When they’re operating, halogen bulbs become extremely hot, so you certainly wouldn’t want to touch them then. 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 weakened when these salts are baked into the glass during the lamp’s operation and the greases would scorch and darken the bulb’s surface.
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 infrared to the green and blue range of the spectrum continuously and smoothly, although 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, molecules, 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.