LIGHT & LIGHTING

Light intensities
Orchids require visible light intensities in the range 700 - 4500 foot-candles, for growth. This is equal to about 10,000 - 50,000 lumens/square meter. In addition, orchids need lower light levels (300 - 400 foot-candles) of (invisible) infrared light for bud formation and blooming.

Required light colors
Orchids, like other plants, need light colors ranging from blue (300-450 nanometer wavelength) to green and yellow (450-600 nanometer wavelength) to red (600-700 nanometer wavelength) and infrared (more than 700 nanometer wavelength).
Orchids will grow if irradiated with light ranging from anywhere between 300 (blue)and 700 nanometers (red). This is called the assimilation wavelength range.

Apsorption spectrum of chlorophyl
Sometimes light requirements of plants are confused with the absorption spectrum of the green colored material 'chlorophyl' in plants. The absorption spectrum of chlorophyl possesses two apsorbtion peaks, one in the blue around 450 nanometers, and one in the red around 675 nanometers. Chlorophyl does not absorp green and yellow light. These latter two colors are reflected by chlotophyl. That's why most plants leaves exhibit a green appearance. Besides chlorophyl, other materials in plants absorp colors coinciding with and/or in addition to those colors absorped by chlorophyl.
Therefore, plants absorp all colors of the visible spectrum, other molecules than chlorophyl also contribute to growth and to other light induced processes taking place in plants.

Assimilation
The plant's assimilation process (fixation of CO2 and water under the influence of light) takes place when the plant is irradiated with light anywhere in the visible part of the light color spectrum between 400 and 700 nanometers. For red light (700 nanaometers), the assimilation efficiency is 2 times higher than for blue light (400 nanometers). Orchids thus combine CO2 and water much faster under red light illumination than under blue light. The color dependent assimilation sensitivity curve is shown in the picture below. CLICK ON PICTURE TO ENLARGE

For inducing buds and blooming, orchids may also need other colors than those causing an efficient assimilation. To induce flowering, plants need additional (invisible) infrared illumination at wavelengts longer than 700 nanometers, and at low intensities.

Role of various light colors
Blue light is required for the proper regulation of the vegetative growth of orchids. Blue light decreases the growth rate; that's why plants grow towards a light source. The darker side of a plant grows faster than the illuminated side.
Besides, relatively low light levels of red and infrared light are needed for bud formation and blooming.

Seasonal influences of light
In wintertime, sunlight availability in geographical areas with parallels in the range 40-70 degrees North and South (e.g. USA, Europe, Japan, Southern South-America, Australia, etc.) is much lower than in summertime.

Reduced light intensity in wintertime
In January, around noon in the Northern hemisphere, the sunlight intensity is about 5 times lower than at noon in June, owing to the low 'height' of the sun. In addition, the total light available for plants, accumulated over a whole day in January, might be 10 times lower than in June, owing to shorter days and increased presence and thickness of clouds. In wintertime, therefore, growth will hamper.

Photoperiodicity
The changing lengths of daylight and light intensities influence the behaviour of plants. Besides influencing the growth, day light shortening or lengthening influence the induction of budding and blooming. Some orchids need shorter days to start blooming, while other orchids need longer daylight periods to induce flowering.
This day light dependent behaviour is called 'photoperiodicity'.
Orchids exist that do not care about change of day length with respect to blooming.
Besides photoperiodicity, other factors like temperaure and humidity may influence the readiness of orchids to start blooming.

Compensation of low light levels
In order to compensate for the low light levels in wintertime, thereby securing sufficient growth levels during the winter, artificial light sources such as lamps can be applied to provide the necessary additional light.

Lamp types
Several basic types of lamps exist: Incandescent, Fluorescent, and High Intensity Discharge (HID).

The properties and applications of the various lamp types have been described in some detail below. The numerical data of all discussed lamp types has been summarised in the tables below, for easy comparison.

Incandescentt light sources
Incandescent (carbon wire; tungsten wire; tungsten-halogen) lamps mainly emit yellowish, reddish and infrared light (the latter being virtually heat!), with very little bluish light. Incandescent lamps resemble to some extent sunlight, but with much less blue light! Incandescent lamps are, therefore, not suited for formative/vegetative growth of orchids.
In addition, incandescent lamps are very inefficient light sources. They emit only 5-15 lumens per watt and thus are expensive to operate. They are available in the power range 10-300 Watts and are cheap to buy; their cost is of the order $'s each.

A special type of incandescent lamp is the (tungsten) halogen lamp. They are a little bit more efficient (10-20 lumens per Watt) than 'normal' incandescent lamps and emit a little bit more bluish light. They are available in powers up to 500-1000 Watts and price depends on power.

Incandescent lamps (including halogen lamps) are in fact nothing more than heaters. They have only one advantage: you can see what you heat, but they are not suited to stimulate a healthy orchid growth. Sometimes, incandescent lamps are used as an additional light source to induce flowering when low levels of infrared illuminationis needed.

The numerical data of all discussed lamp types has been summarised in the table below, for easy comparison.

Fluorescent light sources
Fluorescent tubes exist in several types: 'daylight', 'cool' and 'warm'. Since the eye is 10 times more sensitive for green-yellow colors as compared to blue and red colors, a slight variation in the balance of blue, green, yellow and light output already gives a cool or warm impression.

'Daylight' tubes resemble to some extent the day/sunlight spectrum and emit still more blue light as compared to 'cool' tubes.
'Cool' tubes emit less blue and more red colored light as compared to 'daylight' tubes. 'Warm' tubes emit still less blue and still more red colored light as compared to both 'cool' tubes and 'daylight' tubes.

