Light penetration

Sihcon cells are hundreds of micrometers ( -lm) thick in order to faciUtate handling with minimal breakage, although most solar radiation is absorbed in the first 20—30 p.m. Light penetration decreases exponentially, proportional to, where d is the absorption coefficient of a material and T is its thickness. The values of (X for a given material vary with the wavelength of incident radiation in siUcon, (X is 10 —10 /cm over most of the range of usable solar radiation. 

Dimethyl sulfoxide occurs widely at levels of <3 ppm. It has been isolated from spearmint oil, com, barley, malt, alfalfa, beets, cabbage, cucumbers, oats, onion, Swiss chard, tomatoes, raspberries, beer, coffee, milk, and tea (5). It is a common constituent of natural waters, and it occurs in seawater in the 2one of light penetration where it may represent a product of algal metaboHsm (6). Its occurrence in rainwater may result from oxidation of atmospheric dimethyl sulfide, which occurs as part of the natural transfer of sulfur of biological origin (7,8). 

Production through much of the year will be subject to other constraints for example, the availability of light beneath the water surface. Seasonal differences in day length and periodic fluctuations in the depth of light penetration by active wavelengths often have an overriding effect on the net production rates and the supportive capacity. 

Clearly, unless monomer is the intended photoinitiator, it is important to choose an initiator that absorbs in a region of the UV-visible spectrum clear from the absorptions of monomer and other components of the polymerization medium. Ideally, one should choose a monochromatic light source that, is specific for the chromophorc of the photoinitiator or photosensitizer. It is also important in many experiments that the total amount of light absorbed by the sample is small. Otherwise the rate of initiation will vary with the depth of light penetration into the sample. 

Sunlight can enter a body of water to a depth defined as the photic zone (Figure 7.13). The lower boundary of the photic zone is the region where the light levels have fallen to 1 per cent of their surface value. The photic zone may be as little as 1 m where the water is unclear (due to particulates for example) or may extend up to 200 m. Within this region phytoplankton are capable of photosynthesis whereas below this region no light penetrates and the oceans are dark. Protection from all radiation is possible below 200 m and will allow chemistry and bond formation to occur. 

It is interesting that there is little change in the time required to cure the polymer as the concentration of BP is increased from 0.2 to 1.0 wt.%. These results suggest that the reaction proceeds to completion even before the light penetrates deep into the sample, perhaps by the propagation of a thermal front created by the initial photopolymerization at the leading surface. In any case, these results illustrate that the addition of a thermal initiator drastically reduces the polymerization time for thick systems. 

E. Nakada, Y. Asada, T. Arai, J. Miyake (1995) Light penetration into cell suspensions of photosynthetic bacteria and relation to hydrogen production. J. Ferment. Bioeng., 80 53-59... 

Euphotic Of the depth zone through which light penetrates. The bottom of the euphotic zone is defined as the depth at which less than 1 percent of the incident solar radiation remains, the rest having been either absorbed or reflected. 

Figure 5.9 Decrease in light penetration to the floodwater as a typical rice crop develops (J. Sheehy, IRRI, unpublished data)...

---