Tutorial ~ page 1
Let me apologize, in advance, for any inadvertent errors, and the many known or unknown omissions in the following paper. This is a highly complex subject, encompassing a number of demanding disciplines - and it’s been a long time since I’ve studied the subject.
In order to better understand how and why we see things as we do, and apply this knowledge to lighting
design, a little background about the physics of light, and the physiology of human vision will be useful.
1. Light Energy - what is light?
2. How we see.
3. Light & Colour
4. Quantity of Light
5. Applied Lighting
1. Light Energy
Light is electromagnetic energy. The electromagnetic spectrum is very large, extending from Gamma & X-rays (one hundredth of a nanometer), to Infrared & Radio
waves (one meter & above). The visible spectrum (to humans) of light (visible light - red, orange, yellow, green, blue, indigo, and violet), to which the human eye is sensitive, lies in the wavelengths between around 380 nanometers (short-wave, blues) and 780 nanometers (long-wave, reds); with a middle range (yellow-greens) between. The sensitivity of the eye falls off at the extremities, so 400 nm to 700 nm is a good practical approximation.
The lower frequency long waves have a lower amount of energy (1eV) , and the shorter wave high frequencies have a higher amount of energy (5 eV) . The visible spectrum has energies ranging between 1.8 to 3.1 electron Volts (eV).
These light waves can be reflected, absorbed, or transmitted. Visually Solid objects will Reflect light, and Transparent objects will Transmit light through them. An object that Absorbs all light (no reflection nor transmittal) would be Invisible. The color that we perceive an object to be, is determined by the particular wavelengths of light which are absorbed or reflected by the object. Only the reflected wavelengths reach our eye, and are seen as colour.
Colour is the attribute of visual experience that can be described as having quantitatively specifiable dimensions of hue, saturation, and brightness.
Ie: The leaves of most common plants absorb red, orange, blue & violetlight, and at the same time, they reflect the green wavelengths - and are therefore seen to be green in color. These characteristics of the object are referred to as its spectral reflectance.
2. How we See - Vision & the Eye
The human eye has a simple two element lens. The cornea is the outer element, and the lens is the inner element. The amount of light entering the eye is controlled by the iris which lies in between the two. The light passes through a clear gel called the vitreous humor
and creates an inverted image on the retina at the back of the eyeball .
The retina is the light sensitive part of the eye. Its surface is coated with millions of photoreceptors. These photoreceptors sense the light, and pass electrical
signals indicating its presence through the optic nerve to stimulate the brain. There are two types of photoreceptors, Rods and Cones.
There are approximately 6 - 7 million Cones in our retinas, and they are sensitive to Colours, in a wide range of brightness. There are three different types of cone receptors, which are variously sensitive to short, medium and long wavelengths.
Different light sensitive pigments, within each of these three types, responds to different wavelengths of light. Red cones are most stimulated by light in the red-yellow spectrum. Green cones are most stimulated by light in the yellow-green spectrum. Blue cones are most stimulated by light in the blue violet spectrum. This phenomena describes the Spectral Sensitivity of the eye.
Additionally, we have approximately 110 - 130 million Rods on the retina, which are sensitive to brightness (illuminance), and are used in dim light. Rods are monochromatic, which is why, in dim light, we only see in black and white.
Although there are upwards of 120 million receptors in our eye, there are only some 800,000 fibres in the optic nerve connecting the eye to the brain. The connections cannot, therefore, be simple, and the amount of information sent to the brain for interpretation is huge. The optic nerves cross at the optic chiasma, where all signals from the right sides of the two retinas are sent to the right half of the brain, and all signals from the left, to the left half of the brain. Each half of the brain gets half a picture. This ensures that loss of an eye does not disable the visual system.
And you though you had problems interfacing your electronics...
3. Light & Colour:
The "color" of light sources is derived from a complicated relationship derived from a number of different measurements, including Correlated Color Temperature (CCT), Color Rendering Index (CRI), and Spectral Distribution. In general, colour is most readily described by a combination of CCT and CRI.
White light, from the sun, contains all the possible color variations. Yet, the human eye can only respond to certain colors and wavelengths, and not everyone perceives (sees) the same colors, or exact same shades of a color. When we see, light from the outside world is focused by our lens onto our retinal rods & cones.
