Actually, daylight is a form of incandescent light. Incandescent light is the thermal radiation emitted by a hot object such as the filament of a light bulb of the surface of the sun. But the spectrum of incandescent light emitted by an object depends on its temperature. Since the filament of an incandescent light bulb has a temperature of only about 2500°C, its light is much redder than the light emitted by the 6000°C sun. Thats why photographys taken indoors with incandescent lighting turn out so orange, the light just isnt light, it's orange-red. So you can differentiate between sunlight and the light from an incandescent bulb by comparing the spectrums. Look for the relative intensities of red, green and blue lights. Sunlight will have much more blue in it than light from an incandescent bulb.

Regular (incandescent) light bulbs create light with a hot filament. This light is relatively reddish and contains very little blue, violet or ultraviolet light. Since it comes from a hot, thermal source, this light covers all the wavelengths from infra-red to the green and blue range of the spectrum continuously and smoothly, only its intensity peaks in the red and orange range of the spectrum. Fluorescent lights, on the other hand, create light through the fluorescence of atoms, moleculse and solids. The light is not created by hot materials so it contains certain regions of the spectrum, often including blue and violet light. Depending on the exact make up of the fluorescent lamp, this light may include wavelengths that are particularly important to a plant's metabolic processes.

When they're operating, halogen bulbs become extremely hot, so you certainly wouldn't want to touch them. But even when a bulb is cool, touching it would deposit greases and salts from your skin onto its surface. The aluminosilicate glass used in the lamp's envelope would be weaked when these salts are baked into the glass during the lamp's operation and the greases would scorch and darken the bulb's surface.

A normal incandescent lamp contains a double-wound tungsten filament inside a gas-filled glass bulb. By 'double-wound', i mean that a very fine wire is first wound into a long, thin spiral and then this spiral is again wound into a wider spiral. While the final filament looks about 1 or 2 centimetres long, it actually contains about one metre of fine tungsten wire. When the bulb is on, an electric current flows through the filament from one end to the other. The electrons making up this current carry energy, both in their motion and in the forces tha they exert on one another. As they flow through the fine tungsten wire, these electrons collide with the tungsten atoms and transfer some of their energy to those tungsten atoms. The tungsten atoms and the filament become extremely hot, typically about 2500° Celcius. Tungsten wire is used because it tolerates these enourmous temperatures without melting and because it resists sublimation longer than any other material. Sublimation is when atoms 'evaporate' from the surface of a solid. The gas inside the bulb is tehre to slow sublimation and extend the life of a filament.

Once the filament is hot, it tends to transfer heat to its colder surroundings. While much of its heat leaves the filament via convection and conduction in the gas and glass bulb, a significant fraction of this heat leaves the filament via thermal radiation For any object that is hotter than about 500° Celcius, some of this thermal radiation is visible light and for an object that is about 2500° Celcius, about 10 % is visible light. The light that you see from the bulb is the visible portion of its thermal radiation. However, most of the filament's radiation is invisible infra-red light. While you can feel this infra-red light warming your hand, you can't see it. Because only about 80 % of the electric power delivered to the bulb becomes thermal radiation and only about 12 % of that thermal radiation is visible, an incandescent light bulb is only about 10 % energy efficient. Other types of lamps, including fluorescent lamps and gas discharge lamps, are much more energy efficient.

Metal-halide lamps are actually high-pressure mercury lamps with small amounts of metal-halides added to improve the colour balance. Light in such a lamp is created by an electric arc; electricity is passing through a gas in a lamp and causing violent collisions within the gas. These collisions transfer energy to the mercury and other gaseous atoms in the lamp and these atoms usually emit that energy as light. Overall ,an electric current passes through the lamp and gives up most of its energy as light and heat in the gas. As you've noted, the lamp is relatively efficient, meaning that it produces more light and less heat than ordinary incandescent or halogen lamps. However, metal-halide lamps aren't quite as energy efficient as 'fluorescent lamps' lamps.

What makes a metal-halide lamp so efficient is that there are relatively few ways for the lamp to waste energy as heat. While collision-excited mercury atoms normally emit most of their stored energy as ultraviolet light; the basis for fluorescent lamps; they can't do this in a high-pressure environment. A phenomenon called 'radiation trapping' makes it almost impossible for this ultraviolet light to escape from a dense vapour of mercury, so a high-pressure mercury lamp emits mostly a visible light. Even without the metal-halides, a high-pressure mercury lamp emits a brilliant blue-white glow. The metal halides boost the reds and other colours in the lamp to make its light 'warmer' and more like sunlight.

Next time you watch of those lamps warm up, observe how its colours change. When it first starts up ,its pressure is low and emits mostly invisible ultraviolet light (which is absorbed by the lamp's glass envelope). But as the lamp heats up and its pressure increases, the rich, white light gradually developes. Incidentally, if the power to a hot lamp is interrupted, the lamp has to cool down before it can restart because it only starts well at low pressures.

Whenever you turn on a fluorescent lamp, a small amount of metal i sputtered away from the electrodes at each end of the tube. Thes electrodes are what provide electric power to the gas discharge inside the lamp and sputtering is a process in which fast moving ions (electronicalyy charged atoms) crash into a surface and knock atoms out of that surface. Because sputtering is most severe during startup, a typical fluorescent tube can only start a few thousand times before its electrodes begin to fail. To avoid this expense and hassle of having to replace the tube frequently, you shouldn't cycle the lamp more than once evry ten minutes. If you will only be away for a minute of two, leave the lamp on. But if you will be away for more than about ten minutes, turn it off. Incidentally, the claim that a fluorescent lamp uses a fantastic amount of electric power during start-up is nonsense.