Color Rendering Properties of Light Sources

Do objects have colors of their own? If yes, then why even the most brightly colored objects lose color when taken to darkness?

Following this line of thinking photographer Paul Outerbridge (1896 -1958) concluded that for their color objects depend on light. And that color must be the property of light. In any case, we all know that how good a photograph looks depends upon the quality of light.

Color rendering is related to how objects appear under a given illumination. It is one of the most important characteristics of the light sources. If color rendering is poor, the light source will not be useful for general lighting. The influence of artificial light source on the appearance of the colored object is expressed by the color rendering index (CRI). It tells us how truthfully a light source will make an object appear compared with the natural daylight. The larger the difference in color reproduction, the lower the CRI value of the light source will be. In general, higher CRI values – CRI of 80 and higher – make objects look more natural.

CRI comparison

Lighting sources at the low end of the index, such as low-pressure sodium lamps (CRI 20-30) tend to wash out colors and are best used in applications, say roads, where accurate color rendering is not important. An incandescent or halogen light bulb, on the other hand, is considered to have a “perfect” CRI of 100. Linear fluorescents and compact fluorescents (CFLs) usually fall in the 80 to 90 range.

There are specifications of minimum requirements for both the CRI as well as Lumens per watt for common types of light products. Higher CRIs (CRI of at least 80) are recommended for office applications, still higher for showrooms or where fine designing work is being done. Lower values are permitted for living space, corridors, warehouses and street lighting. Likewise for office atmosphere, minimum 500 lux illumination is recommended for general activities and up to 750 lux for task areas requiring precision work and concentration. The illumination in the immediate surrounding area (band of at least 0.5 m around the task area in the visual field) should be 65 – 75 percent of the task area illumination. The rest of the background area adjacent to the surrounding area should have a minimum one third illumination level as compared with the surrounding area. Note that the ceiling represents a large reflection area and the reflected light creates an impression of homogeneity and most truthfully mimics the natural light. The ceiling illumination can be about 30 percent of the working space illumination value.

What are Wattage and Lumens

Incandescent bulbs create light by passing electricity through a metal filament until it becomes so hot that it glows. They release almost 90% of their energy as heat. In a CFL bulb, an electric current is passed through a tube containing gases. This produces ultraviolet light that in turn gets transformed into visible light by the fluorescent coating (called phosphor) on the inside of the tube. A CFL releases about 80% of its energy as heat. In comparison, LED lighting products use light emitting diodes to produce light very efficiently. In LEDs the movement of electrons through a semiconductor material produces light; a small amount of heat is released backwards, into a heat sink. LEDs thus consume much less electricity to get a certain amount of light output.

People are normally more familiar with lights such as the incandescent bulbs and CFL lamps, but now more economical LED light sources are available. These are solid state devices that emit light with the passage of current.

Wattage: Wattage is connected to the electrical power consumption. The earlier incandescent light bulbs consume too much electricity to produce the desired amount of light. CFL lamps consumed much less electricity for the same amount of light. Now we have LED lights that still consume less electrical power to give you the same amount of light.

Lumens: Lumens (lm) represent the actual amount of ambient light coming from a lamp. The higher the amount of lumens, the more “lit up” a room will be. With the arrival of LED lights, lumen is a better way to talk about “lighting power” than the electricity consumed.

Lumens per watt – luminous efficacy

eye sensitivityThe human eye is sensitive to only a small portion of the electromagnetic spectrum – 400 – 700 nanometers. Wavelengths outside this range are not useful as far as illumination is concerned. Moreover, the human eye is not equally sensitive to all wavelengths in this range but instead shows maximum sensitivity at around 555 nm. This distribution is in the general form of a bell-shaped curve and is called the spectral luminous efficiency function. So, we are only interested in the intensity of this light or luminous flux (lumens). LEDs produce a good amount of Lumens per watt, much better that incandescent bulbs or CFL lamps.

There is another practical aspect of a light source. It needs to be housed and normally shaded in some way to prevent glare, which is an undesirable side effect  resulting from direct viewing of the light source. Therefore some light intensity (or lumens) may be lost due to housing. LEDs generally don’t require shading or reflectors, and thus can provide more useful lumens. The huge potential for energy saving is the main driving force for solid-state lighting because they offer the best lumens output per watt.

Correlated Color Temperature (CCT)

Light sources that are not incandescent radiators have what is referred to as a correlated color temperature (CCT). Color temperature is a way of mentioning the color characteristics of light source – warm (yellowish) or cool (bluish), and measuring it on the absolute or Kelvin temperature scale.

Color Temperature is not how hot the lamp is; it the relative whiteness of a piece of tungsten wire heated to that temperature in degrees Kelvin. On the absolute temperature scale, the temperature is expressed in degrees Kelvin. You get temperature in Kelvin if you add 273 to the degree Celsius. Thus, room temperature of 27˚C is 300K. While CRI represents how accurate a light source is, color temperature represents the character of the light source. At the low end, a color temperature of 2600-2700K creates a warm light character like that seen in incandescent bulbs; a higher color temperature of 4100-5500K creates a whiter light like most often seen in office buildings.

Color temperature is a way of describing the “color” of the emitted light. The term ‘temperature’ refers to a real temperature of a physics concept known as a ‘black body’. A black body absorbs all light falling on it, but conversely, according to thermodynamics, is also a perfect emitter. Here it refers to the filament which is being heated. At some point it will get hot enough to begin to glow. As it gets hotter its glowing color shifts;first red, then orange, then yellow up through bright white. The temperature of the material corresponding to those colors is termed the color temperature.

Light sources that glow this way are called “incandescent radiators.” Their best advantage is that they have a continuous spectrum; they radiate light energy at all wavelengths of their spectrum, therefore rendering all the colors of a scene being lit by them, equally. Only light from sources functioning this way can meet the true definition of color temperature.

Other lights, with a correlated color temperature, do not emit a continuous spectrum of radiations at all wavelengths. As a result, they have difficulty rendering certain colors accurately. Their rendering ability is measured on the CRI scale comparing with the incandescent radiators that are assigned a CRI of 100.

Color rendering, CRI and CCT

High Lumens-per-watt value does not necessarily mean that the light quality is also good. The notion of color quality is a subjective measure.

A common misunderstanding is that high CRI means that the light source will render all colors well, but this is not always the case since CRI is measured only with respect to a reference source. The reference source is either the blackbody curve (for test source CCT< 5000K) or a CIE Daylight source (for test source CCT> 5000K). Furthermore, the color rendering of sources, at extremely low or high CCT, can be very poor even though the CRI score is nearly 100.

CRI is typically used to compare sources of the same type and not dissimilar sources. Further, if you have 2 light sources at 2800K and 6500K and both with CRI 100, you cannot say that both have exactly the same color rendering performance. Thus, CRI should be used only for some indicative purpose.

Since the CRI score is directly related to the spectral power distribution of the light source, it is possible to manipulate the spectrum to your advantage and produce a higher CRI value. Fluorescent lamp manufacturers have been known to “adjust” the CRI by shifting the emission band of their lamp spectra. This can shift CRI by several points. However, a difference of 5 points in the CRI value does not make much difference.

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About Goodpal

I am a firm believer in healthy people (mind and body both), healthy societies and healthy environment. Please feel free to comment, share and broadcast your views -- I like rational and intellectual discussions. Thanks for stopping by. Have a Good Day!
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1 Response to Color Rendering Properties of Light Sources

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