Correlated Colour Temperature (CCT)
is defined as the absolute temperature (expressed in degrees Kelvin) of a theoretical black body whose chromaticity most nearly resembles that of the light source. As a piece of metal (a theoretical Blackbody) is heated, it changes color from reddish to orange to yellowish to white to bluish-white. In practice, most light sources do not duplicate the energy distribution of a black body radiator, so the term correlated colour temperature is used to mean the colour temperature that most closely resembles the light source in question.
While in reality the colour of light is determined by how much each point on the spectral curve contributes to its output, the result can still be summarized on a linear “temperature” scale. The CCT rating is an indication of how "warm" or "cool" the light source appears. Paradoxically, the higher the “K” number (temperature), the cooler the lamp colour will appear. The lower the “K” number, the warmer the lamp colour will appear
When we say a lamp has a Colour Temperature of 3000 Kelvins, it means a glowing metal at 3000 Kelvins would produce light of about the same color as the lamp. If instead, the metal is heated to 4100 Kelvins, it will produce a much whiter light. Direct sunlight corresponds to about 5300 Kelvins while daylight, which has the blue from the sky mixed in, is typically 6000 Kelvins or above. A standard incandescent lamp has a filament at 2700 Kelvins, and therefore (by definition) a Colour Temperature of 2700 Kelvins.
The Colour Temperature of a light source defines its "whiteness", its yellowness or blueness, its warmth or coolness. It does not define how natural or unnatural the colors of objects will appear when lighted by the source. Two colors of lamps can have the same Colour Temperature, but render colours very differently. Low colour temperatures imply warmer (more yellow/red) light, while high color temperatures imply a colder (more blue) light.
Skylight is the light we perceive when looking away from the sun at the sky. Sunlight is the light observed when looking directly at the sun (don’t do this).
Daylight may be thought of as a combination of skylight and sunlight. Daylight varies from a rather low colour temperature near dawn, and a higher one during the mid-day. A colour temperature of 6,500 K, is taken to represent “standard” daylight.
Sunlight and skylight differ in appearance. On a clear day when the sun is overhead, sunlight has an even distribution of all wavelengths, appearing whitish (5000 - 5500 K). As the afternoon progresses, it becomes increasingly yellow, then, depending on atmospheric conditions, orange, and finally, just before sunset, red (1800K).
Skylight ranges from very pale blue to deep blue. The purity and saturation of the blue is influenced by atmospheric moisture, dust and pollution. Generally speaking, the sky is palest when the atmosphere is humid or laden with dust, and it’s deepest blue when the air is dry and free of pollutants. This corresponds to colour temperatures of about 6500 K to well over 20,000 K, respectively, the higher Kelvin temperatures indicating predomination of the blue-violet wavelengths. The colour quality of the sky also varies with latitude, partly because different latitudes have correspondingly different weather
patterns. Countries with drier air tend to have deeper blue skies; those with more atmospheric moisture tend to have paler skies.
The colour temperature of light sources (lamps) makes them visually "warm," "neutral" or "cool" light sources.
Lamps with a lower colour temperature (3500K or less) have a warm or red-yellow/orangish-white appearance. The light is saturated in red and orange wavelengths, bringing out warmer object colours such as red and orange more richly.
Lamps with a mid-range colour temperature (3500K to 4000K) have a neutral or white appearance. The light is more balanced in its colour wavelengths.
Lamps with a higher colour temperature (4000K or higher) have a cool or bluish-white appearance. Summer sunlight has a very cool appearance at about 5500K. The light is saturated in green and blue wavelengths, bringing out cooler object colours such as green and blue more richly.
Some typical color temperatures
1600-1850 K Candlelight
2650 K 40 W incandescent lamp
2700-3000K ‘Deluxe Warm White’ Fluorescent
3000 K 200 W incandescent lamp
2000-3200 K Natural Sunrise/sunset
3000-3200 K ‘Warm White’ Fluorescent Lamp
3400 K Tungsten lamp
3400-3500 K 1 hour from dusk/dawn
5000-6500 K Xenon lamp/light arc
5000-6500 K ‘Day-lite’ Fluorescent Lamp
5500 K Natural Noon - Sunny daylight around noon
5500-5600 K Electronic photo
6000-8000 K Overcast sky - shade
10,000 K Horticultural Lamps
9000-12000 K Blue sky
28,000 K North sky
How Light Affects the Colors of Objects
Color rendering index (CRI) is a system derived from visual experiments. It assesses the impact of different light sources on the perceived color of objects and surfaces. The first step is to determine the color temperature of the light source being rated. Next, each of eight standard color samples is illuminated—first by the light source and then by a light from a blackbody matched to the same color temperature. If none of the samples changes in color appearance, the light source is given a CRI rating of 100. Any changes in color appearance which do occur result in a lower rating. The CRI decreases as the average change in the color appearance of the eight samples increases. Any CRI rating of 80 or above is normally considered high and indicates that the source has good color properties.
