http://cannabis.com/growing/curing-w...chome-101.html

Quote Originally Posted by The Faqs
A small 25x or stronger pocket microscope, which can be picked up inexpensively at an electronics store like Radio Shack, works well for getting a closer peek at your trichome development. We are examining are the capitate stalked glandular trichomes, the coloration of these gland heads can vary between strains and maturity. Most strains start with clear or slightly amber heads which gradually become cloudy or opaque when THC levels have peaked and are beginning to degrade. Regardless of the initial color of the secretory cavity, with careful observation you should be able to see a change in coloration as maturity levels off.

Some cultivators wait for about half of the secretory cavities to go opaque before harvesting, to ensure maximum THC levels in the finished product. Of course nothing tells the truth more than your own perception, so try samples at various stages to see what is best for you & the phenotype your are growing. While you may be increasing the total THC level in the bud by allowing half of the glands to go opaque, the bud will also have a larger percentage of THC breakdown products such as CBN, which is why some people choose to harvest earlier while most of the secretory cavities are still clear.