Fluorescent tubes are characterized by so called 'light radiation temperatures'.
Sunlight itself is characterised by a 'temperature' of about 6,500 Kelvin, being the temperature of the sun's surface. The higher a light source's 'temperature', the more blue light a lamp emits. 'Daylight' tubes possess a temperature of approximately 5-6,000 Kelvin, 'Cool' tubes are characterised by a temperature of 4,000 Kelvin and 'warm' tubes by 3,000 Kelvin.

Virtually all types of fluorescent tubes are suited to some extent for vegetative growth and blooming. However, significant differences in color output exist between the various types.
Although fluorescent tubes can be characterised by a temperature they all emit much less red light as compared to sunlight.

A special fluorescent tube type exist that emits almost only bluish and reddish light and virtually no yellow. This type of tube is very well suited for both growth and blooming.

Fluorescent tubes are about 10 times more efficient (70-80 lumens per Watt) as compared to incandescent lamps. Fluorescent tubes are mostly available in the power range 10-65 Watts and cost of the order of a few $'s each.
Fluorescent tubes are in principle low pressure mercury discharge tubes with phophors coated on the inside of the tube to shift the light colors more to the visible yellow/red colors in trying to imitate the solar (sunlight) spectrum.

The numerical data of all discussed lamp types has been summarised in the table below, for easy comparison.

High Intensity Discharge (HID) light sources
HID lamps for plants are available in three basic types: mecury vapor (Hg), metal halide (MH) and sodium vapor (Na).

Mercury HID light sources
Hg HID lamps exist in low pressure and high pressure types. The low pressure type mainly emits blue light. The high pressure types emit mainly blue light, too, and in adition some green, yellow and red light. Sometimes, besides increasing the Hg pressure, phosphorescents have been added to assist in shifting the color from blue to yellow and red.

Hg HID lamps are mostly suited for vegetative growth (only). They are rather efficient (50-70 lumens per Watt) and powerful (250-1000 Watts). However, they are expensive, of the order of $100s including fixture and ballast. The blue light emitting Hg HID lamps are often used in combination with red ligth emitting sodium vapor (Na) lamps.

A special Hg-Tungsten lamp is the one in which a Hg discharge is combined with a tungsten incandescent wire. This lamp emits blue as well as red light, possessing a combined efficiency of 30 lumens per Watt. This lamps is suited for formative as well as bud growth of orchids, but is not very efficient in electrical to light conversion.

The numerical data of all discussed lamp types has been summarised in the tables 1-3 below, for easy comparison.

Metal Halide (MH) HID light sources
MH HID lamps mainly emit green and yellow light with a bit of red. Their light output appears as 'white' light (white light such as sunlight is obtained if a light source emits all colors simultaneously). MH HID lamps are rather efficient (70-90 lumens per Watt) and powerful (250-400 Watts). Their spectral output, resembling sunlight to some extent, is characterized by a temperature of about 4000 Kelvin. However, MH HID lamps are expensive, their price being of the order of $100s including fixture and ballast.

The numerical data of all discussed lamp types has been summarised in the table below, for easy comparison.

Sodium (Na) HID light sources
Two type of Sodium HID lamps exist: the low and the high pressure types.

The low pressure type emits a single (orange) color centered around 589 nanometers and is available with powers up to 150 Watts. Low pressure Sodium lmaps are the most efficient lamps known, up to 180 lumens per Watt. They are, in principle, suited for bud formation and blooming only.

Sodium HID high pressure lamps emit mainly red light with some yellow and blue. They are mainly suited for bud formation and blooming. They are very efficient (up to 120 lumens per Watt) and powerful (250-1000 Watts).

However, both types of Sodium lamps are expensive, of the order of $100s including fixture and ballast. The red light emitting Na HID vapour is mixed with xenon gas. This lamp emits more yellow and blue and is suited for formative growth and bud formation. The efficiency of this lamps is lower than the low and high pressure Sodium lamps, being 70 lumens per Watt ('only').

The numerical data of all discussed lamp types has been summarised in the table below, for easy comparison.

Comparison of light sources
General remark: both fluorescent tubes and MH HID lamps are very efficient in terms of lumens/Watt. However, a single type HID lamp may still not be optimal and efficient for both formative growth and blooming induction.
Fluorescent tubes with high 'light temperature', Na-Xe-lamps and Hg-Tungsten lamps seem OK since they all emit more or less a 'white light spectrum' (= all wavelengths or colors emitted simultaeously), the latter two lamp types with, however, reduced lumens per Watt values. Fluorescent tubes are rather efficient.
The efficient Hg and sodium vapour lamps should be used in combination to cover all relevant colors adequately.

The data of all light sources mentioned above has been summarized for comparison in the table below (these data are not accurate and only serve the purpose to give you a flavor).

Phyto lumens versus 'normal' lumens
The usually presented values of lumens of light sources reflect the influence of the human eye's color sensitivity curve.
However, plants possess an assimilation color sensitivity curve being very different from the human eye's color sensitivity curve.
With respect to plant growth it is much more interesting to know a lamp's characteristics related to a plant's color growth sensitivity curve. This growth related lamp aspect can be expressed as so called 'phyto lumens'.

In table 4, a summary of calculated phyto lumens is presented for various light sources. The phyto lumens have been calculated from the corresponding 'normal' lumens, taking both the eye's and the plant's color sensitivity curves into account.
In addition, the blue, green-yellow and red light contributions, expressed in phyto lumens, for various lamp types are presented in table 4.

If orchid leaves are turning increasingly lighter green, the orchid might be receiving too much light. A more shaded spot should be selected for that orchid.

If orchid leaves turn increasingly darker green, the orchid might be getting too little light and a brighter spot should be selected for that orchid. Caution: Acclimate plant to higher light level by moving it into brighter light by small steps.