[/u]Colour Rendering Index[/u] (CRI or Ra)
is a numeric indication of a lamp’s ability to render individual colours accurately, relative to a standard. The CRI value is derived from a comparison of the lamp’s Spectral Distribution to the standard (e.g. a black body or the daytime sky) at the same colour temperature
The highest CRI attainable is 100. Typical cool white fluorescent lamps have a CRI of 62. Lamps having rare-earth phosphors are available with a CRI of 80 and above. Technically, CRI's can only be compared for sources that have the same Colour Temperatures. However, as a general rule
"The Higher The Better"; light sources with high (80-100) CRI's tend to make people and things look better than light sources with lower CRI's.
Spectral Energy Distribution
When we look at a light source, the eye "perceives" a single
colour. In reality, we are seeing literally thousands of colors and hues made up of a combination of different wavelengths of light. These different combinations and the relative intensity of various wavelengths of light are used to determine the CRI of a light source.
Spectral Power Distribution (SPD) Curves provide detailed plot of relative power emitted in the different regions of the spectrum. They show the radiant power emitted by the source, at each wavelength or band of wavelengths, over the visible region (380 to 760 nm). Such a plot, with color shadings to indicate the colours corresponding to the different wavelengths, is very useful in providing a visual feel for the colour balance in a lamp. In general, continuous spectra or very full-line spectra produce less distortion of object coloors than a few discrete lines.
- Incandescent Lamps and Natural Daylight produce smooth, continuous spectra.
- High Intensity Discharge Lamps (HID) produce light in discrete lines or bands (used in spectral analysis to identify or fingerprint the material producing the light).
- Fluorescent Lamps produce a combined spectrum - a continuous or broad spectra from their phosphor, plus the line spectra of the mercury
The C.I.E. (Commission Internationale de l'Eclairage, the International Commission on Colour) diagram is based on the idea that mixing varying proportions of three hypothetical primaries (not necessarily red green and blue) can create the sensation in the human observer, of any colour of light. The three "primary" colours are dubbed "X," "Y," and "Z." If we are merely concerned about colour and not about brightness, we can specify just the relative strengths of these three colours, denoted by x, y and z. Since x + y + z must add up to 1 (i.e. 100%) just providing x and y is sufficient to specify lamp colour; the z value is implied. Lamp colour can then be represented on a two-dimensional plot of x and y. All possible colours then fall under a "guitar-pick" shaped triangle in which the perimeter encompasses spectrally pure colours (seen in nature only in rainbows and prisms) ranging from red to blue. Moving toward the center "dilutes" the colour until it ultimately becomes "white". Specifying the x,y coordinates locates a colour on the colour triangle.
Warm vs. Cool
Some people find it confusing that low colour temperature light sources are called “warm” while those with higher temperatures are referred to as “cool.” In fact, these descriptions have nothing to do with the temperature of the blackbody radiator but refer to the way color groups are perceived - the psychological impact of lighting. Colours and light sources from the blue end of the spectrum are referred to as cool, and those toward the red/ orange/yellow side of the spectrum are described as warm.
In Summary (CCT & CRI)
Color temperature and CRI provide some helpful information, but they are not perfect. Color temperature, for instance, fails to indicate anything about how a given light source will render colors. For example, imagine two “cool” light sources with similar color temperatures and color appearances. Suppose light source A produces fairly uniform energy, Suppose light source B, which looks the same, produces a similar spectrum except with almost no light in the red. Red objects which appear natural under light source A will therefore look dull and colorless under light source B even though both lights have the same color temperature.
In general, a high CRI figure means a light source will render colors well. However, since CRI figures are calculated for light sources of a specific color temperature, it is not valid to compare a 2700K, 82 CRI light source to one of 3500K, 85 CRI. In addition, remember that CRI is an average of eight different colors. This means that a light source with a high CRI will tend to render the broad range of colors well, but it is not a guarantee that any specific color will appear natural. Used in conjunction, however, color temperature and CRI can provide excellent benchmarks for the comparison of light sources.
Each element absorbs light of a particular frequency—a particular color.
the attributes of color — which are: the amount of green-or-red, the amount of blue-or-yellow, and the brightness. Note that these attributes are opposites, like hot and cold. Color nerves sense green or red — but never both; and blue or yellow — but never both. Thus, we never see bluish-yellows or reddish-green. The opposition of these colors forms the basis of color vision.
