECTRUM AND COLOUR TEMPERATURE

Bodies heated to incandescence produce light that has a continuous spectrum. This means that all visible wavelengths are present in the light — though not necessarily at the same amplitude. Extremely hot bodies such as the sun — and anything else hotter than about 5500°K (Kelvin) — radiate more energy at the blue end of the spectrum than the red. Lower temperature objects, such as a tungsten lamp filament, radiate more energy at the red end than the blue. Colour Temperature is a convenient way of referring to, and specifying, the relative energy balance between the red and blue ends of the spectrum. Generally, in dimly-lit situations people prefer lower (redder) colour

temperatures, and in brightly-lit situations they prefer higher (bluer) colour temperatures.

The leftmost graph in the set of three shows the spectral characteristics of two common light sources: daylight, and light from a typical incandescent bulb. The daylight has a colour temperature of 7500°K. The incandescent source has a colour temperature of 2750°K — close to the actual temperature of the tungsten filament. Because both daylight and tungsten light are continuous-spectrum sources without big bumps or dips in their spectral curves, they provide ideal illumination for tasks where colour discrimination is important. Unfortunately, daylight does not occur inside our places of work, and producing large amounts of incandescent light requires a lot of electricity. What other options are there?

Gas Discharge Lamps

Gas discharge lamps are much more energy-efficient than incandescent lamps, but in their simplest “clear-envelope” form do not produce a continuous spectrum. The center graph of the three shows the light output from a clear mercury-vapour lamp. Here we have light energy at only five discrete visible wavelengths. Since most of the spectrum is missing, light of this kind provides extremely poor colour rendition.

A partial solution to this problem is achieved by coating the inside of the glass envelopes of gas-discharge lamps with light-emitting phosphors. Fluorescent tubes, for example, have within them a mercury-vapour discharge complete with those spectral lines. The real purpose of the discharge, however, is to light up the phosphor on the inside walls of the tube. The light produced from a typical cool-white fluorescent tube is shown in the graph at the right. The mercury-vapour emission lines are still present, but now the rest of the spectrum has been filled-in with light from the glowing phosphor.

THE BEST LAMPS FOR COLOUR RENDITION

In addition to being rated for watts in, lumens out, and colour temperature, lamps are also rated for their ability to render colours accurately. Lamps are assigned a CRI (Colour Rendering Index) rating from 1 to 100 which indicates the extent to which colours will appear normal or natural when illuminated by that lamp. Incandescent lamps tend to have high ratings — typically 97 — whereas the CRI of the traditional cool-white fluorescent lamp is only 62, and the CRI of a clear mercury-vapour lamp is a mere 22.

Recent advances in phosphor technology have given us highly-efficient fluorescent lamps having CRI ratings of 80 and above. This was accomplished in the new T8 series fluorescent lamps by using a mixture of blue-, green-, and red-emitting phosphors that work efficiently with the S, M, and L cones in the eye. These T8 lamps are available in a range of colour temperatures from 2700°K to 6500°K, and have found acceptance in a wide variety of colour-sensitive situations. T8 lamps having a CRI of 82 and a colour temperature of 4100°K have been suggested for “colour-important industrial applications,” but to find the best lamps for your purposes, some experimentation may be necessary.

If you have HID lighting, and are experiencing colour rendition problems or employee complaints about the colour of the light, consider switching to high-CRI lamps. Some varieties of metal halide, multi-vapour, and high-pressure sodium lamps now have CRIs exceeding 70, and there are a few metal halide varieties with CRIs as high as 80 and 85.

MORE INFORMATION

For further information about energy-efficient lighting, call Ron Estabrooks or Mike Proud at

1-800-236-5193 (toll free).