Light barrier layer

There are another type of photocells known as barrier layer photocells which work on an entirely different principle. They are semiconductor devices in which impinging photons promote the electrons from the valence band to the conduction band across the energy gap. A photovoltagc is generated which can be measured by a voltmeter. Such photovoltaic devices can have a large surface area and are easy to operate. They are commonly used in many simple colorimeters and fluorimeters and as light Otters for cameras. 

This issue has been addressed by the second type of aseptic filling system in which a polyethylene bottle (which can be multi-layered to add a gas or light barrier) is moulded under sterile conditions. While the bottle is still sterile, and the polyethylene is soft, the trim material above the neck finish (moil) is crimped closed to seal the bottle. The sealed, sterile, empty bottle can then be packed and transported to a filling line, which may be in another location from... 

A slit adjustable for both length and width is desirable, but improvisation with black tape can compensate for the absence of proper adjusting mechanisms. A slit width of about 1 mm is needed to provide enough light for the classic barrier-layer photocell, but as narrow a slit as possible should be used to minimize the degree of uneveness in the area covered. Toward the edges of the paper strip the protein (and therefore the dye) concentration of the zones decreases (variably because of different diffusion rates) to zero. The length of the slit should be such that only the central, protein-filled part of the paper strip is scanned (Fig. 

The light leaving the paper should pass through an exit slit to avoid the effect of scatter on the surface of the barrier-layer photocell (Fig. 

It is used for decolorising glass and in photoelectric cells. The electrical conductance of Se in the metallic form is increased as much as 200 times by light. Another application is in the iron-selenium barrier-layer cell which generates a current when illuminated. This system also operates as a rectifier because current flows more readily from iron to selenium than in the opposite direction when an external potential is applied. 

Photodetectors are devices that convert light into an electric signal that is proportional to the number of photons striking its photosensitive surface. The photomultipfier tube is a commonly used photodetector for measuring light intensity in the UV and visible regions of the spectrum. Photodiodes are solid-state devices that are also used in modern instruments. In older instruments, barrier layer cells (also known as photovoltaic cells) were used as photodetectors, because they were rugged and less expensive. ... 

Surface chlorination can produce good barrier layers with less tendency to provide flexure failures. Unfortunately, chlorine reacts too slowly to be practical as a treatment agent in the absence of promotion by UV light. It is clearly not convenient to use UV in the interior of containers, so alternative activators were sought. Chemical activation of the chlorination reaction was shown to be possible using a mixture of 10% 303/90% CI2 as such or further diluted with air or nitrogen at 25°C. 

Three types are used. (I) Barrier-layer cells. These are satisfactory only for simple filter instruments. (2) Vacuum phototubes. These tubes require an external power supply, unlike barrier-layer cells, and their output is usually amplified before measurement. (3) Photomultiplier tubes are easily the most satisfactory detectors for use in flame photometry. The photocurrent is amplified inside the tube in such a way that much lower light levels can be detected and measured accurately than is possible with vacuum phototubes with amplifiers. A stable source of high voltage up to perhaps 2000 volts is required to operate the photomultiplier tubes, but these tubes are almost universally used in high-performance instruments and are essential if the advantages of using narrow band width are to be obtained. 

---