Sources used for general lighting will gradually shift in appearance to become "white" to the viewer, whether they are yellow/white like incandescent, or Lucalox® high pressure sodium lamps - or, blue/white like daylight. Within reason, the human color vision process tends to compensate or fill in for those colors lacking in the spectrum: red in the case of daylight, blue for incandescent, etc.
The eye's previous state of adaptation is also a factor. A warm environment
will look even warmer to the occupants if they enter it from a cold, bluish space. It will look cooler if they have been in a yellowish or pinkish one. Then the eye slowly adapts until the space appears to be lighted with "white" light - no matter what the eye was adapted to previously.
Psychology of Colour:
There is no "best" colour lamp nor is there any formal definition of "true" colour. Each spectral distribution "distorts" object colors compared to another, whether the light comes from a natural source such as sunshine, north skylight, sunset, or electric
sources such as incandescent, fluorescent and HID. The "right" colour source for a given application depends on personal preferences, custom and, to a very large extent, an evaluation of the tradeoffs in efficiency, cost, and colour rendition.
Certain colours are believed to have behavior-altering capabilities. Some colours or combinations of them irritate eyes and cause headaches. For example, bright yellows (either on walls or as the background on a computer screen) are the most bothersome colours, and are not calming or relaxing in any way. Bright colours reflect more light, so yellow over-stimulates our eyes, causing strain and even irritability.
Other colours can alter how or what we eat. Blue is known to curb appetites. Why is this so? Blue food
doesn't exist in nature, with the exception of the blueberry. There are no blue vegetables, and hopefully, if you encountered a blue meat, you certainly wouldn't eat it. Because of this natural colour deficiency, there is no automatic appetite response to anything blue.
There are colours that can put us in a better mood, too. Green is the most restful colour for the eye. It has the power to soothe and comfort. Studies have even shown that people who work in surroundings that are green experience fewer headaches, stomach aches, and other signs of sickness or fatigue.
When people are asked about their colour preferences, there is a great disparity of opinion, but weighted averages of the responses rank the colours in the order:
blue, red, green, violet (purple), orange, and yellow, for both sexes.
Youth generally prefers warmer colours, maturity generally cooler ones.
Psycological or Physiological
All of the senses are involved in consciousness and perception, and there are interesting connections between them. It has recently been reported that sound causes vision to become more acute. Faint lights were more reliably perceived when their appearance was preceded by a sound. (Univ. of California
at San Diego
, as reported in Nature in October 2000).
Full Spectrum Lighting ~ Myth or Magic?
The term full-spectrum was coined in the 1960s by photobiologist Dr. John Ott to describe electric
light sources that simulate the visible and ultraviolet (UV) spectrum of natural light. There are now dozens of electric lighting products marketed as full-spectrum, some promising that they closely simulate daylight and can therefore provide benefits such as better visibility, improved health
, and greater productivity. The term full-spectrum is not a technical term, but rather a marketing term
, implying a smooth and continuous spectral power distribution (SPD) without the spikes and troughs in radiant energy common with most discharge light sources (e.g., fluorescent and metal halide). Full-spectrum products are usually marketed as electric light sources that emulate natural daylight; the explicit (or implicit) message is that "natural" daylight is always better than "artificial" electric light. Some full-spectrum light sources are also marketed as emitting ultraviolet (UV) radiation, as well as visible light.
The following analysis is based upon information generated by the NLPIP (National Lighting Product Information Program), the NRC (National Research Centre of Canada), and numerous other Academic research - and, I believe, represents the best unbiased information available.
Full-spectrum light sources and color perception
Full-spectrum light sources will probably provide excellent color rendering. Color rendering index (CRI) values for full-spectrum lighting sources are typically greater than 90. Color is a human perception constructed from the combination of the spectral power distribution (SPD) of the light source, the spectral reflectance of the materials being illuminated, and the tri-chromatic nature of the human visual system. If there are gaps or large variations in the SPD of a light source, there is a potential for confusion between the apparent colors of objects. Since full-spectrum light sources usually provide radiant power throughout the visible spectrum, subtle differences in the spectral reflectance characteristics of different objects are discernable. So, when color identification is part of the visual task, such as for graphic arts, museums and color printing applications, full-spectrum light sources will ensure good color discrimination.
Full-spectrum light sources and visual performance
Full-spectrum light sources will not
provide better visual performance than other light sources under most circumstances. Visual performance is the speed and accuracy of processing achromatic information (e.g., black print on white paper) by the human visual system. For instance, at the relatively high light levels typically found in schools and offices, visual performance is essentially unaffected by the spectral power distribution of the light source, so full-spectrum light sources are, lumen for lumen, no better than any other light source.
Lighting produced by full-spectrum lamps may be, however, perceived as providing brighter architectural spaces than other lamps. Three factors may contribute to this effect. First, full-spectrum light sources typically have a high correlated color temperature (CCT) of 5000K - 7500K. Lamps with higher CCT values produce greater brightness perception than lamps with lower CCT of the same luminance. Second, most full-spectrum light sources have high color rendering properties, meaning that surface colors will appear more saturated. Greater saturation will also give the impression of greater brightness. Third, the ultraviolet (UV) radiation produced by some full-spectrum fluorescent lamps has a fluorescing, brightening effect on textiles and paper that have been treated with whitening agents. These combined effects on brightness perception may indeed have positive impact on building occupants, but greater perceived brightness can also be a liability, depending upon the expectations of the space's occupants.
Full-spectrum light sources and health
Full-spectrum light sources will not
provide better health
than most other electric light sources. Recent research has shown that human daily activities are strongly influenced by the solar
light/dark cycle. The most notable of these daily, or circadian, cycles is the sleep/wake cycle; but other activities including mental awareness, mood, and perhaps even the effectiveness of the immune system go through regular daily patterns. Light is the most important environmental stimulus for regulating these circadian cycles and synchronizing them to the solar
day. Short wavelength (blue) light is particularly effective at regulating the circadian system; long wavelength (red) light is apparently inconsequential to the circadian system. Thus, to maximize efficiency in affecting the circadian system, a light source should not mimic a full spectrum, but instead should maximize only short wavelengths. Even if a full-spectrum light source includes short wavelength light in its spectrum, it will not necessarily ensure proper circadian regulation because, in addition, the proper intensity, timing, and duration of the light exposure are all equally important for satisfactory circadian regulation.
Light therapy treatment of seasonal affective disorder (SAD) usually involves regulated exposure to a white light source, commonly 10,000 lux at the eye for 30 minutes per day. Any white light source will be effective at these levels, so full-spectrum light source is in no way special for treatment of SAD.
Full-spectrum light sources have no demonstrable benefit for dental health. These claims have no scientific merit.
Full-spectrum light sources and psychological benefits
Full-spectrum light sources may
have psychological benefits, particularly in societies that place value on "natural" environments. One of the claims often associated with full-spectrum light sources is that they are most like natural daylight. Unlike full-spectrum electric light sources, however, daylight does not have a fixed spectrum. Rather, natural light varies with latitude, time of day, season, cloud cover, air pollution, ground reflectance, and, if a person is indoors, window tinting. Nevertheless, it cannot be denied that people consistently prefer natural lighting from windows and skylights to electrical
lights. These preferences are robust and may reflect psychological associations with the natural environment
that produce positive affect in many people. Positive affect induced by daylight may, in fact, help improve mood and motivation and thus increase productivity and retail sales. Full-spectrum light sources offer this positive association with daylight. Although positive psychological benefits from full-spectrum light sources may have been observed in some circumstances, there appears to be no biophysical explanation for those observations. Still, the power of psychological associations (“placebo” effect) cannot be denied, and it is certainly conceivable that cleverly marketed full-spectrum light sources may provide beneficial effects to some people susceptible to that marketing
. There appears to be a strong positive association with full-spectrum light sources that has resulted from marketing, presumably because of the association between full-spectrum lighting and "natural" light.
Some full-spectrum fluorescent lamps are promoted as producing ultraviolet (UV) radiation. This is peculiar, since in general, UV radiation should be avoided. UV radiation fades and deteriorates architectural materials and works of art. Even though full-spectrum lamps might improve the color appearance of artwork, museums specifically require all radiation shorter than 400 nm to be filtered completely from light sources illuminating environmentally sensitive pieces, such as watercolor paintings, and historical artifacts. Except in certain unusual cases, it is also undesirable for people to expose the eye or the skin to UV radiation
. Adverse effects of excessive UV radiation include sunburn (erythema), cataracts, and skin cancer. Several organizations, including the Illuminating Engineering Society of North America (IESNA), the American Conference of Governmental Industrial Hygienists (ACGIH), and the National Institute for Occupational Safety
and Health (NIOSH), have specified acceptable limits for occupational ultraviolet exposure (IESNA, 1996; ACGIH, 1991; NIOSH, 1972).
However, skin exposure to a fairly narrow band of UV radiation, UVB between 290 and 315 nm, can promote the synthesis of vitamin D, which is necessary for proper bone development and maintenance
. However, dietary sources of vitamin D, including dairy and fish
products, provide sufficient vitamin D to have eliminated the incidence of bone-related problems such as rickets, in modern society. These dietary supplements therefore minimize the importance of UVB radiation exposure for most people.
Full-spectrum fluorescent lamps that produce UV radiation use special phosphors with peak emissions in the UVA band (315 nm-400 nm), typically at 355 nm. Although the relative amount of UV radiation emitted by these lamps may be the same as a particular phase of daylight, the absolute amount of UV radiation they emit is quite small. For comparison, approximately 22 minutes of sunlight exposure near midday will produce 1.5 minimal erythema doses (MEDs) of UVB radiation exposure, enough to induce a pronounced temporary increase in vitamin D concentration. One MED is the amount of exposure that produces noticeable skin redness, so the exposure needed to affect vitamin D levels is substantial. Based upon the published data from one manufacturer of fluorescent lamps emulating the UVB content of daylight, it would take at least 30 hours of constant exposure to these lamps when operated at ceiling height to provide 1.5 MEDs. Based on UVB intensity data from another study (Ball, 2002), eight hours in an office or classroom under a claimed full-spectrum lamp will produce a smaller ultraviolet dose than one minute spent outdoors in bright sunlight.
Indoor environments such as offices and schools further reduce UV exposure because most lighting fixtures and architectural materials absorb UV radiation. UVB radiation that does not strike the skin directly is unlikely to reflect off objects, floors, and walls back to the skin. The resulting exposure level will be well below the threshold for measurable vitamin D production. Therefore, fluorescent lamps claiming to emulate the relative UV content in daylight can be disregarded as a viable source of UVB radiation for humans. Since there are no known benefits to human health from UVA radiation, it can be further concluded that the modest amounts of UV radiation produced by these lamps have no beneficial impact on human health.
Ironically, even small but constant amounts of UV (UVA and UVB) radiation will eventually degrade a wide variety of architectural materials such as carpet and cloth, wood products, and printed matter.
One claim occasionally cited as a benefit of fluorescent light sources emulating the UV content of daylight is the enhanced brightness of paper and clothing
treated with whitening agents. Fluorescent whitening agents are used to counteract the otherwise yellow appearance of paper and cloth, making them appear whiter and brighter.
To assess this claim, NLPIP* compared the relative luminance of white paper and of white cloth illuminated alternately by two fluorescent lamps of identical correlated color temperature (CCT), one claiming to emulate the relative UV content of daylight and one without the UV phosphor. By causing the whitening in the cloth or paper to fluoresce, the UV radiation from these lamps should produce higher luminance for the same given illuminance. Indeed, the measured luminance of a white paper sample and of a white cloth sample were 1.7% and 2.3% higher, respectively, when illuminated by the full-spectrum fluorescent lamp with more UV radiation. These effects were also perceptible when alternatively viewed, but any assumed benefits of these relatively small brightness-enhancing effects have never been documented. It should also be noted that lamps emulating the UV content of daylight have about 30% to 40% lower lamp efficacy (lumens per watt) than conventional fluorescent lamps of the same CCT, partly because additional electric power is required to generate the invisible UV radiation.
In summary, there are no known beneficial health effects from the UV radiation generated from these lamps
, but the UV radiation from these lamps can be harmful to many of the materials commonly found in architectural spaces. Further, although the UV radiation generated from these lamps can induce relatively higher luminance in white paper and clothing
, the loss in lamp efficacy needed to produce the invisible UV radiation is substantially greater than the fluorescence-induced luminance resulting from UV radiation.
Other than potential damaging effects on architectural materials from full-spectrum light sources that produce ultraviolet (UV) radiation, the primary disadvantages of full-spectrum lamps are economic. Full-spectrum lamps are often priced several times higher than conventional lamps. For example, a four-foot T12 lamp may cost between $1.25 and $3.60, while a full-spectrum four-foot T12 lamp may cost $14.90.
* National Lighting Product Information Program (NLPIP), at the Lighting Research Center of Rensselaer Polytechnic Institute, Troy, NY.
***E. & O. E. ***
- “Quantity of Light” (illuminance, brightness, glare, contrast, etc ...)
- “Applications” (specific light sources & their use)